I believe the issue with the schrader valve/no schrader valve is the compression is lost due to 2 stroke exhaust cycle and pressure is exhausted out of test hose vs with schrader valve it is contained
Dear Steve, thanks a lot for your videos, I follow you closely from Denmark. I have a compressiontester without a valve. Do you Think i can buy such a valve somewhere?
Well you had me worried there. I immediately ran out to the garage to check my tester. Fortunately it had the Schrader valve. I had tested a chainsaw at 45 PSI and after seeing what you did I wanted to make sure I hadn't condemned it improperly. Another fine lesson from Steve's Small Engine Saloon!
Every one of us without exception will be checking all of our compression testers today. Still working on coffee this sunday morning it can wait till later.
I have had lower end comp testers from Sears that did not work on small engines because they did not seal up enough on lower rpm engines. They would work fine on a car though. Before condemning, try the tester on a lawnmower just to make sure.
I believe the one with a schrader valve holds the compression in the hose so it doesn't have to rebuild that pressure back up, whereas the one without loses the compression out the exhaust each time the piston opens up the exhaust port.
There is a check (Schrader) valve in both - the difference is the location. With tiny cylinders, the volume of the hose averages the cylinder pressure, instead of picking the peak. In large cylinders it matters much less. If the hose volume is same or even larger than the cylinder volume, the peak reading drops down to half or less of the plain cylinder peak. Same hose volume would probably be just 5% or less of the big cylinder volume, and the averaging effect is basically eliminated.
I had no idea..... This will come in handy one day and save me a lot of trouble (and time...and money). Thank you, Steve- This is simply the BEST small engine channel out there!
For your viewers, I bought the Mityvac compression tester exactly like yours and it is awesome. Good to mention too that it comes with 5 spare shrader valve replacements and replacement O rings. Can not remember what I paid for it in Canadian dollars, but it was worth every penny. Cheers Steve
Steve, I learned this lesson the hard way a year ago. I purchased a compression tester without the schrader valve to test on my small engines. Results: low compression. Then I borrowed a compression tester that happened to have the schrader valve and all my small engine equipment tested fine. Of course, your video confirmed what I had suspected.
I've always used the Schrader valve hose for all my testing's. Even automotive use. The only time I remove the valve is when I'm doing a compression leak down test. Great explanation Stevo.
I have an old SNAP-ON compression tester kit that I purchased from a retired mechanic many years ago and have only used it sparingly on various engines of my own over the years. It has the Schrader valve in it....I didn't realize they made a tester without this valve? Learn something every day. Thanks Steve.....good explanation!
The automotive tester has a similar one-way valve, but it is all the way up at the gauge end of the tester, so all the dead space in the hose never lets the air expressed from the tiny cylinder in the saw get up to the actual pressure at the spark plug hole.
Soooo many people argue constantly with me that they are all the same ! That compression is compression…. Wrong ! I believe you will find if you use the schrader valve compression tester with a shorter hose (ECHO makes a good one) you will have to pull it less times to get to your top psi. Was told by my boss years ago that the schrader valve itself is not the same you would find in a car tire, they apparently have a much lower spring rating as to not affect the pressure.. Thank you Steve for this !!!!!! Keep on !
There are "snifter valves" used in plumbing to prevent vacumm inside pipes and to allow some air to enter water pressure tanks that do not have a membrane or bladder to separate the air from the water. Those valves can have (at least) a 3 PSI spring core and 0 PSI core without spring at all, the internal pressure is the only thing that keeps the valve closed and the vacuum opens them. The core used to keep air pressure on bladder tanks has a 60 psi spring. "X PSI spring" means that the valve with allow air to enter when the air pressure reaches X PSI without anything pressing on the tip, the valve core opens on its own due to that air pressure pushing the seat down, assuming the pressure on the internal side of the valve is zero, if it's higher you must add the internal pressure to the spring pressure and if the pressure on the internal side of the valve is negative you have to substract that value from the spring pressure. Of course using a normal inflator that pushes down the tip the valve opens it no matter what the internal and external pressures are.
A poster above hit the nail on the head. With the valve at the spark plug hole, the hose will fill up with the correct compression as the valve will not release the pressure, (takes more pulls to fill the hose to the same pressure as the actual compression.). But the open hose actually adds "trapped volume", (the volume of the head area with the piston at top dead center TDC), and a lower number will be read, just like cutting the combustion chamber of the head for less compression. I modify small 30cc engines for a living, and I know they run at about 180 psi. But my cheap automotive gauge reads 90 psi, (about half). That would be, for example, a compression ratio of 14:1 (Cylinder volume : Trapped volume), to actually be 14:2.
Just bought a Mityvac from Amazon . Works perfect . I wonder how many Testers have been sold because of your video Steve . You should get a commission. Thanks again Steve , You are a STAR.
I think you are exactly right! Without the Schrader valve, you are effectively lowering the compression ratio quite drastically. Which lowers compression dramatically. Think about increasing your combustion chamber to twice as large as it is. Your compression would drop a bunch! By adding a bunch of hose volume, it's effectively making your combustion chamber much larger, hence lower compression. Great job, great video, and excellent information. I've never given this a thought before, I guess, because luckily my gage has a Schrader valve. Thanks!
now ain’t that about something as I was doing a compression test this morning on a Briggs engine and now finding out about the shader valve on a compression tester. very informative thank you good job brother 💪
All compression testers have a check valve in the line somewhere, but the red-box version makes the mistake of putting it in the pipe just below the gauge instead of out at the end of the hose. Why? because that way, they only need one valve for the whole kit instead of one valve per line. Lots of kits have this right, but the photos on Amazon don't usually tell you where the valve is located. And yes, Steve, you got it right about the reason why the red-box kit doesn't work for a small-displacement engine. That added volume in the hose is enough to reduce the compression pressure dramatically. By the way, when the Schrader valve in the end of a compression tester goes south (and they will, eventually), it's not safe to assume that the ones for tires will work as replacements. What you need is a Schrader with a really low spring tension so that the compression will pop it open really easily. Some tool suppliers sell them, and you can usually pick them up from the guy on the tool truck if you work in a shop. The ones sold on Amazon for that purpose are not always right, though; hit up someplace that really does tools instead.
I always wonder about that. Same issue, around 40 lbs, but it ran great. At first, I thought it was the decompression value, but this is the first video I've seen that explains the correct compression tester with a schrader valve. Great video. Thanks
Thanks for putting out this info. I never thought that small engines would need to be a different one. I have done compression test on my small engines and most came out to about a 100 psi they ran good so i wasn't worried about but now thanks to you i will get one for my small engines. I have been working on engines since i was 13 years old now i'm 75 still working on them and still learning you never get to old to learn . Thanks and wishing you and yours a happy new year 🙂
I literally just had this problem couldn't figure out why the cheaper gauge wouldn't work. Now it make sense. Thank you. I bought an OTC set that one works great
45psi????? I about spit my beer out on that one! That old Husky sounded like it should have a kicker on it......haha. Good stuff man. Thank you, I didnt even realize they made a tester without the valve, other than the tapered rubber based units. Every time I stop in here I learn something new. Merry Christmas and Happy New Year to all!
Legend as always. Knew there's a difference and have a tester i only use for small engines but it's good to see your video and people talking about it in the comments. Happy Xmas, merry new year and peas n luv to you and yours. Xx
Great info Steve! I've known this for a long time, but some of the cheapie ones are terribly inaccurate even with the Schrader valve. No regrets about buying that Mityvac a while back. You are absolutely correct about the added volume of the hose. I just did the math: a 24" long hose, 1/4" in diameter will add over 19cc of volume!!!
Yep your right steve. The valve in the automotve tester is near the gauge. Basically what is happing is that when you pull the cord the second time, the air from the hose is pulled back into the engine. The small engine can only displace a small amount of air and is constantly draining and filling the hose as you have pointed out
It is easier to calculate than that. The added volume of the hose is actually the same as adding volume to the trapped volume (head volume with the piston at top dead center). It would be the same as cutting the combustion chamber larger for less compression. The hose does drain back to zero pressure as you noted for every cycle, but fluid dynamics is not really involved, just the volume of the hose.
@@EarthSurferUSA This is a great response. I have my opinions on this, but would love for a community response on this. I thought the study of fluid dynamics involved the principles of flow through a medium. We would love a more in depth explanation on how fluid dynamics is not really involved. Thanks
Fantastic. I had no idea. This will come in very handy now I have some land and have been buying equipment to get the property to speck for a family home. Thanks so much for this.
I think you’re correct on volume being the issue with the non-Schrader valve tester. Timely video as I was going to purchase one soon. Thanks for the tip and have a Happy New Year Steve.
Thank yo so much Steve, this video explains a problem I had with 2 Victa 2 stroke engines . I decided to have both engines Rebored and Honed and fitted New Pistons and rings. I was disappointed to find I was only getting 45 psi after all that time trouble and expense. This video explains why I was only getting 45 psi on the tester. Need to buy a new Tester. Thanks again Steve, thought I was going nuts.
@@Nozinbonsai Yes the Victa does have a decompression valve however ,You just need to remove it and screw in a spare spark plug to block the decompressor.
Hey Steve-O Loco Diesel Engine Tom again. Your explanation makes perfect common sense. Thanks again for all of your help to get me to understand these small engines ! Take Care, Tom-O
By adding the volume of the testers hose you have added combustion chamber volume. This effectively drops the compression ratio. Steve, you are right , the volume of that hose added to such a small combustion chamber will drop that reading. I have seen this happen to various degrees even on small displacement automobile engines too. Take the internal volume of that hose and add it directly to the combustion chamber volume, you can drastically drop the engines compression ratio.
Your theory on why is spot on. The hose is adding to the chamber volume at TDC, thus having more influence on the pressure with the smaller cylinder, effectively lowering the compression ratio (CR). The bottom Schrader (check) valve keeps accumulated pressure from escaping the hose and so not lowering CR after the pressure trapped in the hose maxes out. A shorter or smaller diameter hose made of material that doesn't easily expand would help, as it reduces the loss in CR, but the check valve at the spark plug hole is the best way to get an accurate reading no matter what size the cylinder is.
Yup, everything you said is right on! You're usually using the starter to turn over larger engines as opposed to hand cranking the smaller ones and that too helps with the non-Schrader Valve types being more accurate for larger engines. The Schrader valve types really are the only type to use for either if you want accuracy without second guessing every reading. Just like a quality tire pressure gauge vs. one from the dollar store for $1.99. The cheap one could be off by any amount and hardly any of them actually give the correct reading.
Yeah, man you a totally right that no valve version compresses extra air and that’s why it shows the moon phase instead of what we dare to check. Great observation! Thanks Steve!
Steve, it would have been helpful if you would have connected the schrader valve hose to the 45 PSI gauge and eliminate a bad gauge. There was an air fitting on it so it would have been easy. I agree with your assessment as to why it was so different.
Great info! The quench ratio is greatly chsnged eith small cc compairrd to your truck engine. Also remember when replacing the shrader valve it takes a different one than a regular old tire valve stem.
Many thanks for this video Steve, and greetings of the season to April and yourself from the UK. yes I did buy the wrong one, and yes I did take the chainsaw and outboard apart because of reading around 40psi. Every day's a school day.
Both systems hold the pressure being fed from the cylinder in the tube Steve. It's just that the system with the valve stops it bleeding straight back into cylinder where it will leak out (2 stroke?) on every stroke. Whereas it won't leak back past the Schroeder when it's NOT under a near constant supply of pressure pulses in a pull start engine. The fix would be to purchase an air-hold adapter. I only went and ordered a non Schroeder kit just a few hours ago.
Excellent point about the valve vs. no valve on the compression testing!! Really appreciate the pro tip! Thanks so much for sharing this one!!! Say HI to April also... When the weather gets warmer I will check out which tester I have. Stay warm up in Canada!!! ⛄❄
Well what know about the subject matter with my 35 years of experience of rebuilding all types of engines is as follows, the compression tester with no shrader valve should always read 0 when the engine is not cranking. There is nothing in the hose to hold any pressure at the gauge. The one with shrader valve will hold the pressure in the hose. Both tests have their own place. You have to remember that when an engine run it only gets one compression cycle per stroke cycle weather is 2 or four cycle. The one with the shrader valve reads max compression.
"the compression tester with no shrader valve should always read 0 when the engine is not cranking. There is nothing in the hose to hold any pressure at the gauge." That would mean you would have to read the gauge, during crank rotation at TDC.
I've had a mighty vac vacuum tester since the 80 or 90's. Works great but it dont get used much anymore with newer vehicles. I need a new compression tester Thanks for this video. I work on small engines as well as cars and trucks but had no idea of the differences until now. Thanks again.
I'm glad someone finally touched on this. I've been trying to get a decent compression tester for about 2 years cant seem to get a good one for under 100 dollars Canadian.
Yeah, I got the tester in the red case. Leaf blower read 25 psi but still ran, couldn't understand it till 8 found out about the Schrader valve. What I don't understand is why do you have to buy a whole new tester? Why can't you just buy a replacement tube only with the schrader valve?
I think you can, I have 3 gauges and 2 Hose sets, Both have the schrader valve and both hose interchange with each other, so you might be able to buy a new hose with the schrader valve in it.
Great video Steve, I was wondering if you could do a video about what a good range of compression is on small engines, online people say so many numbers im not sure what's correct, Thanks for sharing all your knowledge 👍🏼👍🏼👍🏼👍🏼
I have been master ase certified for 40 years. I suspect that the tester without the schraeder in the hose actually does have a check valve, probably a schraeder, in the guage body. However, I would caution, if using engine oil to temporarily reseal rings for a second test on a cylinder with low compression, make sure to inject the oil through the spark plug hole then crank the engine to belch out the excess oil before installing the tester. The purged oil can be caught on a shop rag. This second test was known as a dry/wet compression test and comes from the days when the labor to perform a tuneup included a complete compression test to help avoid spending money buying tuneup parts trying to fix an engine mechanical problem. I sometimes use oil to boost compression on a tired engine that cranks but wont start as a diagnostic aid and i haven't killed a tester since the first two after starting the excess oil purge method.
So glad someone made this video! Also.. Don't buy a Harbor Freight one! Always reads low. I bought one a LONG time ago at Auto Zone or Advance Auto I think. Has the valve. Even had to change it, started leaking. Has to be a special one just for this purpose. Yes it's the hose "adding" to the displacement of the engine. I test motorcycles also, wonder at what size they would test the same.
I think I might have to disagree about where the pressure is being measured and why one is different. I think both measure pressure all the way through the hose and up to the display meter, but the one with the valve prevents the compressed air from entering back into the head and leaking out through the piston and rings. When the engine is running there isn't time for the compressed air and gasses to leak out past the rings, but while testing there is plenty of time for the compressed air to leak back through the rings. My opinion. Thanks Steve for another video, I really enjoy your channel.
Yes you are correct. The compression ratio is determined by the combustion space above the piston at TDC. For that 65cc Husky the volume is likely to be near 6cc above the piston at TDC.. If you add 10cc of hose, then you have a volume of 16cc, which you are cramming the 65cc of the Husky into.
I've got the automotive type and never had an issue but when the weather gets better I'm gonna add a schrader valve to mine and see if it improves my readings.
That is a great video. I went to my tool box to look at mine. Mine has the schrader valve at the quick change coupler. I think mine is a Cal-van brand. I bought some brass adapters to check compression on troy bilt 4 cycle weed eaters. Small spark plugs Just has a hole that goes through it. Check the other compression tester hose and see for yourself if no schrader exists. Have a happy new year!
We'll, I could have used this information about three years ago. I had 40 psi on my Toro Tecumseh engine and I just pushed it to the road and bought a new one.
Thank you Steve. Now I know why one of my small engines runs fine even though the compression tested at about 40 PSI. I used an automotive compression tester.
Also good to know to look for that 40PSI level when testing with the non-schrader type. No need to rush out and buy another tester right away but for next time we will know better.
That’s absolutely right. The pressure in the hose needs to build to match that inside the cylinder. I discovered this when I was testing a lawnmower. The Schrader valve was loose and each pull you could see the pressure go up, then slowly go down. Once I tightened the valve, the pressure would build and hold. You must have the Schrader valve for small engines.
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I believe the issue with the schrader valve/no schrader valve is the compression is lost due to 2 stroke exhaust cycle and pressure is exhausted out of test hose vs with schrader valve it is contained
De
Dear Steve, thanks a lot for your videos, I follow you closely from Denmark. I have a compressiontester without a valve. Do you Think i can buy such a valve somewhere?
Well you had me worried there. I immediately ran out to the garage to check my tester. Fortunately it had the Schrader valve. I had tested a chainsaw at 45 PSI and after seeing what you did I wanted to make sure I hadn't condemned it improperly. Another fine lesson from Steve's Small Engine Saloon!
Every one of us without exception will be checking all of our compression testers today. Still working on coffee this sunday morning it can wait till later.
@@IslandwaterjetCoffee should ALWAYS be of the HIGHEST priority! 😂
Make sure the Schrader valve is tight. It could lose pressure and give a false reading. You want to see the gauge pressure build and hold.
I have had lower end comp testers from Sears that did not work on small engines because they did not seal up enough on lower rpm engines. They would work fine on a car though. Before condemning, try the tester on a lawnmower just to make sure.
I believe the one with a schrader valve holds the compression in the hose so it doesn't have to rebuild that pressure back up, whereas the one without loses the compression out the exhaust each time the piston opens up the exhaust port.
I think that's it.
Bingo, exactly
There is a check (Schrader) valve in both - the difference is the location. With tiny cylinders, the volume of the hose averages the cylinder pressure, instead of picking the peak. In large cylinders it matters much less. If the hose volume is same or even larger than the cylinder volume, the peak reading drops down to half or less of the plain cylinder peak. Same hose volume would probably be just 5% or less of the big cylinder volume, and the averaging effect is basically eliminated.
I concur
Ya i was going to say everyone i have seen have a check valve somewhere
Old dog here, learning new tricks! Never knew about the differences with small engines. Thanks Steve (good to see you back)! Happy New Year🇨🇦🇺🇸📆🎉
+1
I had no idea..... This will come in handy one day and save me a lot of trouble (and time...and money). Thank you, Steve- This is simply the BEST small engine channel out there!
For your viewers, I bought the Mityvac compression tester exactly like yours and it is awesome.
Good to mention too that it comes with 5 spare shrader valve replacements and replacement O rings.
Can not remember what I paid for it in Canadian dollars, but it was worth every penny.
Cheers Steve
Thanks Steve. Making the world a better place one video at a time.
Steve, I learned this lesson the hard way a year ago. I purchased a compression tester without the schrader valve to test on my small engines. Results: low compression. Then I borrowed a compression tester that happened to have the schrader valve and all my small engine equipment tested fine. Of course, your video confirmed what I had suspected.
I've always used the Schrader valve hose for all my testing's. Even automotive use. The only time I remove the valve is when I'm doing a compression leak down test. Great explanation Stevo.
I have an old SNAP-ON compression tester kit that I purchased from a retired mechanic many years ago and have only used it sparingly on various engines of my own over the years. It has the Schrader valve in it....I didn't realize they made a tester without this valve? Learn something every day. Thanks Steve.....good explanation!
The automotive tester has a similar one-way valve, but it is all the way up at the gauge end of the tester, so all the dead space in the hose never lets the air expressed from the tiny cylinder in the saw get up to the actual pressure at the spark plug hole.
Great seeing you back Steve , you’re always a must watch ! Thanks for sharing all your knowledge with us .
Old mechanic completely agrees with you Steve Happy New Year.
Soooo many people argue constantly with me that they are all the same ! That compression is compression…. Wrong !
I believe you will find if you use the schrader valve compression tester with a shorter hose (ECHO makes a good one) you will have to pull it less times to get to your top psi. Was told by my boss years ago that the schrader valve itself is not the same you would find in a car tire, they apparently have a much lower spring rating as to not affect the pressure..
Thank you Steve for this !!!!!! Keep on !
There are "snifter valves" used in plumbing to prevent vacumm inside pipes and to allow some air to enter water pressure tanks that do not have a membrane or bladder to separate the air from the water. Those valves can have (at least) a 3 PSI spring core and 0 PSI core without spring at all, the internal pressure is the only thing that keeps the valve closed and the vacuum opens them.
The core used to keep air pressure on bladder tanks has a 60 psi spring.
"X PSI spring" means that the valve with allow air to enter when the air pressure reaches X PSI without anything pressing on the tip, the valve core opens on its own due to that air pressure pushing the seat down, assuming the pressure on the internal side of the valve is zero, if it's higher you must add the internal pressure to the spring pressure and if the pressure on the internal side of the valve is negative you have to substract that value from the spring pressure.
Of course using a normal inflator that pushes down the tip the valve opens it no matter what the internal and external pressures are.
A poster above hit the nail on the head. With the valve at the spark plug hole, the hose will fill up with the correct compression as the valve will not release the pressure, (takes more pulls to fill the hose to the same pressure as the actual compression.). But the open hose actually adds "trapped volume", (the volume of the head area with the piston at top dead center TDC), and a lower number will be read, just like cutting the combustion chamber of the head for less compression. I modify small 30cc engines for a living, and I know they run at about 180 psi. But my cheap automotive gauge reads 90 psi, (about half). That would be, for example, a compression ratio of 14:1 (Cylinder volume : Trapped volume), to actually be 14:2.
Just bought a Mityvac from Amazon . Works perfect . I wonder how many Testers have been sold because of your video Steve . You should get a commission. Thanks again Steve , You are a STAR.
Planning to buy one soon. Great timing. Thanks Steve
I think you are exactly right! Without the Schrader valve, you are effectively lowering the compression ratio quite drastically. Which lowers compression dramatically. Think about increasing your combustion chamber to twice as large as it is. Your compression would drop a bunch! By adding a bunch of hose volume, it's effectively making your combustion chamber much larger, hence lower compression.
Great job, great video, and excellent information. I've never given this a thought before, I guess, because luckily my gage has a Schrader valve.
Thanks!
It is amazing how much knowledge is in your head. I learn something new from you all the time. Thanks, Steve
Glad you did this video on the 2 different types of commpression testers
now ain’t that about something as I was doing a compression test this morning on a Briggs engine and now finding out about the shader valve on a compression tester. very informative thank you good job brother 💪
All compression testers have a check valve in the line somewhere, but the red-box version makes the mistake of putting it in the pipe just below the gauge instead of out at the end of the hose. Why? because that way, they only need one valve for the whole kit instead of one valve per line. Lots of kits have this right, but the photos on Amazon don't usually tell you where the valve is located. And yes, Steve, you got it right about the reason why the red-box kit doesn't work for a small-displacement engine. That added volume in the hose is enough to reduce the compression pressure dramatically.
By the way, when the Schrader valve in the end of a compression tester goes south (and they will, eventually), it's not safe to assume that the ones for tires will work as replacements. What you need is a Schrader with a really low spring tension so that the compression will pop it open really easily. Some tool suppliers sell them, and you can usually pick them up from the guy on the tool truck if you work in a shop. The ones sold on Amazon for that purpose are not always right, though; hit up someplace that really does tools instead.
Will car AC system schrader valves work?
The little Springs have got weigh less than 1 lb of pressure bro.
Press on it with your finger
Good point with the valve. Thanks.
@@CROFTY71 How do you calibrate your finger? :)
I always wonder about that. Same issue, around 40 lbs, but it ran great. At first, I thought it was the decompression value, but this is the first video I've seen that explains the correct compression tester with a schrader valve. Great video. Thanks
You are right Steve! The mass of the air is spot on!
Thanks for putting out this info. I never thought that small engines would need to be a different one. I have done compression test on my small engines and most came out to about a 100 psi they ran good so i wasn't worried about but now thanks to you i will get one for my small engines. I have been working on engines since i was 13 years old now i'm 75 still working on them and still learning you never get to old to learn . Thanks and wishing you and yours a happy new year 🙂
I literally just had this problem couldn't figure out why the cheaper gauge wouldn't work. Now it make sense. Thank you. I bought an OTC set that one works great
45psi????? I about spit my beer out on that one! That old Husky sounded like it should have a kicker on it......haha.
Good stuff man. Thank you, I didnt even realize they made a tester without the valve, other than the tapered rubber based units. Every time I stop in here I learn something new. Merry Christmas and Happy New Year to all!
Good God man, be careful. NEVER spit out beer!
@@richarddykowski1060 It was def a close one! haha
That "Amazon" one probably has a schrader valve, but it'll be by gage, so almost renders it ineffective.
Legend as always.
Knew there's a difference and have a tester i only use for small engines but it's good to see your video and people talking about it in the comments.
Happy Xmas, merry new year and peas n luv to you and yours. Xx
Need all the real help we need to avoid some of my shop expenses; glad to learn from you Steve! Thnx much.
Great info Steve! I've known this for a long time, but some of the cheapie ones are terribly inaccurate even with the Schrader valve. No regrets about buying that Mityvac a while back. You are absolutely correct about the added volume of the hose. I just did the math: a 24" long hose, 1/4" in diameter will add over 19cc of volume!!!
Awesome! Thank You Stella
And we all laughed when our grade 5 math teacher told us we need to know how to calculate the volume of a cylinder.
Another great video of exceptional information on the right tool for the job ..
There may be lots of different tools for the job, but they're not all the right tool for the job. Thanks Steve.
Another Golden Nugget of Info 👍
Yep your right steve. The valve in the automotve tester is near the gauge. Basically what is happing is that when you pull the cord the second time, the air from the hose is pulled back into the engine. The small engine can only displace a small amount of air and is constantly draining and filling the hose as you have pointed out
It is easier to calculate than that. The added volume of the hose is actually the same as adding volume to the trapped volume (head volume with the piston at top dead center). It would be the same as cutting the combustion chamber larger for less compression. The hose does drain back to zero pressure as you noted for every cycle, but fluid dynamics is not really involved, just the volume of the hose.
@@EarthSurferUSA This is a great response. I have my opinions on this, but would love for a community response on this. I thought the study of fluid dynamics involved the principles of flow through a medium. We would love a more in depth explanation on how fluid dynamics is not really involved. Thanks
Fantastic. I had no idea.
This will come in very handy now I have some land and have been buying equipment to get the property to speck for a family home.
Thanks so much for this.
I think you’re correct on volume being the issue with the non-Schrader valve tester. Timely video as I was going to purchase one soon. Thanks for the tip and have a Happy New Year Steve.
Thank yo so much Steve, this video explains a problem I had with 2 Victa 2 stroke engines . I decided to have both engines Rebored and Honed and fitted New Pistons and rings. I was disappointed to find I was only getting 45 psi after all that time trouble and expense. This video explains why I was only getting 45 psi on the tester. Need to buy a new Tester. Thanks again Steve, thought I was going nuts.
Sometimes there's a decompressor which makes compression testing useless.
@@Nozinbonsai Yes the Victa does have a decompression valve however ,You just need to remove it and screw in a spare spark plug to block the decompressor.
Thanks for the info! I had heard many automotive compression testers were not accurate on small engines, now I know what to look for.
Hey Steve-O
Loco Diesel Engine Tom again. Your explanation makes perfect common sense. Thanks again for all of your help to get me to understand these small engines !
Take Care,
Tom-O
Awesome information Steve.
You're a great source of knowledge.
Thanks.
Happy New Year.🎉
Have a happy new year Steve and thanks for all the great advice mate from Britain.
By adding the volume of the testers hose you have added combustion chamber volume. This effectively drops the compression ratio. Steve, you are right , the volume of that hose added to such a small combustion chamber will drop that reading. I have seen this happen to various degrees even on small displacement automobile engines too. Take the internal volume of that hose and add it directly to the combustion chamber volume, you can drastically drop the engines compression ratio.
Your theory on why is spot on. The hose is adding to the chamber volume at TDC, thus having more influence on the pressure with the smaller cylinder, effectively lowering the compression ratio (CR). The bottom Schrader (check) valve keeps accumulated pressure from escaping the hose and so not lowering CR after the pressure trapped in the hose maxes out. A shorter or smaller diameter hose made of material that doesn't easily expand would help, as it reduces the loss in CR, but the check valve at the spark plug hole is the best way to get an accurate reading no matter what size the cylinder is.
Great video. I never considered this. Thanks.
Yes Steve much love and respect from the uk
Yup, everything you said is right on! You're usually using the starter to turn over larger engines as opposed to hand cranking the smaller ones and that too helps with the non-Schrader Valve types being more accurate for larger engines. The Schrader valve types really are the only type to use for either if you want accuracy without second guessing every reading. Just like a quality tire pressure gauge vs. one from the dollar store for $1.99. The cheap one could be off by any amount and hardly any of them actually give the correct reading.
Yeah, man you a totally right that no valve version compresses extra air and that’s why it shows the moon phase instead of what we dare to check. Great observation! Thanks Steve!
Steve, it would have been helpful if you would have connected the schrader valve hose to the 45 PSI gauge and eliminate a bad gauge. There was an air fitting on it so it would have been easy. I agree with your assessment as to why it was so different.
Thank you for your service Steve
Great Christmas gift idea for next year. Thanks and Happy New Year, Steve!
I thought that (this year) 🙂
Very helpful as always! Thank you Steve!
Thanks Steve!
I need to check mine out now!
Wow I didn't even know that was two pressure testers thank you for the video and happy New Years
Yes I had both style testers, always thought the one was way off for some reason ! thanks alot !! no one has ever mentioned this before !
I've never though about it before but I think you are absolutely right!
Thanks Steve Happy New year
Great video and great explanation.
Happy New Year to you and yours.
Great info! The quench ratio is greatly chsnged eith small cc compairrd to your truck engine. Also remember when replacing the shrader valve it takes a different one than a regular old tire valve stem.
Yes! Needed this video 3 years ago before I bought the wrong one.
Many thanks for this video Steve,
and greetings of the season to April and yourself from the UK.
yes I did buy the wrong one,
and yes I did take the chainsaw and outboard apart because of reading around 40psi.
Every day's a school day.
exsactly the mistake i made at the start of my small mower testing , now i know why the low reading , thanks steve
Both systems hold the pressure being fed from the cylinder in the tube Steve. It's just that the system with the valve stops it bleeding straight back into cylinder where it will leak out (2 stroke?) on every stroke. Whereas it won't leak back past the Schroeder when it's NOT under a near constant supply of pressure pulses in a pull start engine. The fix would be to purchase an air-hold adapter.
I only went and ordered a non Schroeder kit just a few hours ago.
What a wealth of knowledge thank you for another great video. Don't work on much anymore, but you're great.
Excellent point about the valve vs. no valve on the compression testing!! Really appreciate the pro tip! Thanks so much for sharing this one!!! Say HI to April also...
When the weather gets warmer I will check out which tester I have. Stay warm up in Canada!!! ⛄❄
April,Steve, Happy New Year. Thanks for all the great tips. God Bless
Well what know about the subject matter with my 35 years of experience of rebuilding all types of engines is as follows, the compression tester with no shrader valve should always read 0 when the engine is not cranking. There is nothing in the hose to hold any pressure at the gauge. The one with shrader valve will hold the pressure in the hose. Both tests have their own place. You have to remember that when an engine run it only gets one compression cycle per stroke cycle weather is 2 or four cycle. The one with the shrader valve reads max compression.
"the compression tester with no shrader valve should always read 0 when the engine is not cranking. There is nothing in the hose to hold any pressure at the gauge."
That would mean you would have to read the gauge, during crank rotation at TDC.
Really interesting and informative.
I've had a mighty vac vacuum tester since the 80 or 90's. Works great but it dont get used much anymore with newer vehicles. I need a new compression tester Thanks for this video. I work on small engines as well as cars and trucks but had no idea of the differences until now. Thanks again.
I got that Mity Vac kit a while back. It is a little pricey but the variety of adaptors is great and very useful
I have had the same problem on boat motor testing. I thought it was just bad guage. Thanks! 👍🏻😁
TY for Sharing
Thanks Steve , Happy New Year to you and yours 👍🏻
I'm glad someone finally touched on this. I've been trying to get a decent compression tester for about 2 years cant seem to get a good one for under 100 dollars Canadian.
Looking forward to a New Year with you and April. Love the videos
Yeah, I got the tester in the red case. Leaf blower read 25 psi but still ran, couldn't understand it till 8 found out about the Schrader valve. What I don't understand is why do you have to buy a whole new tester? Why can't you just buy a replacement tube only with the schrader valve?
I think you can, I have 3 gauges and 2 Hose sets, Both have the schrader valve and both hose interchange with each other, so you might be able to buy a new hose with the schrader valve in it.
Had this issue..thank you Steve great vid sir🙏
Awesome thanks!
I just ordered one on your recommendation!! I've had various luck using the red box one. Sometimes it gives me a good compression and sometimes not.
Great video Steve, I was wondering if you could do a video about what a good range of compression is on small engines, online people say so many numbers im not sure what's correct, Thanks for sharing all your knowledge 👍🏼👍🏼👍🏼👍🏼
I have been master ase certified for 40 years. I suspect that the tester without the schraeder in the hose actually does have a check valve, probably a schraeder, in the guage body. However, I would caution, if using engine oil to temporarily reseal rings for a second test on a cylinder with low compression, make sure to inject the oil through the spark plug hole then crank the engine to belch out the excess oil before installing the tester. The purged oil can be caught on a shop rag. This second test was known as a dry/wet compression test and comes from the days when the labor to perform a tuneup included a complete compression test to help avoid spending money buying tuneup parts trying to fix an engine mechanical problem. I sometimes use oil to boost compression on a tired engine that cranks but wont start as a diagnostic aid and i haven't killed a tester since the first two after starting the excess oil purge method.
So glad someone made this video! Also.. Don't buy a Harbor Freight one! Always reads low.
I bought one a LONG time ago at Auto Zone or Advance Auto I think. Has the valve. Even had to change it, started leaking. Has to be a special one just for this purpose.
Yes it's the hose "adding" to the displacement of the engine. I test motorcycles also, wonder at what size they would test the same.
I just learned something! Thank you, Steve 😅
Brilliant into, cheers John
Very good info as always. Thanks.
Spot on, Steve, as usual!!!!👍👍👍
Great explanation
Great info Steve!
I’m very glad you have covered this problem because I’m tired of having to show them 😊😊😊😊
Thanks for another eye opening video,I didn't know that about the valve
I think I might have to disagree about where the pressure is being measured and why one is different. I think both measure pressure all the way through the hose and up to the display meter, but the one with the valve prevents the compressed air from entering back into the head and leaking out through the piston and rings. When the engine is running there isn't time for the compressed air and gasses to leak out past the rings, but while testing there is plenty of time for the compressed air to leak back through the rings. My opinion. Thanks Steve for another video, I really enjoy your channel.
Thanks for the tip 😀
Yes you are correct. The compression ratio is determined by the combustion space above the piston at TDC. For that 65cc Husky the volume is likely to be near 6cc above the piston at TDC.. If you add 10cc of hose, then you have a volume of 16cc, which you are cramming the 65cc of the Husky into.
Don't see the point of that......
Exactly
Great video, Steve thanks can you buy a hose with a Schrader valve that will work with an automotive type compression test testing kit?
You nailed it!
I have the Mityvac. Haven't used it much but it seems to work good. Leon the Homelite guy has recommended it for a long time.
I've got the automotive type and never had an issue but when the weather gets better I'm gonna add a schrader valve to mine and see if it improves my readings.
THANK YOU!!!! i had no idea there were different versions and that explains why my test was garbage
That is a great video. I went to my tool box to look at mine. Mine has the schrader valve at the quick change coupler. I think mine is a Cal-van brand. I bought some brass adapters to check compression on troy bilt 4 cycle weed eaters. Small spark plugs Just has a hole that goes through it. Check the other compression tester hose and see for yourself if no schrader exists. Have a happy new year!
Thanks Steve, explains alot.
We'll, I could have used this information about three years ago. I had 40 psi on my Toro Tecumseh engine and I just pushed it to the road and bought a new one.
These small gas engines often have a compression release so even with the Schrader valve, they will still read low.
Thank you Steve. Now I know why one of my small engines runs fine even though the compression tested at about 40 PSI. I used an automotive compression tester.
Also good to know to look for that 40PSI level when testing with the non-schrader type. No need to rush out and buy another tester right away but for next time we will know better.
You can also have an engine with a compression release...
@@patthesoundguy True but I don't own an engine with compression release.
That’s absolutely right. The pressure in the hose needs to build to match that inside the cylinder. I discovered this when I was testing a lawnmower. The Schrader valve was loose and each pull you could see the pressure go up, then slowly go down. Once I tightened the valve, the pressure would build and hold. You must have the Schrader valve for small engines.