Vacuum Advance Explained - What it is & How it Works
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- Опубликовано: 1 окт 2024
- Lets take a moment to understand vacuum advance together.
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Thanks, Thunderhead! Excellent explanation! For those of us who work on our own rigs, with only the internet as teacher, videos like this are gold. 1984 4WD Toyota pickup 22R
Dude. This video is perfect.
I have little to no idea when it comes to old-school style setups. And this was super easy to understand, and now know which aftermarket dizzy I need!
I'm running a voodoo cam with. 515 lift roughly.. It's a weekend car should I run the vacuum advance?? I've checked timing with it hooked up and seen 60° of timing... Thx in advance
I would like to see someone answer this I have the same 60
Just subscribed to your great channel. I have a crate 383 stroker in my 77 Monte Carlo that came with mechanical advance. Also installed 700R4 tranny & 3.73 rear gears. 480/470 at the flywheel.
The cam is Lift: .500 / .510, Duration @ .050: 225 / 231, Centerline: 108, Fair idle, Street mild performance usage,
The vacuum advance cannister is connected to distributer but crimped (not hooked up to any vacuum). As it is, my timing settings are maxed out for maximum engine output. Car runs perfect! If I hook up vacuum advance, can I simply leave those timing settings alone?
So for all in I take my base timing plus the max advance of my dizzy which is 15 degrees right? I want around 34 degrees all in so my base needs to be around 19?? I had it set to 13 but she lagged alot and my plugs were quite black... Now at 19 she rips and no pinging that I can hear... Plug straps look ok so far... Major learning curve at the moment
There is a component that you may have passed over. It is the speed of the engine and
the time it takes the fuel air mixture to burn. When at low RPM you don't want
the mixture to start burning too early or you will have the explosion taking
place too early in the compression stroke of the cylinder, making the piston
fight this explosion to complete it's upward movement, if you don't advance the
timing at a high RPM the explosion will happen too late and it will finish the
burn too far after top dead center and the power will be robbed by filling a cylinder
with a piston that is descending. The air fuel mixture doesn't take long to
burn, but that small amount of time has to fit into the mechanical timing of
the piston and valves to make the most of the energy.
where should that distributor hose be connected on a 460 ford with a 4180 carb?????
I have a question about my 66 VW bug.it has 1600 motor,and I would like to know if I can plug my vacuum unit and is it as a regular distributor.
Why would my 1984 f150 302 have small surge in an out at steady load. Thank you
Been following you around YT for a little while and have enjoyed your content, but it's really amazing to me how many times I come to the internet to answer a question and end up finding something from your catalogue that answers it. Appreciate you doing these. The vacuum advance doesn't seem to be working on my 75 F100 but I'm getting ready to sell it. Sounds like I can pass this baton over to the new owner (I'll be glad to disclose it but not something I'm going to worry much about).
What would you do with a vacuum advance port that is hooked up to a computer box? Mine is driver side fender behind battery. Has a channel for the breather tube, too. Electric connections hook into existing harness. Not sure where they end up.
I no longer have a Spark Control Computer on breather. No emissions stuff. Just this box.
So my vacuum doesn’t work. Nor it was connected. But car seems to run just fine.. I bought it like that. Should I fix or leave it alone? Like I said. Runs fine
Your videos are great! Thank you for making and sharing them.
I have an HEI Distributor on my 6 cylinder Corvair that has an Isky 280 cam in it. I'm getting exhaust popping on deceleration and I thought I had exhaust leaks. Someone told me to remove the vacuum from the distributor and guess what, no more popping.
This is the same distributor I used on another engine with the same cam and there was no popping. Why am I getting exhaust popping now?
Someone else says to use a distributor vacuum delay valve. Can you explain how this valve works?
My Distributor is dirty and the vacuum advance is frozen up, I was thinking about sending it to Advanced distributors is that a good place?
Truck I bought is hooked up to vaccum manifold only. The vaccum advance their is no suction from either carb nor distributor? I'm trying to get better mileage. What steps could I take to resolve issue ? Thanks
Great video. What about the advance canister itself. There is an adjustment inside of it. As you know , slide in a small Allen wrench and turning will change things. Could you pass along your thoughts about this? Also (you might laugh at this this) but do you have a troubleshoot engine consulting service? I’d pay you right now for over the phone help. I’m not kiddening.
Most people don't understand how vacuum advance really works, that's really a vacuum retard canister at low rpm, as the engine picks up RPM vacuum drops then it advances . Take a look at when you were sucking on the hose you moved the distributor clockwise which would retard the timing not advanced it.
Hello thanks for this video I have a 1950 Chevy Coupe with a 216 engine that I replace the points and condenser inside the distributor and replaced it with pertronix igniter. The car starts fine and drives but I feel like the car is not running as smooth and as loose as it used to before when it had the points and condenser. I posted a video of the car running on Facebook and some people commented that it could be the vacuum advance. What is your opinion? The car runs like if it is being held back, And like you mentioned on the video I really have to give it gas to run down the road
I have a 1985 Allegro motorhome with a chevy 454. I sucked on the vacuum advance (which is super rusty) and nothing moved. The engine starts but wont stay running without throttle.... this problem started when the ignition module was replaced. That was replaced because it wouldn't start. Any advice?
Quick question on your thoughts: 1977 ford f100. 302 engine. Starts great. Idles great. Revs and drives great. But when coming to a stop at a stop sign or light, the engine dies..... Starts back up with pedal pressed to floor. If I kick it into neutral when coming to a stop the engine will stay running.....
torque converter staying locked up?
Wow, VERY well explained. I agree with below, "no wasted verbage". Thanks sir! Subscribed.
Good explanation I knew nothing about vacuum advance and I can't give a class today either, but it did help me. I can't say that I would be able to tell the difference with or without, but it's always good to know though.
I have a chevy 305 and noticed it doesn't have a vacuum advance, so do i have the mechanical one then ,and should i get a distributor that has one ,answer from anyone appreciated,,thanks
Just go ahead and get a whole new dizzy. They sell new hei distributors on eBay for $50
Hey brother, been having a great time looking through your videos as a new content creator.
I don't want this to come across the wrong way, but I feel you maybe a little off the mark with your understanding of vacuum advance and it's operation. I'm not sure how I can make contact with you, but I will try and make a video in the coming week of my experience with it as a former carburettor and ignition specialist and the different methods in which it operates. Again, I hope this didn't come across the wrong way.
Look forward to hopefully working together with you somehow in the future.
Rob.
Hey rob,
You are probably one one the smart ones - I tried to “simplify” this for folks to a degree because every time I start talking about cylinder pressures as it relates to burn time/ flame propagation, cylinder volume charge density and so on it just goes right over folks head.
Technically when you are cruising you have a much less dense charge - that cylinder charge will have a slower burn time. When you stand on it, cylinder pressures rise and likewise the burn time is much quicker - same concept between a compression test with throttle closed vs open. More pressure = less timing (among other factors), and less pressure = more timing (again, among other factors)
Anyway, I’m a really wordy guy so sometimes I have just tried to present things in a way that will have folks use something properly and not get too invasive - perhaps this was a disservice on this video.
You can contact me at brotherford289@gmail.com if you want to talk - I always enjoy knowledgeable folks.
I need know where the vaccum Gos to from the distributor
Thanks for the insight. I don’t think my mechanical advance is working as I rev the engine. The marks on my distributor only move 3-6 degrees approx. on revving. Is this correct?
I'd bet your mechanical advance is pretty hung up or the springs are wore out. You can pop the cap and should be able to rotate the rotor via the advance mechanism. If it barely moves or fails to return after moved, you can be sure that you have a mechanical advance issue
What afr readings while vacuum advance is engaged?
I have a 351c 4v and it pings on high acceleration loads with vacuum advance connected. When I disconnect it. It won’t ping. What can that be. It has a mild cam. Thanks
Are you running ported or manifold vacuum?
I have a 351 in a 93 bronco. It has a vacuum leak. When parked, I can romp on it with no issues. It just idles a little high. However, on the highway it tries to die if I give it a lot of gas. Could this be caused by something in the vacuum advance?
Question, I have a 72 hurst olds just rebuilt it has a comp cams 42-241-4 cam 490 lift 280 duration 110 lobe. Timing currently set at about 20 Deg ATDC when punched engine is pinging pulling timing back to around 15 no ping but engine won't idle unless set at 800-900 rpm in gear. Please help. Thank you.
Explained everything except what (more timing) means. Does it mean more advance or less. Seems to mean the faster the piston speed the sooner the spark should happen. The term (Vacuum Advance) seems to imply faster piston speed/lower vacuum would advance timing. So the question is, does less vacuum, advance or retard the spark? I saw you suck on the vacuum, but did that advance it or retard timing? Or are you saying the leaner the mixture the sooner the spart shoud occur so as to have time to explode at top dead center? Which leads to why do smog engines mess with timing?
Why would you use the term " pull in ignition timing 2:16 " but when the vacuum is pulling the diaphragm it is advancing timing increasing the numbers,
the term pulling would seem losing timing numbers not increasing numbers as you have stated.....
So I've got a question. The vehicle is a 7600 lb 1981 c30. These heavy trucks run with no cats, no o2 sensors, just a fully mechanical Quadrajet like a 60s or early 70s vehicle. It's at 8*-10* initial timing I forget which. Someone installed an HEI with and adjustable can which I've never messed with.
I have the truck maintained, dialed, and tuned almost perfecto but it's off a hair at "launch".
I like to drive it hard and fast when tuning. Say I'm at a light and punch it. There's a slight delay and then it charges foward HARD. Its way faster than it should be. 😁
I'm trying to get rid of that first little delay. The carb is on point I'm sure of it.
Can I correct that hesitation by going to manifold vacuum rather than ported how it is now? Would playing with the adjustable vacuum pod finesse my tune?
All I need to do is gain torque under load coming from a stop. Pretend I'm loaded with a heavy trailer.
Please and thank you.
Which brand distributor is in the video?
My dad has a 66 F100 that I helped him swap a 390 from a 65 galaxy into. Has a mild cam, headers, Holley 600 and for some reason when the vacuum advance is plugged into manifold vacuum the truck dies. There are no vacuum leaks in the hose and the gaskets are all new. It will also idle only with part choke. Carbs been rebuilt twice. Any help is much appreciated.
Thank you🤔🤔🤔🤔🤔🤔👍👍👍👍👍
Hi There,
Would you put a distributor with vacuum advance on it on a 1985 Chevy C60 with a 366 big block and a Holley 4160? This is a big, slow truck made for hauling rather than speed. I'm wondering if a vacuum advance would help my fuel economy.
Am I allowed to watch your videos being a Mopar guy? I think there are some states in the South that will put me in jail for that.
As far as I know a pushrod engine is a pushrod engine. Ignition is ignition. A holley or Edelbrock carb is a holley or Edelbrock carb.
I wouldn’t even be a ford guy if it weren’t for the ease and extreme affordability of swapping the venerable 80s T5 5 speed into literally any classic Windsor powered ford.
Thunderhead289 I have a 73 olds Delta 88. I have a demon 625 carb. Changed intake manifold. Hei distributor with vacuum advance. I have the 73 chassis manual for the car also. I'm having issues. The book says 12° at 1100rpm. It's not right or I'm not doing something right. I'm getting soot thru tailpipe so I'm thinking timing is off. It's running rich you can smell the fuel. I did the Venturi box idle set screw like you illustratrate in other video. Long story shorter where would you start with troubleshooting. I've been at it for over a month now off and on. Thanks in advance
How do you disable the vacuum advance altogether and then lock the timing in.
That's what stupid people do, but it's done by unplugging vacuum advance and wiring mechanical advance in place.
How much advance typically does the vacuum advance add to the timing if it is set at 12 degrees without. I have a pole school accel dual point distributor on my SBF. I don’t remember how far the timing advanced when the vacuum advance was added. That was many years ago. Right now I have the timing set at 21 degrees. I cant hear pinging when accelerating because the exhaust is to loud.
Would you consider a Melling MTC-6 on a 454 still in the range for vacuum advance working? It's got 112 LSA and .449 lift (I think) duration is like 208/209?
Been awhile since I looked at the specs, it's a stage 2 towing cam in an 81 C30 with a Turbo 400 behind it. Thanks in advance!
I just want to know. If I lightly press on the accelerator and the vehicle feels like its losing power, is my vacuum pump damaged. Ignition timing is right. Fuel air mixture is right. The vacuum hoses are not damaged. When i accelerate harder the engine regains its power. The vehicle is an '87 Nissan 1400. All stock parts with a Hitachi carburettor.
Thank you thunder buddy.
Does the vac connect to the carb or intake?
hey man. ove your channel. I was hoping you. might have some insigt into some performance issues im having. i hsve an 82 GMC vandura. And just recently it started bogging down and losing power but usually just when it shifts into 3rd. I drove it with a vacuum guage attached to manifod vac and wn it bogs down the vacuum goes to around 5 and below. . could this bog be a vacuum issue? i have a vacuum advance on order as im pretty sure mine doesnt work. BUt i also think that it wasnt working for a long time. Also. I had an old timer mechanic convince me recently to bypass all my emissions stuff, and he unplugged all the hoses from my two TVS switches. Do those now need to be removed and plugged off? in other words...could they create a vaccuum leak if left in without any lines attached? im scratching my head for a wekk over this bog
Manifold vacuum is the only source for vacuum advance if you want it to function as intended. Ported vacuum advances the spark too much when accelerating because there is vacuum even when the gas pedal is on the floor which is not correct.
Not true. Once the throttle is opened off idle you will get the same vacuum gauge reading whether connected to ported or direct manifold vacuum.
65sohc Once the plates open, manifold vacuum decreases while venturi vacuum starts to increase from zero. At 1/2 throttle, manifold vacuum is much lower than venturi vacuum. Most people don't understand this principle.
Disagree...When full throttle, manifold vacuum is almost zero while ported is maximum.
The orifice from which the ported vacuum receives its signal is a fraction of an inch above the closed throttle blade, basically at the level of the transition slot. As soon as the throttle is cracked open that orifice is exposed to full manifold vacuum. The venturis are much higher and affect fuel flow through the boosters by increasing air velocity.
When the blades are opened fully, what is the ported vacuum at this point?
Hey man I got one for ya! Okay, I just bought a 1984 f150 Custom short box 2wd with a 5.0 in it. And ive been doing a budget build on it and I just got my mad 6al box and my blaster 2 coil wires up and working, but now when I try and run my tach wire to the back of my factory tach from the tach output onthe msd box I get nothing. Now fyi this truck did not come stock with a tach, I robbed the cluster and wiring harness out of another junk truck and just wired it up to mine and it was working until the msd swap, now nothing. Help me out if you will!
Thank you - I bought a running 1951 Ford Flathead on an engine stand. It ran great on the stand. I got it home and built my own test stand - no problem. The engine runs great. The only thing is that the engine did not come with a vacuum advance unit on the distributor. I ordered one from O'Reilly Auto Parts and it should be in this afternoon. I've never installed a vacuum advance unit, but I'll give it a try. Thanks.
Thanks ThunderHead - another very helpful video, much appreciated. I am running a Blueprint 347 with the pertronix billet vacuum advance distributor. I am running ~12-14 degrees initial and 32-34 degrees total mechanical with my vacuum advance connected to manifold vacuum. I get about 16 inches of vacuum at idle. Everything is great, except I am getting spark knock when cruising on the highway when I tip in to very light throttle. Am I just running too much mechanical timing? Should I run vacuum advance off of ported? Do I need an adjustable vacuum advance canister to lessen the advance this is giving me during this tip in scenario? I'm stuck.
Thanks for your help and your experience!
Hello Thank you for the info. Which vacuum port do I connect to on my Edelbrock 1406? Passenger or driver side? Stock sbc 283
Thank you. Nice video
Could you please tell what do you mean my mild, stock and big cam ?
I didn’t get that part .
I have a sbc and I have a eselbrock carburetor with vacuum advanced hooked up to it. But I do have a stock cam and a eselbrock manifold with a 650 cfm 4 barrel. Should I just use manifold port instead of ported.? I'm going to also set my distributor at 8 degrees time advance.
Were to connect on carburator
Hi from Venice Beach. .....How do I know how much vacuum my vacuum cannister is rated at.?
The only way to know for sure is to connect a timing light. Check timing with vacuum advance disconnected, then with it connected at full vacuum. To get full vacuum at idle and thus max out your vacuum cannister you will likely need to use either a vacuum pump, such as Mighty Vac, or suck on the hose like Thunderhead289 did.
You need to go to the NAPA listings. There are MANY vacuum advance units for different applications (same motor different years).
The 63 nova I just bought has a t where the vacuum for the vac advance is being shared with what I assume is a pcv valve and they both go to manifold. Any potential issues here? I’m chasing down a loss of power on acceleration and after rebuilding carb, changing mechanical fuel pump, changing points contacts, and a new vacuum advance the last thing I can think is it that or a timing issue.
I bought a 70 Impala with a lot of stuff wrong with it. Got most of the major things fixed, but I'm having a hard time knowing where to put my vacuum hose from valves to the 2 bbl carb. Bought a Haynes manual and that was pretty useless and I can't find any stock photos of a 69 or 70 with the air cleaner off. I just want it to run right. Not fast
SwitchDoctor it goes straight from the distributor to the front pass. side port on the carb.
Borrow or buy (Harbor Freight has decent cheap ones) a vacuum gauge, connect it to a port, test if it has vacuum (needle is up at idle) and when you rev the engine the needle should drop (no vacuum). If you connect it to a port that does that you should be good.
Lance Uppercut woo wait. mine is the opposite.. idle no vac. half throttle 15 in. gm hei .
@@danwells7691 If you have no vacuum at idle and then vacuum starts as soon as you open the throttle a bit that is ported vacuum.If you then punch the throttle vacuum should instantly go to 0.
Where do you connect the hose?
Great video. It left me with a question though. If i buy an vacuum advanced distributor, and wanted to go with a racier cam profile, could I just disconnect the vacuum part and plug it and still use the distributor, or does that distributor need the vac to function properly.
Like if I bought an vacuum operated 123 distributor which is without points and controlled by a microprocessor and revcounter to set the timing, then i should be able to unplug the vacuum if I wanted a more tracked spec engine, like a new cam and such? I'm actually buying a 123 distributor for my classic mini as "we speak", but want to buy the correct version.
Thanks
One of the best explanations I've seen. I have a 1979 Viking Deckboat with a Chevy small block 305 in it. I bought it not running thinking I could fix it. Well so far it's been a hole in the dirt (can't get it in the water) I'm throwing money into. Initially the guy I bought it from said it just needed a fuel pump. So I replaced the pump and then found the Starter was bad. So replaced it. Then I found the Spark Plug wires were not right. So fixed that. Then as I was playing with those I found the Rotor in the Distributor would turn in my hand. I looked at it and it seated on the shaft good and actually looked fairly new. So I pulled the Distributor and grabbed the bottom gear and the Rotor still turned in my hand. Now I'm kinda stumped. This Distributor doesn't have a Vacuum Advance on it and I can't find an old style, similar Distributor. Some have said just replace it with a modern electronic ignition but all the Distributors I can find still have the Vacuum Advance on them. I don't have the experience to switch it. I think the coil should be changed also but I really don't know what to do about this ignition system. Stumped in South Carolina!
? Hi, I have a Series 3 Landrover, it has a non-return valve on the pipe close to the vacuum, is that required...
Dude!! I love how ur explanations r straight & to the point its very clear u completely understand thus u kno how it works!! I understand wen u speak & no studdeding u dont have alot of "um,um" my only question is where u been & how come iv never seen or heard of u well no more i have subscribed fantastic work pal keep em comn!
Can you do a video about the vacuum amplifier, i have a laye 70s dodge and have no clue how it works.
2:30 aww fuck yeah
Thank you. Very well said.
So if you run ported you don’t use manifold vacuum??
This is an old video where I taught along conventional lines - I have since said heck all that and teach how I do things because honestly conventional carb teaching sucks and doesn’t perform as well
my car has a distributor, ut it has two vacuum lines. please may I know where to fix those two vacuum lines the carburetor
Where does the vacuum advance hose hook2 on the 302 Ford 84 model on the carburetor and is it both to have suction all the time or just when you give it gas
Great video! A carb tuning how to would be great
He has excellent videos on setting up Holley four barrels. OEM carbs are not tunable. If a problem has been reliably isolated to the carb it is either repair or, more likely, replace.
I have a question my car feels like its ristricted like to only go 45 miles an hour its a 235 engine i have a stainles metal line from the carb to the advance. .so my friends say the line is to thick its a 1/4 line they say the line restricks the distributor from turnning more so the car can go faster is that true .. I hope I make sence please any one let me know
Why can't you find anyone with distributor machines? They use to make getting timing so easy.
With a bigger cam, can you run a vacuum pump to get manifold vacuum back and use a vacuum advance?
Thanx
Great as usual thunderhead! At some point I'd like to hear what you consider a mild street cam vs a mild street/race cam vs a all out race cam.your about the only 1 listen to as most leave things out from what I was taught. I use to put TV cans in my street motors but lately I've used comp cans hi energy. What do you prefer?
I have a vac advance distributor I want to. Do away with advance suggestions
So I know this was two years ago but my girlfriend has an 82 GMC sierra s15 and we can not figure out where to attach the vacuum hose to the carb... any suggestions on which carb port it is?
Most carbs have a full time vacuum port and another that supplies vacuum over about 1000-1200 rpm or thereabouts. I've always found I got better results using the full time port (usually lower down on the carb). This was with both small and big block Chevy engines with headers and mild aftermarket cams in fairly light cars. I got much crisper throttle response and the side benefit of slightly better gas mileage. Your engine/vehicle combination my prefer the ported vacuum (usually higher up on the carb). Why not try each for a day or two of normal driving and see which you find better?
Just be sure to block of any unused ports.
Let me know your results if you would.
You took tooooo long explaining this man. I got a freaking headache listening to this!! 😡
Oh ye of small mind.
Dude calm down. This guy is trying to explain to you about how vacuum advance works and here you are being a dummy
Is vacuum advance only for carburetors?
Good stuff
Great explanation for me. Thank You!
Nice ford galaxie i have a 63 ford galaxie with 390 bb in it and a 67 ford ranger with 352 bb in it matter fact my ford truck looks just like that blue one next to the what is that a 65 galaxie? i have a few others also, 49 chrysler desoto strait v8,,67 plymouth fury 440 a 63 ford fairlane 500 with 260,,67 impala 327,,1970 impala with 350 a 1970 camero with 396 and a 93 dodge ram 318.
All gutless compared to anything now. Cars have come a long way.
Thank you. I have a 1952 Buick...it's my first classic, and I am trying to troubleshoot issues with it. I'm sure a replacement vacuum advance cannister exists, but I am glad to know that it's not everything.
Question: Just completed tune-up on 318 Mopar Engine 2 barrel Holley; idles and revs great until I hookup vacuum advance; starts missing when reving RPMs; What's the problem? Remove vacuum advance and runs great again!?
Either the vacuum advance is adding too much timing (likely) or the vacuum advance diaphragm is ruptured and causing a vacuum leak (unlikely).
what would you recomend for totel timing on a 1979 351w 2150 motor craft 2bbl c4 trans chines short tube fox body headers duel 2.5 exhaust h pipe glass packs no smog junk on 87 octane?
32, then 34, then 35 deg. @ 2,500 rpm. Test drive at each. Choose whichever one performs best at that point. Just saying.
Question .. my Cleveland 351 bored 30 mild cam stock crank , 650 edlebrock my little prob is when iam at cruising with very pressure on the pedal I get a pop through the exhaust it happens in 3 rd and 4 gear . Just thought I would pick your brain .. thanks
seems like too much cruising ignition timing off hand. several things can play into this including air fuel ratio. if you have vacuum advance, remove it and see if the issue persists - this will help diagnose the problem. get back with me here.
Thanks ,will do
Well I remove the vacuum advance hose , plugged off both ports one on carb other distributer and problem gone , can the advance be changed or adjusted because the vacuum advance can be tuned with a Allen key
ah , good deal. so it was bringing in too much advance. the adjustable units are a bit tricky. if you lesson their advance, you will also lower the vacuum at which they will drop out of the timing equation. so for example, you have one pulling in 15 degrees of timing that drops out at 13 inches of vacuum. adjusting it will make it add in only say 10 degrees, but now it drops out at 10 inches of vacuum. my issue is that they are still adding in extra timing now at a higher load then they were before. its just gray area. this is the point where you have to listen for pinging or stumbling issues.
Well in the gray area , there is no ping or stumbling . Now is it possible to adjust the timing so I can run with the advance hooked up to improve fuel economy and easier starting ( just that when the advance was hooked up the choked worked , one press on the pedal and it fired up and run on high idle then when warm it kicked down)
Very good explanation.
Is there a tool to open a vacuum advance
hi , can you teach me about vacuum advance screw inside canister ?
What does the vacumm advance connect to?
I have an 79 F350 with a 460 that someone put an Edelbrock 1405 on ..right now it runs ok when "cold" but when I drive it for 20 min or so then when I try to let it idle it acts like it wants to die. Put a vacuum gauge and it hovers in 15mg which is "late timing". At "cold" it pulls 21mg or so, seemed healthy...I had been adjusting the idle mixture screws because even when I first bought the truck it seemed to run rough at idle - shaking every so often. I figured that was just partly from having such a big engine.
I spend some time messing with the idle screws to get max vacuum (got about 21mg or so) and lowered the idle rpm to around 700. Of course putting it in gear on automatic tranny that RPM drops even more so now becuase it is less happy when warmed up, when in gear is when it acts like it wants to die, so at stop signs, etc. So I don't trust it to take it through town too much.
So now even after my attempted adjustments it runs worse than when I got it when fully warmed up. I tried to check base timing but some idiot painted a giant white stripe on the line so I cant read it . the engine label says stock is 8 degrees. I have to get down in there and clean up the line markers and repaint them
Yesterday I decided to test the vacuum advance (which in this case they connected to ported vacuum). Well it turns out it just sucks air, and doesn't move - I can hear the air just go right through it. So, I need to replace that - so that seems to probably be why I have a really bad idle. And it sounds like I need to connect the advance to manifold vacuum, and then maybe I will get proper idle at full warmup.
It seemed to run better when I first bought the truck (still ran rough at idle though, you could feel the shaking every so often) but I think that is because they just raised the idle RPM to overcome the crap idle. They had the idle around 1000ish
I'm hoping putting on a new vacuum advance unit and connecting it to manifold will solve my crappy idle issues. It seems to run fine other than that right now, no problems accelerating or taking off from a stop sign even. Just bad loping and wanting to die after its fully warmed up. Throws me off a bit that it seems to run better when cold even with the advance not working.
Could be two things - generally if something runs better cold than it does warm with the choke fully open is because the mixture is too rich. a colder engine needs more fuel since the intake is cold so fuel does not atomize as well. once the engine warms, less fuel is needed for a running ratio since the fuel is atomizing much more efficiently.
on the other hand, if you have excessive underhood heat, the fuel will start vaporizing in the lines, and even if it doesnt vaporize, it gets very warm and less dense which alters the AFR. id imagine with a 460, you dont have a lot of room under the hood. search "thunderhead289 return line" i really battled this in virginia till i added a return line to my galaxie.
also, always have a clean air filter, a dirty unit is restrictive and works as a choke to a degree and will create some of the same symptoms
I hacked away at it for a while today, got the new vacuum advance installed, checked the timing seemed OK, checked for vacuum leaks, didn't seem to find any.
Finally I pulled the idle jet rods out of the carb and blew air through the holes..and now - seems to run much much better. I saw a video on RUclips where they show how to do that and thought it was worth a shot. Maybe I just had some dirt in there, don't know. It seems to be running OK for now, or at least, when I took it to the gas station, it was running pretty good and didn't want to die at stop signs.
Thanks for the responses! Your videos are excellent
I was going to guess the vacuum cannister was shot.
Informative video what’s your recommendation 302 sbf with an RV cam looking at an MSD ready to run vacuum advance with no need for a trigger box
Thanks I found out why my carb was backfiring under load with a bad diaphragm
I have a small block chevy and car runs when vacuume advance is unhooked but car dies and back fires when I hook it up can u tell me what would cause that....
The first thing that comes to mind is that that vacuum advance diaphragm is ruptured. When you connect it you are introducing a big vacuum leak.
Do you have to plug both end ? At the vacuum and carb?
Only need to plug carb. Canister gets no vac without being hooked to a source (carb)
How do I clean out jammed vacuum advance ?
Another great video...very informative.
btw love the channel!
great video. so if two new vacum advanced distributor cause backfires in the exhaust at a certain rpm/timing it could be the vacuum ?
No, not the vacuum. Backfiring from the exhaust is from a rich AF mixture. Backfiring from the carb is a timing issue.