Vacuum Advance

[klwebb]

A family friend told my dad that the reason my truck bogs down when you hit the gas too hard is the vacuum advance? i can slowly accelerate and it does fine, but my dad wants me to be able to get out into traffic quickly so i dont cause a wreck. can someone explain what a vac advance is/how it works (and where it is) and how i can adjust it? i have a 350 with a 4barrel rochester carb

or does anyone have any other ideas why it bogs down like this? yeah its a slow truck, but it shouldnt die when i want to get up and go

[DT1]

I could be the vacuum advance, initial timing or the Carburetor accelerator pump.

[basketcase]

The vacuum advance is the little can with the rubber vacuum hose going into it on the side of the distributor. It's purpose is to advance the ignition timing at high manifold vacuum times to increase fuel mileage. High manifold vacuum is achieved during slight to moderate carb opening like during low speed driving or steady state cruising.

Once you stomp on the gas and the carb opens fully, manifold vacuum drops to zero or near to that. The end result is that the vacuum advance unit plays no part in helping your truck accelerate. Your advance curve is then controlled by the centrifugal advance weights and springs inside the distributor. The centrifugal advance unit will increase the advance curve as rpm increases up to a predetermined limit. However it doesn't start to advance the timing until the rpms increase and it's usually all in by 3000-3600 rpm. So, off idle, the centrifugal advance is just laying there, so to speak, until the engine starts to rev. If it doesn't begin to advance after the engine starts to speed up it could cause sluggish acceleration.

Here's a couple of things to check. Assuming your tune up is not years old, take off the distributor cap and grab the weights. Pull them out sideways and check for the inside of the distributor to rotate slightly. If it does your ok. If not it's froze up and need repair.

Check your initial advance with a timing light. It should be in the 6-8 degree area for a stock engine. If it's way retarded your truck will be dead off idle.

If your timing is set right and it's still dead off idle check your harmonic balancer to be sure that the ring hasn't slipped. The outer ring that has the timing mark on it has a thin rubber elastomer between it and the hub to absorb vibrations. When that elastomer fails it can allow the outer ring to slip or even come off and tear the crap out of things, in worst cases.

To check if it's in the right place take out number one spark plug. Next insert a piston stop into the spark plug hole. A fat as possible Phillips screwdriver works well. Make it long enough that it can't fall inside and is easy to grab a hold of and be sure to insert it the same for both checks. Turn the engine in the direction of rotation until it's coming up on the compression stroke with both valves closed. While one person is turning the engine another can hold his thumb over the spark plug hole. When air is being pushed out it is coming up on the compression stroke. Stop and insert the piston stop. Now continue to turn the engine slowly just until the piston hits the piston stop and stops. Don't force it, be gentle. Make a chalk or crayon mark on the balancer right next to the "0" on the timing mark. Now turn the engine in the opposite direction until the piston is coming up again and stop as soon as it hits the piston stop. Mark the balancer right next to the "0" again. Exactly half way between those two marks is top dead center and that's where the permanent timing mark should be. If it's there the balancer is ok. If it's off then you need a new or rebuild balancer because it has slipped and you will never be able to set your timing correctly not knowing where the mark should be. Even though you just made a new mark it could still move every time you start the engine. If it's off just a degree or two that is common.

If this all checks out ok your problem is in the carb. The accelerator pump is a good place to start there.

Tom

[DT1]

Originally Posted by basketcase

The vacuum advance is the little can with the rubber vacuum hose going into it on the side of the distributor. It's purpose is to advance the ignition timing at high manifold vacuum times to increase fuel mileage. High manifold vacuum is achieved during slight to moderate carb opening like during low speed driving or steady state cruising.

Once you stomp on the gas and the carb opens fully, manifold vacuum drops to zero or near to that. The end result is that the vacuum advance unit plays no part in helping your truck accelerate. Your advance curve is then controlled by the centrifugal advance weights and springs inside the distributor. The centrifugal advance unit will increase the advance curve as rpm increases up to a predetermined limit. However it doesn't start to advance the timing until the rpms increase and it's usually all in by 3000-3600 rpm. So, off idle, the centrifugal advance is just laying there, so to speak, until the engine starts to rev. If it doesn't begin to advance after the engine starts to speed up it could cause sluggish acceleration.

Here's a couple of things to check. Assuming your tune up is not years old, take off the distributor cap and grab the weights. Pull them out sideways and check for the inside of the distributor to rotate slightly. If it does your ok. If not it's froze up and need repair.

Check your initial advance with a timing light. It should be in the 6-8 degree area for a stock engine. If it's way retarded your truck will be dead off idle.

If your timing is set right and it's still dead off idle check your harmonic balancer to be sure that the ring hasn't slipped. The outer ring that has the timing mark on it has a thin rubber elastomer between it and the hub to absorb vibrations. When that elastomer fails it can allow the outer ring to slip or even come off and tear the crap out of things, in worst cases.

To check if it's in the right place take out number one spark plug. Next insert a piston stop into the spark plug hole. A fat as possible Phillips screwdriver works well. Make it long enough that it can't fall inside and is easy to grab a hold of and be sure to insert it the same for both checks. Turn the engine in the direction of rotation until it's coming up on the compression stroke with both valves closed. While one person is turning the engine another can hold his thumb over the spark plug hole. When air is being pushed out it is coming up on the compression stroke. Stop and insert the piston stop. Now continue to turn the engine slowly just until the piston hits the piston stop and stops. Don't force it, be gentle. Make a chalk or crayon mark on the balancer right next to the "0" on the timing mark. Now turn the engine in the opposite direction until the piston is coming up again and stop as soon as it hits the piston stop. Mark the balancer right next to the "0" again. Exactly half way between those two marks is top dead center and that's where the permanent timing mark should be. If it's there the balancer is ok. If it's off then you need a new or rebuild balancer because it has slipped and you will never be able to set your timing correctly not knowing where the mark should be. Even though you just made a new mark it could still move every time you start the engine. If it's off just a degree or two that is common.

If this all checks out ok your problem is in the carb. The accelerator pump is a good place to start there.

Tom

That's a great writeup and I agree with 99% of it. However, If you are running a PORTED vacuum advance, it actually helps with initial acceleration. Since the the engine is pulling full vacuum at idle and the advance port is above the throttle plate, the instant the throttle plate opens, the port is exposed to the high vacuum (If only for a split second) and distributor advances just enough to get the vehicle moving. As the throttle is opened further the vacuum to the advance module decreases and the mechanical advance takes over.

[fwayne]

Carb accelerator pump my vote. Especially if its old, the ethanol gas deteriorates those pump seals shine a light down inside the carb venturi. open the throttle and you should see a very strong squirt. If its weak or dribbles its the accell pump. Funny thats the same description you might have describing a prostrate gland problem!

[67 chevelle]

take the distributor cap off , take the vacuum hose off the carb , suck on the hose ,, no jokes please ,, if the rotor moves your advance is working , if it does not , replace it

[jjzepplin]

He would have to have some pretty stout lungs for that thing to move. I just collapse it manually and put my finger over the vacuum hose entrance and let go. If it makes a sucking sound and returns it is probably good. He could also have a stuck shaft and mechanical is not working. But my first guess would be the accelerator pump. Look down the carb and give it a couple pumps of the throttle and observe large amounts of fuel or none at all.

[67 chevelle]

yes bogging can be so many things , my first guess would be carb , then vacuum leak , then distributor

[70STOVEBOLT]

"He" is a she...

[57larry]

I'd check the vacuum first, carb next

[klwebb]

So, I've had time to read all these responses but not enough time to try all of them

I did snap a video, I think it is the accelerator pump. Which sucks cuz I spent $130 getting it rebuilt by a guy who's been doing it for 30+ years.... It looks like it's slowly leaking, and I think air is coming out of it. I'll have to post the video from the computer since my dumb phone won't let me....

[DBK81]

Accelerator jets have a little gasket under them perhaps this was missed on the rebuild and is causing strong flow issues? Just a thought.

[Alex V.]

I rebuilt a Rochester 2G for a friend and overlooked the check ball in the bottom of the accelerator pump well. It ran fine but, no matter what, stumbled when you tipped into it very hard at all. I went round and round with it, making sure all the carb's calibrations were set right, checking timing, idle mixture, etc. - after I looked close enough at the parts blow-up to see that check ball and popped it back in, all was right. Not sure if 4G's or Quadrajets have such a part, but it's one thing.

On vacuum advance, I found this writing very enlightening on the ins and outs:
http://www.stevesnovasite.com/forums...ad.php?t=11689