At low rpms (pulling off from a stop) which advance system is more in play, vacuum or mechanical?
The engine continues to hesitate when pulling out. While resetting the timing today we tried pulling a vacuum on the vacuum advance unit, but didn't get any advance on the timing. Could this be why it has trouble on acceleration? Shouldn't we see some advance even with the little vacuum we can create bu sucking on the tube?

'69 302 2v

 

Ported vacuum gets vacuum only at part throttle. Accelerating from a stop shouldn't be using much advance at all, because you're at a low rpm, high load situation. Unless you're starting off at 1500rpm, the mechanical advance probably isn't going to be advancing the timing, and you're not cruising at part throttle, so the VA shouldn't be operating either.

The VA should only come into play while you're cruising at part throttle. It's there to advance the timing during low-load cruising situations to increase efficiency and decrease emissions. Under moderate acceleration, vacuum goes away and you lose the vacuum advance.

That said, you should get some advance by sucking on the tube. If not, either your VA linkage is stuck, or the diaphragm isn't holding vacuum.

 

i use the sucking method as well although it's kinda disgusting :-)
switch engine off, pull distributor cap and remove the rubber hose from vac to carb on carb side. if it's a hard line, just plug a new rubber line to the vac canister.
then suck on the rubber line and you should see the contact braker plate in the distributor moving. if you suck on it and you don't feel the slightest bit of resistance and nothing moves then the diaphragm is fried (get a new vac canister)
If the plate moves you're halfway there. during sucking block the rubber hose with your tongue. it should hold the vacuum you created. it should not move back on it's own. if it does->again the diaphragm is fried.
As well as Tad suggests try to move the braker plate with a screwdriver the same way the vacuum canistrer should do it.
If the diaphragm is ok then make sure the line beween that and carb is ok as well.

To confirm what you said avove: at idle you should have none on only very very little vacuum on the advance. Only when you quickly rev up the engine it should create vacuum but falling down straight away.

Kalli

 

I took off the vacuum advance and took it apart. It had three "shims" tightening up the spring. They were nothing more than split lock washers. With the washers in, I couldn't budge the linkage to the breaker plate no matter how hard I pushed on it. When I removed the "shims" I could make it move slightly by sucking on the hose, but the internals would lock up and wouldn't let it release when I released the vacuum. I borrowed a vacuum pump today, and also confirmed it would not hold vacuum.


Basically, the shims were too tight, the diaphragm has a leak, and the internal mechanism is binding up. Sounds to me like just about everything that could possiblly go wrong with it, did. Picking up a new unit this evening.

Yes the breaker plate moves smoothly, but I wouldn't call it freely. It has a little resistance to it. Any rule of thumb on how much vacuum it should take to move the breaker plate?



Thanks for the help.

 

Huge improvement!!! Even my 15 year old son noticed the low end improvement as the rear end came about 1/4 of the way around on him as he was pulling out of the neighborhood! I think I may have created a monster.

 

LOL, good to hear

 

heheheee :-) nice one.

 

i'm having the same problem with mine. i just switched to a pertronix flamethrower distributor and have noticed when trying to pull out normally from a stop it bogs around until i gas it more then it finally moves off. i wouldn't think that a new distributor would have a screwed cannister but i might check this tomorrow.