Help - Anybody who knows old school (carb)

[N20LT4Bird]

I've got a friend here who has an 87 TA with a small block chevy. Its got a holley 800 and an edelbrock rpm air gap manifold. Not too sure of the cam specs, but its not too wild. It has a normal HEI distributor with a new vacuum advance.

I dont know this carburetor stuff too well, so I'll throw this out to you guys.

Its having a problem with the timing advance. I think he set the basic timing at 8 degrees, but as soon as you give it some gas, the advance kicks in and you've got full advance by 2000-2500 RPM. What is making the advance kick in so soon? He knows the carb is too big, but I dont think thats the cause. Could the intake not be matched to the cam or something causing vacuum to go wild? I know its hard to answer that without cam specs, but is this a common thing? ANY help would be good. Thanks.

[M3EATER - GG '00 WS6 T/A]

The vacuum advance onthe distributor is intended to give more advance under light load conditions (high vacuum). This give better milage. As soon as the vacuum drops (you boot the throttle, or go up a hill, etc...), the advance goes away.

Sooo....... your problem (of having too much advance too soon ....)is NOT the vacuum advance .... or anything to do with the carb set up.

This is all the centrifugal advance set-up in the distrubutor. Although, having full advance by 2500 rpm doesn't sound bad, for full acceleration.

Are you having problems with it knocking due to too much advance?

What is the total advance coming up as?

What octane is in it?

What compression?

(Ok, Fred... your turn...... )

[N20LT4Bird]

I am going from memory from our last conversation, hes coming in the shop later to work on it. I asked him about ping - he said no. I think he said 36 degrees of advance.

So, this is normal for the advance to kick in all the way? How do you check full advance?

I wish I could use a scanner!

[WICKED LT1]

There are more than a few different vacuum advance canisters that were used over the years by GM. All of your timing should be in by about 3600rpm on a street motor. A total of 36 degrees sounds good for a small block. There are adjustable vacuum advance canisters out there by Crane and a few others that let you tailor the advance curve rate and tip-in for your combo. I would have to look up some of the factory ones, but I seem to remember some of the older emissions canisters that would dump all the advance in pretty quick. HTH


-Paul

[Joe 1320]

Usually the stock vacuum advance unit is set up for a ported vacuum source. Most times it is connected to a vacuum source that doesn't allow a vacuum signal until a certain throttle opening, temperature, load or combination of those variables. usually when someone changes the intake manifold and carb setup, the factory ported source goes bye bye and the distributer is hooked up to a constant vacuum source. That is what you are getting here. An easy fix would be to pick up an adjustable vacuum advance and dial it in the way you want. You can also pick up a weight and spring kit for the distributer and taylor the centrifugal advance (mechanical) as well. This used to be an art in the old days before the computer was given control of advance and timing.

[N20LT4Bird]

Yea, thats what we wound up doing - going without the vacuum advance. Seems to run OK, but he does need to dial in the weights and springs. Thanks for the input guys.

[Joe 1320]

The vacuum advance will aid in fuel economy.... I also suggest searching for a ported source. I cannot remember which source on the carb is ported, but there should be one.

[WICKED LT1]

Carb Vac source

Originally posted by Joe 1320
The vacuum advance will aid in fuel economy.... I also suggest searching for a ported source. I cannot remember which source on the carb is ported, but there should be one.

Usually, the port that is below the throttle blades is constant, and the port above the throttle blades is timed. A cheap vacuum gauge, which you almost have to have, to tune these will tell you for sure. HTH


-Paul