[GoGoat]

My tri-power on my 66 GTO starting surging lately. Didn't do this for several years after I went thru the carbs. It surges at low RPMs in gear (4 spd). Thought I would get some ideas what to look for before I tear into it.
Thanks

[Dick Boneske]

A common problem on our Pontiacs. But, why it began to do this after working OK for several years is a mystery.

Usually, surging at low rpms with light load on the engine is caused by too much vacuum advance at low engine loads, not carb related. Easy to troubleshoot--remove and block the vacuum line to the distributor and drive it to see if the problem goes away. If it does, try less advance, try ported vacuum, or replace the vacuum mechanism with the adjustable Accel unit. Initial advance usually works out best at 8-12 degrees, mechanical should add 20-22 degrees, and the vacuum advance should add no more than 15 degrees for a total no-load advance of less than 45 degrees. These are ballpark numbers only, but too much total advance will result in surging--even noticeable in neutral when holding the rpms at about 3,500-4,000 rpms.

If you determine the problem is not the vacuum advance, the problem is almost surely the center carb. I've fixed the surging in a few cases by adjusting the idle mixture screws about 1/2 turn out from max vacuum or idle rpm. On engines with a lumpy cam, the problem can be fixed by cutting about six turns off the power valve spring. The weaker spring keeps the power valve from opening at low rpms with the lower vacuum reading due to the increased camshaft duration and overlap.

All of the above is assuming the end carbs are not leaking air when closed. This can be checked by putting your hand over the each airhorn, one at a time, and noting if idle changes. If it does, that end carb is leaking air past the throttle plates.

[GoGoat]

Unhooked vacumn advance and car manners improved 100 percent. Info on car
66 GTO conv
4 sd tripower 355 rear comp cam XE284H
With vacumn unhooked and pluged
16 deg initial 28 deg total
with vacumn hooked up
30 Int 44 total
Has Crane cams adjustable vacumn
I have played with turns out. Improved the probelm but still has some.
Seems like I need the vacumn advance to come in like above 3500
Below that I want the better manners?

[GoGoat]

forgot to include. The motor is a 462 455 bored 30 over

[geeteeohguy]

I had to install an adjustable vacuum advance recently on my '65 GTO with a tripower 389 for exactly the same problem. Fixed it entirely.