I have a 89 TPI 305 Vert - the cruise control was working fine untill one day it stopped - Funny That . What happens now is that when you go to use it the car pulses i.e revs up then drops frequently - It is like there is a vacuum leak but I have replaced al the vacuum lines & they test up ok. I have also checked all electrics & they are all ok. I put a vacuum gauge on the line out of the vacuum chamber & the vacuum is good but when you rev it up the vacuum drops away????
Any suggestions?

 

Can you post a picture of what your cruise unit looks like? Is it mounted up near the radiator near the charcole canister?

 

Yes that is exactly where it is located

 

Mine will surge as well and the unit is brand new. It looks almost identical to yours. The only thing I can figure is that there is a small leak in the system somewhere which causes the unit to search for the right speed (mine locks the carb throttle linkage based on engine vacuum so far as I can tell) to keep. I would assume yours is the same issue.

So far as the vacuum dropping away...check the vacuum coming out of the intake, the brake booster is a good reference or another one low on the intake or throttle body/plenum if you can find it. If everything is blocked off properly or doesn't leak, the vacuum should increase as the RPM's increase.

I've never personally checked how the vacuum reacts coming off the ball itself.