With the engine running, If I pull the vac. advance hose off the carb. there is no rpm change at all. I thought rpm's should drop. Hei dist. bushings and springs look worn. I cleaned the weights and put it all back together still no change. Is this correct, or do I have a dizzy, or a carb. problem?
468, stock hei, holley carb., if that matters any. Thanks for any help before I buy something I don't need..:confused:

Your engine should drop rpms if the vacuum advance is working properly. Check for vacuum in the line. If no vacuum, possible carb problem. You can use direct manifold vacuum also vice off the carb. If you have vacuum in the line, pull the cap back off the dizzy and with a short piece of vacuum line on the vacuum canister of the dizzy, suck on the line to see if the advance moves. If it does move, that's not the problem. To check the mechanical advance, pull the rotor off the top of the advance weights and try and move the advance shaft. If that doesn't move, then the shaft is frozen and the whole dizzy shaft will have to come out and re-lubed.

I suspect you have it connected to a ported source which is not exposed to vacuum under idle conditions.

I have the vacuum line connected to the side of the carb. just above the site hole. When I pull the line off I have vacuum at that port for sure. But even with it sucking air there is no drop.
I did try the sucking on the line with the cap off trick, but it took a whole lot of sucking to get it to barely move. Feels like the whole shaft moves pretty easy by hand, but I'll take it back apart and double check..
Thanks for the replys so far..

If you have very low manifold vacuum at idle, the vacuum advance may not be working properly. Most stock distributors come with canisters that expect at least 15” Hg. If your cam is lopey enough that the vacuum never is that high, you may not get any advance from the vacuum advance. The solution is to get another vacuum canister that will allow the advance to come in at a lower vacuum level. The Echlin/Napa VC1810 allows vacuum to come in at 8” Hg., and might be what you need. Get a vacuum gauge and find out how much vacuum the motor produces at idle to be sure. As mentioned above, you need to use manifold vacuum as well. Good luck.

I tried to tune the carb. with a vacuum gauge. The highest reading I could get at idle on front of carb.(full manifold vacuum?) was right at 11".
I have a lunati bracket master II camshaft, but looking for an excuse to change.:) I'll try the new vac. can.. Thanks

Are you connecting the vac advance to the same port you took the vac reading on?

Are you connecting the vac advance to the same port you took the vac reading on?


No. Took reading from port on front of carb., where I have the vac. line to the transmission ran to. Should be full manifold vacuum, if I read the directions right. Is this the correct port to hook to when tuning a holley carb. with a vacuum gauge?
I have not checked the other port with gauge. But I will.

Lorent - with that cam, the advance is correctly hooked to ported vac which at idle will not change timing or idle speed. Your initial question did not indicate a specific problem; is there one?

Check your other port with your vacuum gauge. With a big cam and low vacuum it is going to want a lot of advance at idle, re: manifold vacuum. So I have to disagree with ya on that Corky. What you don't want though is the vacuum advance fluctuating, so checking your timing once you get things sorted out with the can is the check.

Lorent - with that cam, the advance is correctly hooked to ported vac which at idle will not change timing or idle speed. Your initial question did not indicate a specific problem; is there one?


The only problems I was trying to figure out was my idle is never the same after the engine warms up.Example, I can set my idle in park with warm engine to lets say 1000rpm. Put it in gear and sometimes no rpm drop, sometimes drops to 750. Drive the car to town stop at a stop light sometimes it will idle down anywhere between 600 & 1100, it varies all the time.
And it runs a little warmer than I would like for it to. Other than that it seems to run fine, no knocks, pings or dead spots. Just thought a vac. advance problem might cause these issues. I'm a real DA when it comes to timing and vacuum. Thanks for all the help.

rpms shouldn't vary when at a stop. perhaps there is a vacuum leak somewhere?

rpms shouldn't vary when at a stop. perhaps there is a vacuum leak somewhere?

If you hook the vac can to direct port with that cam, the idle speed will vary because the engine will not produce enough vac to completly open a stock vacuum advance. This is what causes the idle speed to vary. There are two options - hook the vac advance to ported vacuum or replace the vac advance unit with one that fully opens below idle vacuum.

Ask me how I know this? ;)

If you hook the vac can to direct port with that cam, the idle speed will vary because the engine will not produce enough vac to completly open a stock vacuum advance. This is what causes the idle speed to vary. There are two options - hook the vac advance to ported vacuum or replace the vac advance unit with one that fully opens below idle vacuum.

Ask me how I know this? ;)


The can is hooked up to ported vac..
I'm playing with different advance springs and new vac. can now. I'll let yall know how it comes out. Thanks for all the help.

One other thing to keep in mind, if you are idling in the 1K~ range and put the very lightest springs in, they can allow mechanical advance to start at that point, something you don't want. The timing light should be show no variation in timing at idle.