Join Date: Mar 2002
Location: Austin, TX, USA
Re: Hyfire 6A with a Mallory HEI Dist. - won't idle or stay running
First off, if you're going to buy a vacuum gauge, don't over look the hand pump type. They should be about the same price as a vacuum gauge and will serve double duty allowing you to use them for things like checking AC damper servos, EGR valves, vacuum advance canisters etc.
Idle vacuum varies depending on the cam. I see you posted your cam specs, but not being a cam guru, I'm not sure what to make of it. A stock motor will have an idle vacuum of about 17" while a cammed motor will have idle vacuum from 10" down to 8". It varies greatly.
What you want to see is steady vacuum with no significant flutter in the needle. Things like vacuum leaks, improperly adjusted or burned valves can cause the vacuum to read low and / or flutter.
When setting your idle mixture, you're looking for an increase in vacuum (small change ~ 1 or 2 "Hg) as you find the optimum mixture. If you search you'll find some good descriptions of the procedure.
Vacuum advance should cause the idle vacuum to increase also. If you get a hand vacuum pump, you can even apply it directly to the vacuum advance canister to characterize the amount of advance at various vacuum levels.
Long story short, idle vacuum is a good health check that will help you rule out a number of problems affecting idle and off-idle performance. Although you may have another issue like a leaky carb, too much fuel pressure, blown power valve, or other things causing the motor to load up. But I think you'll find the idle vacuum reading valuable feedback as you troubleshoot.
68 Coupe, 350 w/ Edelbrock Performer RPM heads, cam, intake, 700R4, Dave's small body HEI