72_351C

I have a Holley 750 double pumper and a Mallory Comp 9000 Unilite distributor with vacuum advance. I have the vacuum on the distributor hooked up to one of the two ports on the carb. Sometimes, and only sometimes (but more often than not), when I am running at higher rpms (higher than idle), when I let off the gas, the idle does not go back to the 675ish rpm, instead it goes somewhere between 1000 and 1100 rpm. I can kick the gas petal and it will go back to "normal" but I am questioning why it won't go back all the time. The one constant I have found is that when it is fast idling, if I disconnect the vacuum on the distributor, it goes back immediately. The carb does not have a choke, so I know it isn't that and the fact that I can disconnect the vacuum during a fast idle leads me to believe it has something to do with vacuum, but I am uncertain about what to do to fix it and why it's happening. Any ideas?

geos sho

Carb is leaking down to rich most likely i run msd mech advance dead on all the time vacuum advance could be hanging remember clev had dual vacuums advance and retard.Geo

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my need is speed

72_351C

I used to run mechanical, but went back to vacuum for some reason or another. If you think it's running too rich which is what is causing it to not idle back to "normal", then why would pulling off the vacuum line when it's fast idling take it back to normal idle? Could be I guess, but I don't understand why. There is more vacuum at higher rpms, hence the advance it timing, but when you let off the gas, then I would think it would drop the vacuum pressure and the timing would return to normal. Also, if the advance was hanging, then it would "not" go back to normal by removing the vacuum line (because it would still be hung). With that being said, it's pulling too much vacuum still (at least I think) and that's why it's fast idling. With that being said, then perhaps you are right in the fact that it's too rich, hence the higher rpms.

btw - This thing is by no stretch of the imagination stock!

zaneabel

Just something that may help, make sure the vac. hose is connected to port vacuum on the carby and not manifold vacuum / can check with a vac.
gauge.

9psi EB

how big of a cam are you running, it might be that the throttle plates are too far open at idle, exposing the vacuum port at idle therefore its advancing the timing. If this is the case, you might need to drill 1/8 holes in the primary plates

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zaneabel

I have normally only found that to be with vacuum secondary carburettors.
Not with double pumpers/mechanical secondaries, as the DP carbys usually
can accomodate large camshafts. Whereas the vac. secondary carbys idle transfer
slots are normally highly exposed with large camshafts. Before drilling holes
in your primary throttle plates, you can try adjusting the secondary throttle
plates so they open slightly, this can reduce the transfer slot position on the primary
throttle plates.