im stumped-need help idle problems

ringermt · Jun 27, 2007

1970 SS with 402 (unknown cam, not stock)

I recently replaced my old hei distributor with a new msd hei distributor (street fire style with a 4 pin ignition module) . I have the car set at 16 degrees initial and it runs very well at idle with the vacuum advance unhooked and the line plugged. My RPMS at idle and vacuum advance unplugged are a little higher than they where previously. Right now they are at about 1300 RPM in park and 850 RPM in gear whereas before they were 1100 and 750.

My problem lies in hooking up my vacuum advance. When I do, my idle in park goes to over 2k RPM.

I have no vacuum leaks that I can find and I am at a loss with what to do about the fast idle.

Any help appreciated, thanks!

Mike

77 cruiser · Jun 27, 2007

You're probably putting in another 15-20 deg. with the vac. adv. just slow the idle back down & try it in gear to see if it hold enough vac. to idle the way you want. Might have to limit the can to 10-12 deg. depending on what it wants.

Dave427 · Jun 27, 2007

Which port are you plugging it in on? If you have a Holley , use the port on the side of the metering block. Make sure its ported vacuum , above the throttle plates.

Dave

SWHEATON · Jun 27, 2007

Yup,just wat Jim stated above is very likely the issue,do what he suggested.

Scott

ringermt · Jun 27, 2007

Thanks for the replies. If I understand correctly, adjust my idle back down to 110 rpm in park/ 750 rpm in gear with the vac. advance unhooked and try hooking it up again? Is that right?

As for the carb, it is a Rochester.

77 cruiser · Jun 27, 2007

Set the idle with the vac. adv. hooked up.

Jerry70 · Jun 28, 2007

You're likely dealing with both vacuum and centrifugal advance issues. When you hook up the vacuum advance the extra advance increases rpm. That extra rpm causes the centrifugal to add more advance which increases rpm some more. It doesn't stop there though. That increased rpm increases vacuum which in turn increases the amount of vacuum advance and that causes more centrifugal to come in, which causes more vacuum to come in. This might go on until one or both advances max out.

If the vacuum advance is hooked to a ported source (and the carb idle position is properly adjusted) you shouldn't have any vacuum advance at idle. If it's hooked to manifold vacuum, you need a canister that fully deploys at idle vacuum (the B28 only needs 8"). Another common problem is light advance springs when combined with engines that need high idle speeds (like big with cams). Typically, springs that allow total sooner also start advancing sooner. In my experience, the lightest MSD springs started centrifugal below 800 rpm.

im stumped-need help idle problems

ringermt

77 cruiser

Dave427

SWHEATON

ringermt

77 cruiser

Jerry70

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