Thread: Oscillating idle

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ok when my engine is idling the RPM are Oscillating up and down from 700-1,000 i have ran 3 carbs on it 2 Q-Jet's and a holly 600cfm i have checked for vacuum leeks and have found none the timing is set to 18 all in by 3,000 when i am running it above 1,300 RPM's the oscillating stops.

Sounds like a lean surge...have you fattened it up yet?

well 1 of the Q-Jets was off my buddys car and it was doing the same thing on mine

i have a base timing of 18 and all the timing is in by 3,000

vacuum is reading from 20 to 15

and the vaume is @ 22 @ 1,500

it is a bbc 402 with a edlebrock performer 2-0 intake

all so throw the base timing down to 16

the Distributor is an HEI
i set it to 10º no help just makes it run rougher
i dont know what the totle is right now.
I tried the carb spray right at the intake "nuthing"
What is the lowest idle you can get? "i guess 750rpm" but becus of the vac going up and down it is hared to set the air/fule mixture i set the screws 1 1/2 turn out if i back them all the way out nuthng happens if i turn them all the way in it will stall out

I might be wrong, but it sure sounds like a vacuum leak to me.

Originally Posted by DennyW

I had to read back a little. That means you cannot get enough fuel for the idle circuit, which means the carb needs to be a little bigger, or the power valve. If you can back the mixture screws all the way out, and it doesn't change, that tells me there there is not enough. It should go rich at that point.

yeah it did that to the 2 q-jets 800cfm and the holley 600cfm

Originally Posted by DennyW

Then you almost have to have a vacuum leak like NTFDAY is indicating.

Are you sure the intake angle against the heads are correct ?

yeah i am shure of that. i all so added a super thin smear of RTV around every intake port and i am runing MR. gasket ultra-seal i am LOST and it is late to i am going to give it a try toarow with my frend

well if it stop at 1300 then you past you idle trans slots or very close?? so it may need more fuel ? hard to think a big block would need that?? and it can have a in take leak to ? if the heads and block was cut the intake will not sit down on the china walls .this will stop it coming all the way down .you some times at the very least cut the bottom of the intake for this .and the intake can leak from the lifter valley to ??????

Before overkill sets in; try either the carb spary or non-chlorinated barkecleaner spray (preferred) as a slow, very slow dribble among the venturi'is as it runs. It will either pick up what you're missing or choke it down to a stop. At least you and us will know where to go.

Originally Posted by pat mccarthy

well if it stop at 1300 then you past you idle trans slots or very close?? so it may need more fuel ? hard to think a big block would need that?? and it can have a in take leak to ? if the heads and block was cut the intake will not sit down on the china walls .this will stop it coming all the way down .you some times at the very least cut the bottom of the intake for this .and the intake can leak from the lifter valley to ??????

i did have the block and heads cut but olny .005 to cleen them up. I all so added a super thin smear of RTV around every intake port on the intake and heads

Originally Posted by nitrowarrior

Before overkill sets in; try either the carb spary or non-chlorinated barkecleaner spray (preferred) as a slow, very slow dribble among the venturi'is as it runs. It will either pick up what you're missing or choke it down to a stop. At least you and us will know where to go.

i did the carb spary thing
???Might it be the Distribuor??

Originally Posted by BBMonte

i did have the block and heads cut but olny .005 to cleen them up. I all so added a super thin smear of RTV around every intake port on the intake and heads

yep you did say that i have not ever seen any big blocks that square up at .005 .but still easy to check the intake look and see if there is any gap in the front and back of the intake were you put the rubber things that blow out or RTV them that is the china walls look and see if there is some gap from intake to block ?

Get a vacuum gauge, they are pretty cheap. Hook it up to the intake and read the movement of the needle.