'77 Vette high idle

[Jim H]

Hi guys,
I have been battling this problem for some time now, a high idle that happens after I drive the car. I have checked all the obvious possibilities. If I start it up and let it warm up, the idle is below 800. After driving it a short distance the idle in neutral goes to 1100-1400 and if I shut the engine off and restart, it goes back down to about 800. Yesterday I noticed something weird, I have been wondering if maybe something was keeping the secondary throttle plates from closing completely so after a drive yesterday with the car in the garage and idling at around 1200-1300 I pulled the air cleaner and pushed the secondary air valve open to get a look at the throttle plates and as soon as I did that the idle dropped to below 800. Wow, what a surprise...seems counterintuitive but it happened. And repeatedly opening and closing it gave the same result, closed - high idle, open and idle drops. The secondary throttle plates appear to be fully closed.The carb has been modified according to Cliff's Recipe 2 and aside from the idle problem is working great.

Any thoughts?

Thanks
Jim

[Cliff Ruggles]

Remove the baseplate and re-index and center the secondary throttle plates to make sure they are fully closed.  Insure there is some room between the linkage and it isn't holding them open slightly.  I spend a lot of time in this area when building them and you will have running issues if they don't full close and have too much light showing around them.

Also check the distributor.  Idling high then returning to a slower idle speed once you've shut it down and restarted is a classic symptom of someone installing light springs on the weights or one of those cheap POS HEI distributor re-curve kits.......Cliff

[Jim H]

Hi Cliff,
Thanks for the reply. If the secondary throttle plates were cracked open wouldn't the idle go up if the air valve is forced open? It seems bass ackwards to me. The distributor is an MSD 8362, the weights move freely and I do have light springs installed but if I put stiffer ones in it does not help the high idle.

I made a diy smoke generator a while back to look for vacuum leaks. Didn't find any but I plan to fire it up again and see what the air flow is above the secondaries as I manipulate the air valve.

Jim

[Kenth]

I would check function of the PCV-valve first.
Sometimes on new rebuilds the return springs on secondary throttle shaft needs some light lubricant to function properly, as in fully closing the throttle blades, especially after WOT.

A wild guess is that atmosphere is closing the throttle blades when you open the air valves manually.
(I wouldn´t put money on that!)

[Jim H]

Thanks for the input, Kenth.

PCV valve is good and yes taking a look at the linkage is definitely a good idea as Cliff also suggests. As for the pressure differential, since the secondary plates are fixed by the linkage I don't think that could happen, but it's an interesting thought.

[Cliff Ruggles]

Nothing should happen to engine speed if you manually open the secondary air flaps at idle speed.

This indicates a problem with air getting past the throttle plates in the baseplate.

Do not rule out the distributor, put a timing light on it and make sure the timing is ROCK solid at idle speed.  I can't budge at all cranking to idle and shouldn't start to advance at all until right off idle.

Aftermarket "high performance" distributors are troublesome for that deal as the mechanical advance mechanisms have too much leverage.  I don't even work on them here for that reason.  By the time you get strong enough springs on them to hold them back at idle they refuse to advance correctly.....Cliff