Idle stop adjustment, or idle mixture?

[JCoop]

I've adjusted the idle speed screw on my Holley 750 so it idles nicely at 750-800 RPM. I've also adjusted the idle mixture screws; initially to 1.5 turns out, then a very fine adjustment (another 1/8 out) to get best rpm. Since there was a slight hesitation upon acceleration from idle, I've also adjusted the primary accelerator pump so there's hardly any slop, it moves as soon as the throttle is moved. When I coast to a stop after cruising, engine at normal temps, idle stays at 1200, even as I blip it. Then she very slowly goes down to 1000, then to 800. I've fooled with return springs too, hard, soft, long, short, etc. It still does it.
Anything I'm missing? Any suggestions?

[Tongue Pirate]

Are you running a choke? Sounds like something is hanging up keeping the butterflys from closing all the way. Can you simulate it when you are running the trottle by hand to see what is happening? Do you have vacuum secondaries or manual?

[Woodz428]

As Tongue Pirate alluded to the high spead idle cam on the choke may be sticking. It also sounds like you could have a vacuum leak or lean idle circuit, when you drop off throttle the engine is still pulling fuel from other than the idle circuit to compensate for the leak or lean circuit.

[Excaliber]

Perhaps the mechanical advance in the distributor is not returning as quickly as it should. OR the mechanical advance is coming in at to low an rpm and CAN'T return until engine speed begins to drop? Kind of a catch 22 there...

By simply advancing the timing, even slightly, the idle rpm will increase.

[JCoop]

Thanks all for the analysis. No choke, mechanical secondaries. And the vacuum advance is plugged. I DID split the vacuum source for the PCV and brake assist servo. It was "T" fitted to the base of the carb, so I got a fitting to the manifold and ran the brake servo there and kept the PCV to the carb. I'll check those connections for leaks.
I haven't tried to replicate it at rest and with the air cleaner off. I'll check that too. It seems to happen when the temps have gone up to op temps; not when cold.

[Zoom This]

I have had the same idle issue on my 418W in SPF 1735. Old motor did it, new motor does it. New carb, new MSD Pro billet distributor. Still idles high then slowly drops at a stop. Believe me, I've done everything possible to the distributor and carb. My theory? Motor and idle mixture is just fine when the car is moving and cool air is being ingested. When the car comes to a stop, the motor is now inhaling hot air. This equates to a richened fuel mix at idle, thus bringing the mixture to a rich condition and a lowered idle. It's the same thing as turning the idle mix screws in a half turn or so when the hood is down and the car is at a stop. Try driving down a long hill when the motor is at full operating temp. Put the car in neutral and watch the idle. It stays at 1200 rpm. At least mine does. Bring the car to a stop and within 10-15 seconds the motor is now idling where it was intended to, 900-950 rpm. I'm done chasing a percieved mechanical problem. It's the heat under the hood.

[JCoop]

That must explain why it only happens when the motor's hot.
I wonder if the "turkey pan" intake would solve the problem.

[Mike Simard]

Zoom this has a great idea. I would have guessed it to be linkage related, SPFs do have to be adjusted to not force the carb closed at idle and there's always the loose motor mount culprit. Since I've been datalogging stuff and have an air temp sensor in the air cleaner here's what I noticed: The air temp while driving is usually 85 degrees, after stopping for a few minutes and restarting it will be up to 140 degrees while moving slowly in a parking lot, that's enough to cause the air to be considerably less dense. Jcoop, when you tune an engine with the hood up at rest you're dealing with cool air. If you're setting it by rpm you may want to try setting it to the maximum vacuum instead. You could get fancy and keep a vacuum gage hooked up, watch it with the hood closed under driving conditions and sneak out occasionally to make an adjustment to the idle screws. BTW, if you have off idle hesitation it's likely that the transfer slot is overexposed at idle which may also contribute to sensitive idle behaviour. You have to get the slots such that the throttle plate exposes a minimum at idle, it should look like a square with no more length exposed than its width.

[Zoom This]

Don't bet on a turkey pan to solve the idle issue. Hot air still rises up to the scoop and circulates around the motor confines while a little may escape the hood scoop. I just live with a 1200 rpm idle at at stop sign, since it does go down within seconds. I had to carry a screw driver around to set the idle speed and mixture screws to get it right. The whole Cobra motor thing when you're trying to manually adjust fuel settings for the street on 550+ HP is challenging. As Mike mentioned, make sure to keep the transfer slots away from the idle mixture equation as much as possible. If you get too far up on the idle screw you open up the transfer slot and you quickly become over-rich at idle. I wound up having to drill 3/32" holes in each of the 4 throttle blades to get more idle air past the idle mix port in the throttle bore. Good luck.

[Michael C Henry]

I fought the run on problem .I tried to add an electric dashpot .It was in the way and only fit on the wrong carb. What I've found is more initial advance will let you shut the throttle plates a little bit more to achive the same idle setting . The less idle the less runon. I have a Mallory HyFire VI ignition box. It has programable start retart. I have 8° programmed in and normal 650-700-rpm idle at 12° bftdc so It starts with only 4° bftdc below 500 rpm. Works pretty well.Fast Idle for warm up is the fast idle cam on what would have been the choke. I think I have too high a power valve setting .The engine surges at a cruise on the freway in third at 25 mph. I think I need a lower number power valve that will open a little later in the acceleration proscess.