Carb Idle Speed Increases after warm up. Why?

[Bill M.]

I have a 1966 427-L72 425 hp. Non altered.

The carb is a Holley # 3886101-E1, List 3247 dated 6-3-4 (March 4, 1966)
Metering block 4901-3247, 6R3551B-4519.

The intake is completely stock as is the the engine.

The car runs fine with no issues except the idling speed of the carb after warm up.

The idle is set to the factory settings and remains there until driven for a while and then the idle will
go to approx 1000 RPM and stay there until the next cold start up. Mind of its own.

Return springs, cabling, firewall opening etc were all checked out with no binding or other issues.
I can push the pedal down real fast when stopped but the carb idle speed still stays at 1000 rpm.

A few times I had to stop and turn the idle down but the problem still exists. The main problem is that when I turn
the car off it will sit and "diesel" for awhile.

Has anyone ever had this problem or have any suggestions of what it might be.

[Gary B.]

Re: Carb Idle Speed Increases after warm up. Why?

Bill,

Check out this thread. Someone had similar issues and one person said it could be a vacuum leak.

https://www.forums.ncrs.org/showthre...eak+idle+speed

Gary

[Duke W.]

Re: Carb Idle Speed Increases after warm up. Why?

The run-on is caused by the ported vacuum advance so total idle advance is limited to initial rather than initial plus full vacuum advance. This configuration was established to increase EGT for California emissions, and since L-72 was a low volume option they used the same carb. on 49 state deliveries. The increased EGT increases combustion chamber boundary surface temperature, and this can cause preignition, which is the cause of the run-on. Of course when this happens you should put the transmission in first gear and ease out the clutch.

The '65 L-78 has full time vacuum advance, since there were no tailpipe emission standards that year.

If not already done I recommend you convert to full time vacuum advance. First verify the the carb has ported vacuum advance by removing the vacuum hose at the carb. nipple and plug it with your finger. Idle speed should remain the same. Then check vacuum at the carb nipple. It should be zero or very low.

Assuming ported vacuum advance is verified obtain a 1/8" tee, a couple of feet of 1/8"vacuum tubing and a 1/8" vacuum cap. Use the vacuum cap to plug the OE vacuum advance port on the carb, then use the tee and vacuum tubing to tee the vacuum advance hose into the choke vacuum break. Save the OE choke vacuum break hose and make a new harness to connect the choke vacuum break source to both the choke vacuum break and VAC.

If the OE type 360 12 VAC is still on the distributor check that it meet specs... see the CSM and AMA specs. Do this with the cap removed and ensure that the stationary pole piece rotates smoothly as you apply vacuum. If it doesn't meet spec or is another ID number, let us know.

If it's okay, with the engine fully warmed up go through the idle speed-mixture adjustment adjustment procedure in the CSM and target 900 as the final idle speed. If it's an OE equivalent cam, vacuum should be about 14". Report your reading.

You will also need to adjust the fast idle. Use the CSM value, which you set with the engine fully warmed up, and upon cold start the idle should stabilize 100-200 revs below where you set it with the engine fully warmed up.

Duke

[Mark E.]

Re: Carb Idle Speed Increases after warm up. Why?

"The idle is set to the factory settings and remains there until driven for a while and then the idle will
go to approx 1000 RPM and stay there until the next cold start up."

Clarify this please.

It's normal for curb idle speed to increase a few hundred RPM as the engine comes up to operating temperature. Are you setting idle speed to spec at operating temperature?

If yes, then make sure the choke is firmly and fully open after warm up. If not, the fast idle cam may engage unexpectedly.

The VAC on many engines is connected to ported vacuum. This does not cause erratic idle speed.

[Duke W.]

Re: Carb Idle Speed Increases after warm up. Why?

The VAC on many engines is connected to ported vacuum. This does not cause erratic idle speed.

Not true!

A "sticky" VAC can cause erratic idle behavior. Consistent idle behavior requires consistent total idle advance, but if the vacuum or centrifugal advances are sticky and do not always achieve the normal idle advance consisting of zero vacuum advance on ported applications or full vacuum advance on full time applications and zero centrifugal at idle, idle speed will vary nearly every time the engine is allowed to return to idle speed.

There are some exceptions like the 365/375 HP 327s. Centrifugal starts at 700, but these engines will not idle that low with acceptable idle quality due to the high overlap of the 30-30 cam. 900 is about the minimum and this will add 2-3 degrees of centrifugal at idle, but the idle is stabie with the usual +/- 50 RPM lope.

Few understand what the vacuum advance does and why it is important, and in the course of a regular tuneup they are rarely checked for meeting specs, and some OE VACs are not suitable for some engine configurations because they don't pass the Two-Inch Rule that applies to engines with full time vacuum advance.

Duke

[Mark E.]

Re: Carb Idle Speed Increases after warm up. Why?

Good point. A sticking VAC mechanism could cause idle speed to vary. But isn't this true when connected to ported or manifold vacuum? My point is changing from ported to manifold vacuum will not fix this problem.

Also check for loose or binding throttle shafts.

[Timothy B.]

Re: Carb Idle Speed Increases after warm up. Why?

Mark,

Changing from a ported to manifold vacuum signal will greatly help the problem of run on.

The higher idle advance from the initial and manifold vacuum advance will allow the engine to run cooler and more efficient thus the idle speed will increase. This will require the throttle blades to be closed more to reset the idle speed back to spec, less fuel and air is drawn into a cooler combustion chamber so when the ignition is turned off there are no other hot sources of ignition and no extra fuel to ignite causing the run on.

Some of the low hp/low overlap cams will run well with ported vacuum advance for just the opposite reason in that the throttle blades need to be opened more because when you have air from the PCV and use full manifold vacuum advance the idle is difficult to get low enough to 5-600. That being said the Corvette engine bay is very air flow restrictive to cool things and manifold vacuum advance works better with idle speeds around 700RPM.

[Mark E.]

Re: Carb Idle Speed Increases after warm up. Why?

Timothy,

I understand what you're saying, but using ported vacuum doesn't cause the OP's problem of varying idle speed.

Running less advance at idle can increase temperature, but a properly tuned engine with no vacuum leaks won't typically have a run-on problem. This symptom is the engine's way of saying something is wrong.

As an aside, many engines are happier and more responsive using ported vacuum for VAC. Changing to manifold vacuum may cause a stumble when flooring it from idle. I believe that's because when vacuum suddenly drops at WOT, VAC retards timing just when the engine wants more advance. I've found that throttle response can often be improved using ported vacuum for VAC combined with more static timing. This provides a smooth advance curve as RPM increases at WOT. Also, some engines tend to "buck" at idle with too much VAC, which limits static timing and performance.

Exhaust temperatures at idle may be a bit higher using ported vacuum. This may be an issue for a taxi cab or if you're in a parade, but most Corvettes see limited idle times.

So I encourage folks to experiment with ported and manifold vacuum using different static timing settings and see what works best for your engine. If you do advance static timing, be sure to check total centrifugal advance. You may need to shorten the advance curve.