Fast/slow idle speed Holley 3810

[Bradley D.]

Yesterday I drove the car about 10 miles and the idle speed increased from the normal range of 750 to 1000. It was a hot day 85-90 outside temp.
When I got back home I adjusted the idle speed from 1000 to 750, now today on start up the idle is 500. The L79 doesn't do well at that level. Again a hot day so no choke involved. I have done a search but most are concerned with PV numbers and no one seems to really address this specific issue.
This has a divorced choke. All that seems to work when it gets cold enough.

I think its something like when it gets really hot, the carb sucks more air(from a leak or warped baseplate), leaning out the gas and thus increasing engine speed. I also develop a whistle from the carb about this time. I've replaced basplate gasket, and the engine/ carb were rebuilt a number of years ago, but probably has only a couple K miles since rebuild. I've owned the car just over a year and am just starting to 'learn' some things about it.
Performance is great. Not a hiccup of any kind and pulls strong.
Any thoughts are appreciated.
Brad

[Monte M.]

Re: Fast/slow idle speed Holley 3810

Brad,
When it is hot, have you tried the old trick of spraying carb cleaner or something similar around the base plate, vacuum ports, vacuum hoses, and so on, to see if the idle fluctuates at all.

[Bradley D.]

Re: Fast/slow idle speed Holley 3810

Brad,
When it is hot, have you tried the old trick of spraying carb cleaner or something similar around the base plate, vacuum ports, vacuum hoses, and so on, to see if the idle fluctuates at all.

I've sprayed the carb with no effect, but not sure about vacuum lines. I concentrated on the carb itself. I'll give it a try.

[Michael W.]

Re: Fast/slow idle speed Holley 3810

Your choke should still be closing for a cold start, even in hot weather. No choke= very low idle speed or stalling until the engine warms up.

[Timothy B.]

Re: Fast/slow idle speed Holley 3810

Bradley,

You have to start with the simple stuff first, make sure the choke coil is adjusted per the book and then when the engine is hot adjust the fast idle speed on the high step of the fast idle cam to the spec setting, this adjustment is set with a hot engine. After that with the engine still hot set the curb idle speed and go through the A/F emulsion adjustments.

[Gerard F.]

Re: Fast/slow idle speed Holley 3810

Brad,

I know I mentioned this item in a former post, but maybe try adjusting the position of the secondary throttle plate and the mixture screws in the metering block when you set the idle.

On a 3810, the car does idle from the secondary side in addition to the primary side. The mixture screws in the metering block control the fuel to the secondary side and the position of the secondary throttle plate controls the air to that side. If the secondary throttle plate is too far open, and/or the mixture screw is too far out. It will idle faster. Too much air through the secondary, then screws up the primary.
side.

Try playing with the sec throttle position and the mixture screw. The throttle plate screw is hard to get to on the car as it is under the base plate, but you might be able to get to it with a small right angle flat blade screwdriver. The secondary throttle plate on a 3810 should be no more than a half a turn on the screw from a fully closed position. When you turn in the mixture screws in at normal idle, you should feel that lean roll idle, then turn them out 1/4 to 1/2 a turn.

[Jim T.]

Re: Fast/slow idle speed Holley 3810

Bradley another thing you can check is the centrifical advance. It could stay fully advanced if it needs cleaning when the engine and outside temp gets hot and retract with cooling overnight. A good cleaning of the centrifical advance and a light lubrication could be helpful.

[John H.]

Re: Fast/slow idle speed Holley 3810

I also develop a whistle from the carb about this time.

Brad -

The "Holley whistle" is almost always the sign of a warped baseplate or bottom surface of the main body; a thorough rebuild should include machining or plate-surfacing those areas to dead flat.

[Dennis O.]

Re: Fast/slow idle speed Holley 3810

I'll be watching this thread, as my '67 L79 has the same problem. Other than this, it runs perfectly. I'll try some of the remedies; it'll be interesting to see what comes up.

[Bradley D.]

Re: Fast/slow idle speed Holley 3810

Its been a few months but I think I've fixed my idle problem, but the whistle still comes back (sounds like it comes from the primaries) when the engine gets hot. I plan to take the carb off and rebuild it, or have it rebuilt this winter. I question the base plate being warped since the gasket should compensate for that and I do get a good crush on the gasket.