mownhoj

So this video says (at 2:38) turning the screw clockwise creates a leaner mixture:


This article says "turn both screws all the way counter-clockwise, which will make the mixture as lean as possible." :

http://www.ebay.com/gds/How-to-Tune-...5701629/g.html

I'm inclined to believe the folks at Jegs over eBay any day of the week, but thought I'd ask you guys.

Thanks!

joe_padavano

The idle mixture screws are simply a needle valve. Screwing them in (righty tighty) makes the passageway for the fuel smaller, which leans the mixture. Keep in mind that this only affects the mixture at idle and very small throttle openings. Once airflow through the venturi is sufficient to pull fuel through the main jets, the idle circuit has little or no effect on mixture.

Mark71

I noticed that my carb (a Holley) runs very rich when the engine is first started. Even after the choke opens, it remains around 11:1 on my AFR gauge - enough to make your eyes water, but no black smoke. When the engine reaches operating temp, after say 5-10 minutes, things lean out quite a bit and it idles much better.

In fact, if I adjust the idle mixture screws while it is still cold, it will only barely idle or even stall once it warms up.

Is that behavior normal? Does it have something to do with the charge temperature of the air or fuel entering the carb?

joe_padavano

You should only adjust the idle mixture screws with the engine at operating temperature and the choke fully open. Once that is done, don't touch them. All other cold start adjustments are done with the choke and fast idle screw.

mownhoj

Yes, that's the whole point of the choke if I understand correctly; to make the engine run rich until the combustion chambers get hot.

Joe thanks for the info on the rich lean. I'm taking the car to a shop tomorrow. It sputters when I press the accelerator and I'm ready to let a pro tune it and more importantly to let a pro look at the overheating issue.

Greaser007

Joe,
Thanks for the reminder of the idle adjustments being made at Operating Temperature.

I have been patiently dialing-in a Solex-clone carburetor on my '46 ****** jeep, and had come across a thread where the service manual stated that the idle adjustment should be made with the engine at operating temps.

I'm with Mark71 with my Holley's being Cold-Blooded on a cold morning, and take about a 10-minute warm up.

Len