Holley 4 barrel idle adjustment

[The motor guy]

I had a mechanic tweak the idle mixture screws and idle speed on my 350 Chevy and since it's a boat the idle speed will be higher on the hose vs in the water due to back pressure from the water. I'm worried the idle speed might be a tad low when I put it in the water.

My question is I can raise the idle speed without having to readjust the mixture screws again, correct? I read online that anytime you adjust the idle speed you have to readjust the mixture screws, but that didn't make sense cause that main idle speed adjustment screw doesn't have anything to do with the mixture screws.

 

[marc1]

Originally Posted by motor_oil_madman
My question is I can raise the idle speed without having to readjust the mixture screws again, correct?


Yes

 

[NYEngineer]

Yes again.

 

[ragtoplvr]

The backpressure will affect the idle mixture slighty. \The marine carb is jetted for the backpressure and marine applications so should not be touchy
Any mixture change will be slight. Just raise the idle speed and if it runs ok then you are good.
Lots of youtube videos on readjusting the idle speed and mixtures, it is very easy if there is a problem, which there probably will not be any.

Rod

 

[The motor guy]

I wonder if that one jet being a little too rich was the cause of the gas smell at idle that I would occassionallysmeel especially when restarting.

 

[ragtoplvr]

modern gasoline is designed for fuel injection, it seems to boil rather easily in carburetors. That could also be your odor.

Rod

 

[Chris142]

Opening or closing the throttle blades to adjust the idle speed WILL have some effect on the idle mixture. The idle transfer slot will be either more or less blocked depending on whether you raise or lower the idle speed. Opening the blades even a tiny bit allows more air which = leaner mixture.Its very likely that you will have to readjust the mixture screws.

 

[atikovi]

Generally you adjust the mixture screws to get the highest vacuum or idle speed first, then the idle speed screw can be adjusted to bring down the speed to the desired rpm.

 

[The motor guy]

Originally Posted by Chris142
Opening or closing the throttle blades to adjust the idle speed WILL have some effect on the idle mixture. The idle transfer slot will be either more or less blocked depending on whether you raise or lower the idle speed. Opening the blades even a tiny bit allows more air which = leaner mixture.Its very likely that you will have to readjust the mixture screws.


How is it any different than me giving it more throttle to just go faster? I can't sit their and adjust the mixture screws after time I change the throttle position. That's all the screw does.

PS: I only only know how to how adjust carbs on single cylinder two stroke motors, not multi cylinder 4 strokes.

 

[atikovi]

Originally Posted by motor_oil_madman
How is it any different than me giving it more throttle to just go faster? I can't sit their and adjust the mixture screws after time I change the throttle position. That's all the screw does.


You do understand the idle mixture and idle speed are adjusted by different screws? Not clear by what you mean above.

 

[NYEngineer]

Motor Oil Madman, It's actually VERY simple to do mixture screws on a Holley. Get a vacuum gauge, hook it up to manifold vacuum and with the mixture screws run all the way in and then backed out a turn and a half, start on one screw and turn it out until you get the highest vacuum signal on the gauge, then do the same thing on the other screw. This works on two corner and four corner idle screws.

I recently had a guy call me with a car he just had done and had a 950 with four corner idle circuits. The guy that built the engine dialed in the carb... Poorly.
I did the vacuum gauge thing and by the time I got to the third screw I was able to close the throttle blades enough that they were back over the transition slot and the car ran better and better. I went around a second time and the car ran fantastic.

One suggestion I have for you is get the slowest idle you can maintain dependably so your stern drive or transmission doesn't take a beating going into gear from a fast idle. Many times, with an engine in good condition, I can get a 650 rpm idle.

 

[The motor guy]

That's what it's set at now. Once you put it in gear though it goes down to like 500 and it runs, but it's almost grunting.

 

[das_peikko]

That's an interesting way of going about it. I've always used RPM to adjust mixture because the small engines I've worked on don't have a provision to hook up a vacuum gauge. Some of the carburetors even have the intake manifold as part of the carburetor.

 

[The motor guy]

Any harm in kicking it up 100-150rpm so it runs smoother?