Farm Equipment - Zenith Carb Adjustment Help Needed

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Zenith Carb Adjustment Help Needed

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Category: Allis Chalmers
Forum Name: Farm Equipment
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URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=26793
Printed Date: 22 Nov 2024 at 4:58pm
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Topic: Zenith Carb Adjustment Help Needed

Posted By: C.Wilcox
Subject: Zenith Carb Adjustment Help Needed
Date Posted: 05 Mar 2011 at 6:43pm

I had the Zenith carburetor on my 1945 C rebuilt and just put it back on the tractor today. It started up fine, but ran real rough at a real high RPM. Pretty much would have been a racing speed if had it been in gear. It sounded almost as if it was missing and the exhaust was pretty black in color. I throttleld the engine back as far as I could and it was still racing right along so then I tried adjusting the idle adjustment screw like the guy at the shop told me, but that didn't help at all unless I ran it in real tight and then the tractor would choke off. So I set that back to 3/4 turn back from tight which is where it started. I then tried opening the throttle up all the way and then adjusted the main jet, but still couldn't seem to get rid of the black smoke and the engine still ran rough so I set that back to 2 1/2 turns off of tight. The only thing I could do to reduce the RPMs was to pull the choke back to almost nothing and then the engine speed would come down, but it seemed to gallop worse than ever when I did that so I'm guessing I was just starving it for fuel. So my questions now are:

1- Why is it idling at such a high RPM and how do I fix it?

2-The main jet controls the fuel/air mixture and should be the key to fixing the black smoke right?

Replies:

Posted By: Chalmersbob
Date Posted: 05 Mar 2011 at 8:47pm

You are running very rich. I think 1 1/2 turns out is about a starting point. You have a replacement card if it has an adjustable main jet. The stock card has a fixed jet.

You could try loosening the set bolt at the throttle quardrant, and adjust that rod to see if that helps. I don't have a Zenith book for an adjustable jet card, but Marvel/Schrieber said to start a 1 turn out from bottom. Bob

Posted By: Rogers
Date Posted: 05 Mar 2011 at 8:53pm

For a Zenith carb you start 1-1/2 turns out on the upper air needle and 4-1/2 turns out on the lower gas needle. This should be good enough to allow the engine to run.

After starting open the engine to about half throttle and let it run for a minute to warm up. Close back on the lower main gas needle until the engine begins to run lean starving for gas. Then start opening back up on the lower gas needle until the engine begins running rich keeping up with how many turns you opened it back up.

Leave the main gas needle set half way between lean and rich. I have found that the final main gas needle adjustment is about ¾ to 1 turn open from where the engine started starving for gas, and the air needle ends up staying at 1-1/2 tuns out.

I hope this helps.

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Think for yourself and be your own expert. Be willing to change your mind; however, willingness to change your mind doesn’t mean that you will. Blindly following any path is the pinnacle of insanity.

Posted By: C.Wilcox
Date Posted: 06 Mar 2011 at 10:26am

Thanks guys, I'll give this a shot and see how it works. Will these actions possibly help with reducing the idle RPMs or is that an issue with the magneto?

Posted By: Gerald J.
Date Posted: 06 Mar 2011 at 12:30pm

Idle speed depends on the governor, the throttle spring, the governor and throttle linkage, and the idle speed screw on the throttle shaft.

Advanced ignition timing will speed up the idle speed and cause hard cranking from firing before top dead center.

Gerald J.

Posted By: Rogers
Date Posted: 06 Mar 2011 at 3:19pm

I agree with everything Gerald said. I assume that it was not idling high before the carburetor was rebuilt, so I would check the setting on the idle speed screw and throttle linkage first before I started looking at the governor. It is easy to get something wrong in the linkage when putting a carburetor back on.

Posted By: Dick L
Date Posted: 06 Mar 2011 at 4:18pm

Remember also that the idle screw does not set the idle speed. The stop screw on the back sets the speed by the amount of fuel it lets into the engine with the throttle held closed against that stop. The idle screw adjust the mixture so it runs smooth at the stop screw setting.

Posted By: Gerald J.
Date Posted: 06 Mar 2011 at 10:51pm

Actually you want the idle speed screw all the way out when adjusting the idle mixture. If the throttle plate isn't fully closed some gas comes from the figh speed (power) jet and the interact more.

The governor will hide the effects of adjusting either mix so if you adjust the idle mix by engine speed nothing much happens. If you hold the throttle closed and slow it down below specification, with the speed screw, then you can adjust the mix for the fastest speed. Then go adjust the high speed jet again holding the throttle plate constant or else watch the throttle shaft and adjust for the governor closing it the most. Then go back and adjust idle. They DO interact and it takes half a dozen shots at each sometimes to get them both to about optimized.

Then you have to check to see if the engine will speed up from idle or take a load at working speed without stumbling. If it stumbles its probably lean on the power jet and needs more.

I've adjusted the power jet for better plowing speed when I had the tractor geared up enough the governor wasn't controlling it, what I call, "pulled off the governor," where the governor has the carburetor wide open but the load is slowing the tractor. MF calls that an overloaded tractor. Then I've found richening the power jet gets more power at the cost of fuel economy. And in piston aircraft they run full rich for power for take off, then lean the mixture for economy when well away from the ground. Same thing applies to a tractor engine.

Gerald J.

Posted By: Dick L
Date Posted: 07 Mar 2011 at 4:41am

I'll have to try that. Goes against every instruction I have ever read but who knows you might be on to something.

I have been working on and adjusting carburetors for over 50 years and have not had problems with the way I have done it.

I have always adjusted the idle first then the power jet (if it is adjustable) most B's or C's do not have an adjustable power jet.

Kinda hard to set the power jet first without an adjustment.

I was shown over 50 years ago to set the stop screw with a 1/16th or so open butterfly before installing a rebuilt carburetor so it will start and run at a fast idle before any adjusting. Worked then and worked on three tractors last summer at my age of 72. I have two in the pail and one ready to drop in when it warms up outside. I am betting that they will work correctly when I am done as well.

Must be the air is thinner where you are at.