Model B Mixture screw problem
Printed From: Unofficial Allis
Category: Allis Chalmers
Forum Name: Farm Equipment
Forum Description: everything about Allis-Chalmers farm equipment
URL: https://www.allischalmers.com/forum/forum_posts.asp?TID=127255
Printed Date: 16 Dec 2025 at 9:03pm
Software Version: Web Wiz Forums 11.10 - http://www.webwizforums.com
Topic: Model B Mixture screw problem
Posted By: goatflamm
Subject: Model B Mixture screw problem
Date Posted: 14 Aug 2016 at 9:19pm
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Hello All. I am new to this site as I am new Allis Chalmers owner. I have taken a look at some of the forums while working on my tractor and I have found a lot of information and really enjoy reading all the knowledgeable and professional feedback given on the forums. Therefore, I have a question that I am hoping somebody can help me with... I recently bought an AC B and I have gone over pretty much everything to get it running. Checked timing, compression, spark, fuel, new plugs, rebuilt carb, new battery for starter(has magneto), and generally cleaned the fuel system the best I could. Today after finally getting all that completed, I got the tractor running. It was running decent at idle, but not perfect, and when I got on it and drove it, it begins to sputter and cut out until it finally shuts off. I have played with the idle speed adjustment and the mixture needle. I noticed tonight that when I adjust the idle mixture screw while it is idling, it does not change it at all. I took the carb apart a couple times to see if something is plugged up (including taking the jets out and cleaning them) and cant find anything that is blocked up. The only thing I didn't remove(because I cant get it out) is the well vent, but
I did shoot carb cleaner in it and it shoots out just fine, so I don't think anything is clogged in there. Why would the mixture screw not be making any change in the way it is idling? I started with it 1.5 turns off seat, and both screwed it the whole way against seat and backed it all the way out, with no change either way. It just idles the same way the whole time. Hopefully somebody on here with more experience than me can help. Secondly, where do I fill the oil bath air filter cup up to. I can't seem to find any definitive guideline on where to fill it to. I filled it so the whole cup is filled up to the holes in the center tube in the cup. Maybe this has a little bit to do with my other problem? Thanks to anyone who can help.
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Replies:
Posted By: goatflamm
Date Posted: 14 Aug 2016 at 9:22pm
| I forgot to add that on the bottom part of the carb, there is a small hole directly behind the well vent. Does this hole actually go anywhere? I ran a wire down through it and it seems just to be blocked off at the bottom and doesn't go anywhere. |
Posted By: Gerald J.
Date Posted: 14 Aug 2016 at 9:38pm
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You have to defeat the governor to adjust the mixture, otherwise the governor will correct for changes in speed by moving the throttle plate. Usually that means simply holding the throttle against the idle speed screw on the throttle arm. Alternatively you can look for the governor moving the throttle and adjust the mixture for the governor closing the throttle the most, indicating the best mixture. Gerald J. |
Posted By: goatflamm
Date Posted: 14 Aug 2016 at 9:44pm
| Thanks, I will give it a try to tomorrow. |
Posted By: goatflamm
Date Posted: 14 Aug 2016 at 9:52pm
| I also read about and saw pictures of an anti-surge spring that goes from the throttle linkage to a post right under the exhaust manifold. Will this do the same thing? |
Posted By: Dick L
Date Posted: 15 Aug 2016 at 6:46am
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If you are having a problem understanding where the little hole goes in the bottom of the bowl means you have not had the carburetor power jet out to clean behind it. The gas flows down and thru the power jet then up the discharge tube into the neck of the carburetor as a mist from the vacuum of the turning of the engine. The idle screw adjusts the air mixing with fuel at idle in the idle circuit. The adjustment screw at the bottom if it has one adjusts the mixture of the fuel that mixes with the air drawn up thru the venturi while the engine is running. If the carburetor is clean with all passages clear and you do not see a difference in the adjustment your idle screw it means your stop screw has the idle set to fast. The stop screw on the back of the carburetor sets the idle speed. The idle screw then is adjusted for the engine to run smooth at that speed, (only) After the idle is set at the desired slow speed you then adjust the power jet to where the engine runs the fastest rpm and then you turn it in to where you detect a slight slowing of the rpm. Just that simple on that engine and carburetor.
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Posted By: goatflamm
Date Posted: 15 Aug 2016 at 8:18am
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Thanks again for the additional information. The whole at the bottom of the carb that I was talking about isn't the hole in the bottom of the bowl. The hole I'm talking about is on the bottom casting just in front of the bowl, but not in the bottom of the bowl, right behind the well vent. It may not be anything but a whole from machining, but I just wasn't sure. Also, are there any recommendations for the filling of the oil bath air cleaner? |
Posted By: goatflamm
Date Posted: 15 Aug 2016 at 8:57am
| My carb also only has one adjustment (besides the idle speed screw on the back of the throttle lever). |
Posted By: Dick L
Date Posted: 15 Aug 2016 at 9:13am
That hole is a drain hole for over flow. It should have a small screen on the under side. Then you only have to adjust slow idle. Again if the passages in the idle circuit are clear and you do not see a difference your stop screw is set faster than a slow idle. You are drawing enough air thru the engine that you are only using the power jet. In your case a non adjustable power jet. A non adjustable power jet means that you have no mechanical way of adjusting it above idle. The jet can be changed or if you know what you are after can be drilled out as an adjustment. This should help some if you do not have a drawing.  |
goatflamm wrote:Posted By: goatflamm
Date Posted: 15 Aug 2016 at 10:12am
| Thanks for the diagram and the info. That is way better than the diagram I had. |
Posted By: Leon n/c AR.
Date Posted: 15 Aug 2016 at 10:47am
| Have you checked the throttle shaft for wear, if loose it will let vacuum of engine pull air in and you will not be able to adjust carb. Cup of oil bath should have a line on the inside of cup most also say fill by line. From what you said it may be slightly over full. Leon |
Posted By: goatflamm
Date Posted: 15 Aug 2016 at 12:32pm
| Yes, I replaced the throttle shaft and packing when I rebuilt the carb. As far as the oil bath cup goes, I noticed it says "oil level" on the outside flange of the cup where it flanges out to fit over the top part of the air cleaner assembly, but I assumed the flanged line is just to fit over the fitting. I could not find any line inside the cup like I was expecting. |
Posted By: Dick L
Date Posted: 15 Aug 2016 at 12:51pm
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Without looking I think it is about in the center of the hole in the center cup.
I have one that is not marked as well. Most the fill line is a ridge that sticks out around the outside. Full will draw oil into the carburetor. |
Posted By: goatflamm
Date Posted: 15 Aug 2016 at 1:31pm
| Ok thank you I'll have to take a better look and see if I can find it. Thanks again for all the help. |