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Farmall M Carb Problems


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I have narrowed down the problem of this tractor I'm working on to its carburetor. It runs/idles rough and runs very rich. If you turn the fuel screw in anymore than 3 turns, when you throttle it up its just like either turning the switch off or it revs up on 2 cylinders. If you take the air screw out at running speed and cause a vacuum leak it will smooth out and run good. Owner put a tractor supply carb kit in it and I have gone through and sprayed out all passages with brake cleaner and compressed air but it doesn't make a difference. I have a super H carb that I put on it and it runs fine. I have tested the float and it works fine. 

 

I think the problem lies somewhere in the top half of the carb, more particularly maybe a stopped air passage or fuel getting into the air passages from somewhere

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21 hours ago, LipRipper3006 said:

I have narrowed down the problem of this tractor I'm working on to its carburetor. It runs/idles rough and runs very rich. If you turn the fuel screw in anymore than 3 turns, when you throttle it up its just like either turning the switch off or it revs up on 2 cylinders. If you take the air screw out at running speed and cause a vacuum leak it will smooth out and run good. Owner put a tractor supply carb kit in it and I have gone through and sprayed out all passages with brake cleaner and compressed air but it doesn't make a difference. I have a super H carb that I put on it and it runs fine. I have tested the float and it works fine. 

 

I think the problem lies somewhere in the top half of the carb, more particularly maybe a stopped air passage or fuel getting into the air passages from somewhere

well have you soaked the carb in gunk overnight ? that spray stuff is not the ticket. 

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I had a main jet needle that was ground off center, didn't seat no matter what, cheap carb kit I guess. 

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Read the thread several times. I think your telling us it's rich in all circuits. In which case I would attach clear tube to carb drain and check actual fuel level while running. The air screw trick indicates that idle air is stopped somewhere. That is not necessarily the top half of the carb. Air is mixed with fuel in the bottom half. Use your aerosol can with straw and follow the flow from air horn to screw and down through air bleed in bowl back up through idle fuel orifice.

Reading back what I just typed. There are two passages for idle air. One goes to the screw to supply additional air that you can adjust. The other goes to the idle air bleed orifice to premix idle fuel.

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I bought a cheap set of the smallest drill bits I have ever seen at Menard's.

Those work great for probing all the jets and passages in those carburetors.

My H was running well but not perfect.

I took carb off again and went through all the passages with the bits and it then ran like a new one.

You have something plugged somewhere.

If you see a hole, make sure it is open by probing with the bits and then using either air or carb cleaner under pressure to make sure it is open.

 

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14 hours ago, Diesel Doctor said:

I bought a cheap set of the smallest drill bits I have ever seen at Menard's.

Those work great for probing all the jets and passages in those carburetors.

My H was running well but not perfect.

I took carb off again and went through all the passages with the bits and it then ran like a new one.

You have something plugged somewhere.

If you see a hole, make sure it is open by probing with the bits and then using either air or carb cleaner under pressure to make sure it is open.

 

Had a set for cleaning torch tips , worked great for this, had a little aluminum pencil size drill holder like an exacto knife holder. You can chuck a bit in it and store it in the handle when done. 

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Is the float set correctly? Is it rubbing on the sides of the bowl?  Is the choke operating correctly? Turn the lower (main fuel) adjusting screw in all the way to its seat and back it off 1.5 turns. Top screw should be seated, then backed out one turn for initial adjustment. 

IMG_20220502_1429315.jpg

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If It was mine I would just take the carb back off buy the complete comprehensive kit; strip the thing down to nothing sandblast it inside and out blow it out really well put it back together and finish by painting it 2150 red. Then you know what you have. 

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8 hours ago, JDpartsman said:

If It was mine I would just take the carb back off buy the complete comprehensive kit; strip the thing down to nothing sandblast it inside and out blow it out really well put it back together and finish by painting it 2150 red. Then you know what you have. 

Does the paint make it run better? 

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Don't sand blast it, use baking soda. It's going to take longer, but it will dissolve when washed out with water. I've sand blasted them before and have plugged up tiny passages. 

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7 minutes ago, MinnesotaFarmall said:

Don't sand blast it, use baking soda. It's going to take longer, but it will dissolve when washed out with water. I've sand blasted them before and have plugged up tiny passages. 

...plus blasting removes the "pickling" treatment that was meant to prevent the cast iron from wicking gasoline from the bowl.

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I place my disassembled carburetors in a solution of Simple Green in an ultrasonic cleaner.  That makes them like new.  Cleans out those stubborn passages.

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8 hours ago, Farmall Doctor said:

...plus blasting removes the "pickling" treatment that was meant to prevent the cast iron from wicking gasoline from the bowl.

I worked at a dealership in the 1970-1980 timeframe.  We would sometimes get a carburetor in the shop that would seep gas at the gasket and just generally leak.  If you put the dry bowl in a vise and put gas in it, you could see the gas “walk” right up the side of the bowl and our older mechanic called them “walking carburetors”.  The only cure we knew was to put on a new bowl.
 I always wondered what caused this..  Is there a way to re pickle a carburetor? 

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Does the carb have the idle jet in it, the small brass tube that threads into the top of the fuel bowl, lots of times people forget to install, this would make the idle circuit very rich like you are saying.

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Just saying you guys who are bashing sand blasting a carb have no idea what you are talking about!!! I have rebuilt many carbs after blasting them and I have had 0 issues and as a matter-of-fact lookup Burry Carb service they do mainly John Deere carbs but this guy has rebuilt hundred of carbs and not just Deere and every single cast iron carb is completely disassembled and sand blasted, then reassembled and painted. He was the guy who gave me the trick. The reason you have to paint the carb is because the sandblasting takes it down to bare cast causing it rust and look like garbage. Another thing Do you really think the pickling has remained completely intact on these carbs for 70 plus years and who knows how many rebuilds or times these carbs have been apart. 

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10 hours ago, JDpartsman said:

Just saying you guys who are bashing sand blasting a carb have no idea what you are talking about!!!

LOL! Ok, Guy. I have rebuilt about 500 carburetors here in my own shop in the last 12 years, and I have no idea how many in the shop I worked at for 16 years previous to that.

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Sand gets into all kinds of passages it should not......  Dip them in a bucket of carb cleaner or toss them in an ultrasonic cleaner.   No issues.  I rebuild at least a hundred carburetors a year since I purchased my cleaner.   (Never should have mentioned I had it!)  The only time my rebuilds have issues, is if the fuel tank has contamination issues.  I always recommend an inline fuel filter to my customers. 

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I've sand-blasted every cast-iron carb I've rebuilt, probably a dozen or less. Guess I wasn't smart enough to know you weren't supposed to do that?! Have had no problems so far. Might have to look into having Shiek clean them up for  me in the future?

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Put a new kit in it and it ran fine for a while. Ran it up and down the road. It accelerated fine but once it got at a steady rpm it seemed to have a miss. Thought it may be timing so I tried adjusting that. Now it's back to running about the same minus the black smoke at idle. Once you pull the throttle back it's like you hit the kill switch. I can turn the air screw all the way in and the tractor is still running. I can run the fuel screw out until it is blowing black smoke rings, it still wont take throttle. I am thinking it may be a vacuum leak from the intake manifold. Either that or something is fishy with the ignition system, maybe a bad condenser or coil. It is still a 6v system.

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1 hour ago, LipRipper3006 said:

Put a new kit in it and it ran fine for a while. Ran it up and down the road. It accelerated fine but once it got at a steady rpm it seemed to have a miss. Thought it may be timing so I tried adjusting that. Now it's back to running about the same minus the black smoke at idle. Once you pull the throttle back it's like you hit the kill switch. I can turn the air screw all the way in and the tractor is still running. I can run the fuel screw out until it is blowing black smoke rings, it still wont take throttle. I am thinking it may be a vacuum leak from the intake manifold. Either that or something is fishy with the ignition system, maybe a bad condenser or coil. It is still a 6v system.

What happened to it ran fine with a different carb?

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