M carb question

Doc 2 · Jan 7, 2015

Rebuilding an M carb. When the main metering screw is turned all the way in the tip is 11/64" away from the face of the tube holding the main jet. Is this correct? It seems as if the screw would be ineffective. When I tore this apart the nut holding the main screw had bad threads near the inner end and the screw was way out so I have nothing to compare it to. Thanks. Brian

rustyfarmall · Jan 7, 2015

(quoted from post at 14:32:30 01/07/15) Rebuilding an M carb. When the main metering screw is turned all the way in the tip is 11/64" away from the face of the tube holding the main jet. Is this correct? It seems as if the screw would be ineffective. When I tore this apart the nut holding the main screw had bad threads near the inner end and the screw was way out so I have nothing to compare it to. Thanks. Brian

If that screw is turned all the way in, it should completely shut off any flow of fuel. Probably you need to get a new brass nut.

Janicholson · Jan 7, 2015

I was under the impression that the load screw on an M adjusted the richness over a range, but was not the only passage for fuel into the main nozzle. Jim

John M · Jan 7, 2015

Sounds to me like you didn't remove the old packing.

pete 23 · Jan 7, 2015

The screw should completely shut off the fuel. All fuel, load and idle circuits has to pass through that main jet. No other passages for fuel to enter main nozzle or idle ports. You may have to remove some of the packing or you have the wrong screw. Lot of people don't realize there are at least two, possibly more screws. They have a different taper on the pointed end and are different length. The original fire crater sleeve and piston set came with a new main jet and screw that matched the different taper of that jet. I have run into a few carburetors where the screw did not completely shut off main jet but they would reach far enough to adjust the fuel flow. Like I said though. You may have to remove some packing.

Doc 2 · Jan 7, 2015

All the old packing is out, the nut is all the way in, the screw is all the way into the nut with the spring fully compressed. This tractor has been on the farm since new in 1947. I know the engine was majored in the early 60's. I assume fire craters were put in, 120# compression now. Carb could have had a change over then.I'll look for a longer screw.

My Dad and uncle bought this M new in 47. When my Dad quit farming in 50, my uncle bought his share of stuff. Tractor was used regularly into early 70's. Sat for a long time stuck, was freed up and used a little bit. Sat again outside. 2006 got it under cover and just now getting going on it. This is the tractor all us cousins who didn't grow up farming learned how to drive a tractor on. Lots of sentimental value and trying to do it right. Thanks for the replies. Brian

John M · Jan 7, 2015

Theres your problem, theres no spring on that screw, just on the idle mixture screw and the idle speed screw.

Doc 2 · Jan 7, 2015

That's it! The new packing will hold the screw tight. Thanks. Brian

glennster · Jan 8, 2015

i just went thru this with my 52 m. tractor always ran rich from the day i got it. no amount of load adjustment helped. had to start the tractor with the throttle at idle and no choke at all. found the same problem with the main load screw. ordered new screw and nut from i-h. the old screw was shorter than the replacement. someone must have replaced that at some point with the wrong one. runs fine now.

M carb question

Doc 2

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rustyfarmall

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Janicholson

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John M

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John M

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glennster

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