Farmall M Runs When Choked - Need Help

Greetings fellow antique tractor enthusiasts! I'm writing my first post in this forum in need of some assistance or advice. I'm trying to resolve a problem with a 1948 Farmall M. In a nutshell, I can get the tractor started, but in order for it to keep running it must remain nearly fully choked, otherwise it will stall out. I can keep it running by tweaking the choke for several minutes in this condition, but have only done so for diagnostic purposes. Throttle movement during this time has no effect. The tractor runs reasonably smooth (ignition, timing, etc. all seem OK), but obviously not correctly in this condition. After multiple searches in these forums for clues to the problem and having made several attempts to resolve the problem, I'm rather stumped. I’m by no means an expert on these and have been learning some as I go. I am reasonable mechanically inclined and have some experience with an Allis Chalmers B also (learned to love and hate magnetos….but that’s a story for another forum ). I’m fairly comfortable with working on/diagnosing problems, although I’ve not done major engine repair or teardowns. I'll detail what has been attempted thus far and provide some more general information on the tractor itself.

[b:ebdb71c36a][color=blue:ebdb71c36a]Tractor Details & background:[/color:ebdb71c36a][/b:ebdb71c36a]

- 1948 Farmall M (Serial #FBK-169321X1) with gas engine, distributor/coil ignition (non-magneto) & standard governor (don't know exact type).
- Carb. is IH E12 updraft model #50983DC with 8557D throttle body.
- Tractor is kept outside year round through four season conditions and has been for several years.
- Tractor was running fine last Fall, but sat through most of the winter without being run. I'm fairly sure the gas was run out of the bowl in late Fall in prep. for Winter to avoid any moisture/float issues.
- Had major engine rebuild in mid-90's by previous owner and has probably had approx. 150-200 hrs. run time since

[b:ebdb71c36a][color=blue:ebdb71c36a]Work done thus far (in no particular order):[/color:ebdb71c36a][/b:ebdb71c36a]

- Completely removed, disassembled & cleaned carb. (except venturi port removal from throttle body) with carb. cleaner and compressed air. Disassembly included all jets/ports, etc. No obvious signs of blockages. Fuel line inlet filter was a bit dirty, but was easily cleaned. Carb. ports were subsequently cleaned/inspected several times after uninstalling/reinstalling carb. and experiencing similar poor running conditions.
- Replaced needle valve and seat with kit purchased at Tractor Supply Company (I know some may debate the quality of parts in this kit, but on inspection/installation they looked identical)
- Tested float for leaks (had no fuel in either float and did hot water immersion test - no bubbles)
- Replaced float bowl and throttle body gaskets
- Checked butterfly valve/throttle shaft for proper motion and sticking concerns both before and after carb. removal and reinstallation - all seemed smooth and operating properly with respect to the governor. Note both governor side housing and carb. float bowl assembly were removed during reassembly and testing.
- Verified proper reassembly and mating of throttle shaft assembly to throttle body (matched up shaft notch to butterfly plate notch seat)
- Checked manifold bolts for any unusual looseness - all seemed OK - did not remove manifold or make a serious attempt to check for manifold leaks
- Checked choke for smooth operation - all OK
- Checked fuel flow to carb. (line only) and with float bowl assembly with needle valve and seat in place. Had what seemed to be good consistent flow on both accounts. Note fuel line has an inline filter installed and has run fine for years with this in place.
- Checked fuel flow with gas cap installed and loose - no difference in flow to carb. Both good.
- Checked governor area for obvious signs of damage - none found. Note governor spring seemed in good condition, although likely some governor parts have wear. Governor weights moved freely as did governor spring and rockshaft levers. I don't have too much experience with these, but understand the basic operation.
- Spark plug condition was checked. There was minor carbon buildup (nothing obviously unusual) and gap was .003” off at .028”. Gap was reset to .025”.


[b:ebdb71c36a][color=blue:ebdb71c36a]Other observations:[/color:ebdb71c36a][/b:ebdb71c36a]

- OK, this may sound a bit odd, but both my Father and I recalled that when operating the throttle lever the idle position was always closest to the driver and full run was toward the front of the tractor, farthest position away from the driver. What’s odd is that it seems to be the exact opposite from what I just described (lever close to driver = high throttle / lever away from driver = idle position) as confirmed by watching the throttle plate and governor linkage. This just seemed unusual, but maybe we are just not remembering correctly for some reason.
- Noticed slight abrasions on the float assemblies to the lower inside of each ballast (sticking float??). Bent each ballast slightly away (carefully at the tang!) from the abrasion site thinking that it would eliminate this possibility. Float seemed clear of any obstruction when placed in bowl in approximate proper position prior to re-assembly.

Work on the tractor will be somewhat weather dependant given the time of year hear (upstate NY), but I’ll do what I can. I’ll try to report back on any follow-up work, particularly if I get this resolved! Thanks in advance for any constructive advice or thoughts.

Thanks everyone for the replies and helpful advice. I'll comment on a few posts where I can, but will need to spend some more quality time with the tractor when I have a chance to check out some other suggestions.

[color=blue:84b3532561][b:84b3532561]Throttle/throttle plate/governor[/b:84b3532561][/color:84b3532561]

- Seems like I'm getting conflicting information on the throttle position. One indicates that the way I described mine working is correct (idle position = farthest from driver / high throttle = closest to driver) which I think is the case based on the governor/throttle plate positions I've described. I could see them being mixed up at the bell crank had I disassembled them, but I have never taken them apart from when the tractor was running fine, so I couldn't have reversed them. I'm currently working on the assumption they are not reversed at this point, but will investigate further.
- I was pretty careful on reassembly of the throttle shaft. Note that I think I forgot to mention that I did not remove the throttle plate from the throttle body. There seemed to be no binding or slop/play in it. Basically, it seemed to be working perfectly fine so I decided to leave it alone. Worth a recheck though!

[color=blue:84b3532561][b:84b3532561]Carb - Gas/Air flow[/b:84b3532561][/color:84b3532561]

- I still haven't completely ruled out something with the carb. such as a blockage or float problem, but I've sure spent a lot of 1:1 time with this thing so far. It does just seem like it's not getting enough gas, but as several posts have suggested the root cause could lie elsewhere (manifold leak, loss of venturi, points, etc.)
- I initially have set the main jet to 3 turns out during carb. reassembly procedures, but did experiment with turning the screw out probably twice that or a little more when trying to diagnose fuel flow issues. It had no effect at all, which brings me back to blockage/lack of venturi investigating.
- Fuel filter - As noted I seemed to be getting good fuel flow to the carb. and float assembly, although I obviously can't see the float in operation once it's all back together. It has run fine with the current filter in place in the past, but replacing it at this point makes sense. It's cheap and rules out one more potential problem area.
- Carb. fuel inlet filter (the brass screen one that screws into the carb. that the fuel line screws into) was removed and throughly cleaned. I could see daylight clean through the screen once clean. It had some minor buildup, but is clean as can be now.
- Only checked manifold bolts for tightness (all good) and checked condition of carb. gaskets, both of which were in reasonable shape. I replaced them anyway. Further work on this front is warrented at this point.

[color=red:84b3532561][b:84b3532561]So, my next steps I believe would be are............(starting with the easiest stuff 1st)[/b:84b3532561][/color:84b3532561]

- Check point setting and condition
- Replace fuel filter with gravity fed compatible filter (or maybe remove completely?)
- Test compression on all cylinders
- Test for gasket/manifold leaks (propane/carb. cleaner)
- Inspect bell crank assembly & inspect/reassemble throttle shaft assembly & settings
- Reinspect/clean carb., particularly mian flow pathways? (ack!...not again!! :shock: )

Thanks for any additional comments or advice!

I figured I'd followup on my original posts as I'm not sure why replies to this post keep getting scattered through the forums. I've finally got the Farmall running again! Work was spotty given weather conditions and other committments. Turns out it was after all a plugged carb that was the problem all along. Even after multiple cleanings the carb did not come clean. It wasn't until a couple of carb cleaner soaks and finally removing the venturi (that took a while) that the unobvious blockage was cleared. Tractor runs very smooth now. After trying multiple suggestions provided by members of this forum (many thanks!) was I able to at least eliminate many other possibilities and turn my attention back to the carb. I'm not certain exactly what port was plugged, but obviously a main fuel supply port was partially blocked. In reviewing my original post, I mistakingly noted that this had a distributer setup, which was incorrect. It has a magneto. Sorry about that! In testing the magneto I discovered that it was giving a very weak spark that was corrected with a rebuild. All other potential problem areas such compression, manifold leaks, sticking float, etc. all checked out fine. Just thought I'd send a final update on the resolution of this for anyone else's benefit. Thanks for all the great suggestions and help.