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Farmall M Runs When Chocked - Need Help

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11-22-2009 15:46:17

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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 :o ). 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.

Tractor Details & background:

- 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

Work done thus far (in no particular order):

- 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”.

Other observations:

- 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.
This post was edited by Sparetime at 15:49:06 11/22/09.

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11-23-2009 19:15:42

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 Re: Farmall M Runs When Chocked - Need Help in reply to Sparetime, 11-22-2009 15:46:17  
Had same problem with 47 HV, Rebuilt card and cleaned fuel system without results. Replaced points and condensor. Fired right up and runs fine.

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11-23-2009 05:19:33

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 Re: Farmall M Runs When Chocked - Need Help in reply to Sparetime, 11-22-2009 15:46:17  
My M Farmall was doing about the same thing as yours with the exception of the throttle being backwards. I went through 5 carburators before I finally got one that made it run right. I would put one on and it would do the same thing as yours (run with choke out only). I took each carb apart and everything looked fine, but I knew something was not right so I kept sending the carbs back to the rebuild place. On the 5th carb, I put it on and it ran perfect. On the bad carbs you could take out the air adjusting screw on the side and it would run fine and as soon as you even put the screw close to its hole it would begin to cut off. Try that with yours and see if it does the same thing.

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11-22-2009 19:27:03

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 Re: Farmall M Runs When Chocked - Need Help in reply to Sparetime, 11-22-2009 15:46:17  
Sounds to me your venturi is missing.

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wayne in mn

11-22-2009 19:06:24

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 Re: Farmall M Runs When Chocked - Need Help in reply to Sparetime, 11-22-2009 15:46:17  
Your throttle is backwards; I would suggest that the bellcrank is hooked up backwards. Look between the exhaust pipe & the valve cover; the rod from the operator's platform attaches to the lower hole and the governor rod is in the upper hole.
Note: type in "Farmall M" in the search box on the Case-IH page. Quotation marks not needed. Select 1939 M, select carbureted engine, select governor controls for a parts layout.


Note: #1, 2, & 3 adjustments are done with the engine not running when the carburetor has been removed & replaced. 1. Loosen the 2 screws that are holding the governor tube to the governor housing. The clamp is slotted to allow adjustment. Tighten the tube-to-carburetor screws; gently tap the tube so it "centers" itself, & then tighten the tube-to-governor screws. 2. Remove the top cover on the governor housing (has the pipe going to the cylinder head) and check governor to carburetor synchronization. Remove the cotter pin so the clevis pin can be removed. With the operator's throttle lever set wide open and rotating the horizontal rod upward to wide open throttle position, the clevis pin should just slip into the rod that is in the tube. This horizontal rod should rotate freely allowing the carburetor throttle plate to move from idle to wide open. Adjust the clevis height if needed & lock the nut. You may need small fingers & right-angle needle nose pliers to make the job easier. Return throttle lever to idle. Re-attach the top cover.
3. Carb adjustment: idle air screw is set at 1½ turn out to start with; high speed mixture screw is 2½ or 3 turns out.
4. With the engine warm & running at wide open throttle (WOT), turn high speed mixture screw (on the bottom facing down to the rear) in until the engine begins to starve for fuel. Then turn the screw out until the engine begins to blubber. Now find the "sweet spot" in between the two settings. If you are working the engine (plowing, etc.), then the high speed screw needs to be turned out ½ turn or more beyond the "sweet spot" to be sure the engine isn't starving for fuel and can develop its rated power.
5. The same procedure can be followed for idle air mixture with the addition of setting the idle stop screw (just to the rear of the governor shaft tube) to adjust the low speed RPM.
On the "H"& “M” carb, the idle air mixture screw (faces to the front just below the idle stop screw) is an air adjusting screw so turning in will richen the mixture and out will lean the mixture.
Repeat the carburetor adjustment procedure again to make sure “all is well”.

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El Toro

11-22-2009 18:50:33

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 Re: Farmall M Runs When Chocked - Need Help in reply to Sparetime, 11-22-2009 15:46:17  

That fuel filter may be the problem if its not for gravity flow fuel system. Open that main jet adjustment screw near the bottom of the carburetor. That's how you enrich the fuel mixture on these tractors. Even garden tractors have them similar to the one in this picture. Hal

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Bernie Steffen

11-22-2009 17:32:21

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 Re: Farmall M Runs When Chocked - Need Help in reply to Sparetime, 11-22-2009 15:46:17  
Lots of info for first time post. Only one thing I might add. Did you check the screen in the inlet fitting on the carb? That will cause this problem. Hope you gitter fixed. Bernie Steffen

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pete 23

11-22-2009 17:16:15

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 Re: Farmall M Runs When Chocked - Need Help in reply to Sparetime, 11-22-2009 15:46:17  
After checking what previous posters suggest I will just add, don't over look rest of ignition system. They can do weird thing to you some times. Last week I went out on a job, Farmall 400 gas tractor. It started acting up for my guy and he said he was able to keep it running by pulling choke but eventually it would not run any more. Turns out points were not opening at all by then so no ignition. I just opened up the points a bit to get it so he could use it to auger some corn and told him I would need to rebuild the distributor as shaft was very loose. Also found out CASEIH now offers a bushing rebuild kit so will find out how that works out as we could never get bushings for them all the years I worked at a dealership.

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D Slater

11-22-2009 18:34:43

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 Re: Farmall M Runs When Chocked - Need Help in reply to pete 23, 11-22-2009 17:16:15  
Pete bushing IH 373577R91 was around for a long time. They didn't make the part # easy to find.

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pete 23

11-22-2009 19:04:41

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 Re: Farmall M Runs When Chocked - Need Help in reply to D Slater, 11-22-2009 18:34:43  
I'm going to look up that number and see if that is what they ordered for me. Thanks, learn something every day.

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Jerry RV

11-22-2009 17:12:15

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 Re: Farmall M Runs When Chocked - Need Help in reply to Sparetime, 11-22-2009 15:46:17  
I have a 48 M also. The only thing that I can help with is that when you pull the throttle towards the driver it is at high throttle. Lever away from the driver is idle.

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11-22-2009 16:21:04

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 Re: Farmall M Runs When Chocked - Need Help in reply to Sparetime, 11-22-2009 15:46:17  
Nice first post!!
I believe that the fact that you can keep it running means there is fuel supply. ( I further agree with Scotty, make sure the filter is for gravity flow systems, NAPA has them)
Manifold leaks to the outside will also show up using a propane torch with the nozzle (mixer) unscrewed from the pipe. Using pure propane (a small hiss from the pipe) as a feeler around the manifold and carb can pinpoint exterior leaks by letting it have enough fuel to run (usually) much better. If it has an internal leak to the exhaust side, it will not help at all. Some of the internal leaks (at the heat interface where the manifolds are connected above the carb can be seen by removing the exhaust pipe and looking into the hole with a little LED light. It is an ares that can crack, and rust through. Other times it must be taken off to see or test with water to assure no interconnect is present between the intake and exhaust sides. Best of luck, and welcome. Jim

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11-22-2009 16:05:12

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 Re: Farmall M Runs When Chocked - Need Help in reply to Sparetime, 11-22-2009 15:46:17  
Wow! I don't recall a post that ever started out with that much information. 'Grats and thanks for being so thorough.

Your problem is most likely going to be too much air or not enough gas. On the first, you do need to check for an air/vacuum leak somewhere around the manifold. It could be as simple as loose or deteriorated gaskets, either at the head or at the bottom connection to the neck of the carb. Worse cases might be a crack or hole in the manifold, or a blown head gasket (a compresssion check might be in order to eliminate the latter). The easiest test is to get it running, and then spray a little aerosol carb cleaner into the area. If the motor stumbles or bogs down further, that would indicate a leak, and a more careful control of the spray from that point will help you locate it more specifically.

It does sound as if you've worked over the fuel supply side. You could try eliminating or replacing that inline filter (be sure any new filter is compatible with a gravity feed system -- many are designed to require the pressure of a pump and make for problems on the gravity feed tractors).

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11-23-2009 18:50:53

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 Re: Farmall M Runs When Chocked - Need Help in reply to ScottyHOMEy, 11-22-2009 16:05:12  
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.

Throttle/throttle plate/governor

- 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!

Carb - Gas/Air flow

- 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.

So, my next steps I believe would be are............(starting with the easiest stuff 1st)

- 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!
This post was edited by Sparetime at 18:57:12 11/23/09 2 times.

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El Toro

11-24-2009 14:38:15

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 Re: Farmall M Runs When Chocked - Need Help in reply to Sparetime, 11-23-2009 18:50:53  
Hold your hand over the throat of the carburetor while a helper tries to start the engine and see there's good suction on your hand. If there's good suction I think you may have did someting during the carburetor rebuild that causing the problem.. Hal.

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