Help! M does not run smooth

Over the years, I posted questions of problems with my “experienced” tractors and you have never failed to solve the problem. I have another one and hope you help solve this mystery.

I have a 1948 M with a 325 Dual loader, Schwartz front end with full PS and an add-on two speed splitter which gives me the ideal forward and reverse speeds for picking up round bales after harvest – it’s primary job around the farm. It’s a perfect job for this tractor and I love being in the fresh air after baling.

Here’s the problem… The engine does not run smooth. When it idles, it sounds like its missing, it’s a bit better a low-mid range rpm, worse at higher rpm. When I put it under a bit of a load (I grab a bale and take off quickly, going uphill), it seems to smoothen out and sounds like it’s hitting on all four. When the “pressure” is off, it’s back to running uneven.

I have changed plugs (not wires), which has not helped. I have gone through the carb, no difference. Someone suggested a bad valve so I checked compression on each cyclinder – all four are equal, all hold (for at least 10 sec) and have about 80 psi – so not sure if it’s a sticky valve. Is this a correct assumption?

Another clue… The temp guage shows it runs cool, and I assume the guage works. If I forget the winter cover on front, it will get hot. When I get the temp up, it does seem to run better. When I’m in the field, the guage shows it to be below normal temps. Could I have a bad thermostat? Would that explain the symptoms?

Someone else suggested a leak in intake manifold, would that create these symptoms? Could it be multiple problems? Any other ideas?

Thanks,
Jim

80 lbs is awfully low compression. I'd run some top engine cleaner thru 'er and see if it helps.

Allan

They have a large bypass, so thermostat does marginal good, but helps.

I would make sure the plug wires are good. It could also have a worn distributor bushing, or many other things.

I would bet your spark advance is all over the place. Worn out shaft bushing, weights, broken springs etc in distributor. This causes over advanced spark and never at the same time twice in a row. Most over looked part of tune ups and repairs on these old tractors. Original M battery ignition has a 40 degree advance, 4 inch aluminum pistons should have a 30, fire crater 22 degree with a gasoline head. Also, the driving gears and shaft that turn the distributor may be loose shaking the dist apart.

I had an H with similar issues; thought the valves were needing attention. However, when removing the manifold to take off the cylinder head, one intake runner fell to the floor. It had completely broken off and until the engine was warm the issue did not show up.
Use a propane torch (not burning) and flow the gas around the manifold area when the engine is acting up as you described. JB-80 or other similar combustible gaseous product will work also.

If you have a manifold leak it should run better if you give it a little choke. It will smoke but run smoother. Cheap and simple test.

I agree with 80 psi compression is low. Check your plug wires and also the condition of your distributor cap. Also check your points, they should not be burnt or corroded and set to the proper gap. Another thing that you could check is your valve to rocker arm clearances. They do wear and need to be adjusted over time and often overlooked. For what its worth, I had an M that I could not get to run right no matter what I did. It turned out to have a cracked head and a bad head gasket. After fixing it, it starts and runs perfect. M's where bad about developing cracked heads. Funny thing about mine was, very little water would disappear from cooling system. Non the less though, there would be a few drops of antifreeze in the bottom of oil pan. To check that on yours, just let the tractor sit for a few days and then loosen the oil drain plug just to the point that it will drip (don't take it out) and if antifreeze drips out you likely have a cracked head or bad head gasket. This creates compression problems, and moisture in the combustion chamber fouls the spark plugs.

When was the last time the valves were adjusted? Very recently?

Our 1937 Allis Chalmers WC years ago ran smooth under load, but it would blubber at idle. A veteran mechanic (long ago deceased) told me to open the plug gap to 40 thousands. I did - and the tractor has run smooth at idle ever sense. If this works on orange it will work on red. Tom

I've had a car act the same way and it had sucked a valve guide. I'm thinking something with valves. If timing were all over the place I would be inclined to think that would show up in the power band. Swap that head out.

You guys never fail me! You’ve offered many very helpful suggestions! Let me respond on a few things and maybe it will narrow the problem…

I had no idea what a normal compression should be. I talked to a neighbor and he also said that 80 is very low, and may be part of the problem -- or another symptom. With compression that low, should I be thinking an overhaul?

Adjust valves: I had a cracked head (the tractor, not me) some years ago and bought a used head from a neighbor. I did not check it, did not do values – but I did check valve adjustment when I installed it. I realize now I should have done the values and had the head checked. I did buy a new intake manifold, but as I think about it, it has not run as good as it should since I replaced that head. A big mistake on my part…

Worn distributor: A couple of years ago, I bought a new distributor with electronic ignition. Are there other parts that could be worn that drive it? Or, should a new one solve the worn distributor theory?

Plug wires: Easy fix and something I can do.
Cracked head/antifreeze. Tractor has not been run for a couple of days, I will try this test.
Manifold leak: I will try the (propane) gas test, another easy test. And YES! It seems I can make it run better with the choke.

Low compression/engine cleaner: This one I don’t understand, what would I use and how would that help? I bought this tractor in the 80’s and it’s never been overhauled since I’ve had it, perhaps it’s due?

Plug gap: Another easy thing to try, I’ll give that one a try along with the other tests.

Again, thank you all for your helpful suggestions! I will report back on what I learn. If my responses and added information will help solve the problem, please let me know.

If I had to buy a head, what should I expect to pay? How about overhaul, any ballpark numbers?

Thanks again for all of your help!

Jim

Diagnostic update:

No antifreeze in oil.

I put oil in each cylinder and rechecked compression. Compression increased from 80’s to about 90. Is that meaningful? What should compression be?

I reset spark gap to .40, seems to idle better….

When I had it idling, I tried the propane test around the intake manifold – every nook and cranny. I directed the raw gas around all surfaces, including carb gasket and all around the head gasket. There was no change in idle speed. If there was an increase, it was so subtle I could not hear it. Should it be obvious? Then I tried starting fluid, same procedure. When I woke up on the ground, it was still idling. Okay, the ether did not render me unconscious -- I’m kidding! Again, no discernable idle speed increase with starting fluid sprayed on those surfaces. I did not, however, try either of those combustibles on the head, should I do that too?

Jim

Another update:
This afternoon, I put the M on the manure spreader and spread two loads. I spread in 4th gear, full throttle, which made it work. I had to “open” the carb a bit and tweaked the fuel settings, but when I had a full spreader load, in 4th year, full throttle, it sounded good! When I emptied the spreader, and it was without load, it began to “sputter” again.

Does this put another piece into the puzzle?

Jim

That sounds more like a point setting / timing issue. Possibly the advance weights in the distributor maybe sticking. It's minor, since it run good under load.

If you have a timing light hook it up. The timing should be close to top dead center when idling slow and it should gradually advance to it's maximum before you get to full throttle. You said you replaced the head, didn't happen to put on a low comp dist head did you. That could be your problem with that low of comp pressure and contribute to the rough running under no load. There are five different advance curves on those IH distributors,(if that is what you have). A 16 degrees, 22, 25. 30 and 40. I have seen some pretty ragged running M's with low comp head and way too much spark advance.

How would I know if it's the "wrong" head?

I don't have a timing light, but have a neighbor that's good with M's, and I'm sure he has one. I'll check on the timing.

Thanks!

Jim

I feel that you are describing classic symptoms of a worn out distributor. My M behaved in much the same way. I found another distributor that seemed to be nice and tight, installed it, and the symptoms went away.

We replaced the distributor a few years ago and
upgraded to an electronic ignition. It's been a very
frustrating problem!

Thanks for your thoughts!

Jim