John Deere M Loss of Power/Backfiring

ndgregor · Dec 20, 2016

So I decided to take the John Deere M out for a spin yesterday. I recently restored this tractor about 6 months ago. Started right up, and went out to the field. The tractor was operating fine with plenty of power for about 25 minutes, when all of a sudden it started to lose power and start to sputter. It would sputter and not hold a steady engine speed, regardless of throttle position. It then began to backfire through the exhaust, at which point I shut it down. I thought it had run low of gas so I walked back, grabbed a can and filled the tank. I restarted the tractor, and the same issues were observed so I shut it down again. Chained it up and dragged it back to the barn. Yesterday (although the battery was low on juice) it would not start. I also noticed what appeared to be a small amount of oil leaking from the exhaust/intake joint manifold gasket. I知 not sure whether it was an exhaust or intake port.

I have not started to inspect, but was wondering if any of you folks had a guess on what I might start with? To me it seems like it suddenly lost timing, and but I am not sure how that happens at random and so severely.

Stimw · Dec 20, 2016

Does it have points/condenser? First place I would look.
Water in fuel second place.

Billrog · Dec 20, 2016

Check points for sure as Stimw said.

DJ54 · Dec 20, 2016

If the ignition checks out OK, I believe I'd have to remove the valve cover, and have a peek. You could have a broken valve spring, letting the exhaust valve hang open a bit. When that cylinder fires, it would let it backfire out through the exhaust valve.

Did you get into the engine at all when you restored it..?? If you didn't at least re-ring it, rings could be worn enough to let a little oil by, or coming down around the valve stems, because of worn valve guides, pulling oil in on the intake stroke, if it is hanging open. When it backfires, it could be blowing the oil out the exhaust port.

Ken · Dec 20, 2016

DJ54 said:
If the ignition checks out OK, I believe I'd have to remove the valve cover, and have a peek. You could have a broken valve spring, letting the exhaust valve hang open a bit. When that cylinder fires, it would let it backfire out through the exhaust valve.

Did you get into the engine at all when you restored it..?? If you didn't at least re-ring it, rings could be worn enough to let a little oil by, or coming down around the valve stems, because of worn valve guides, pulling oil in on the intake stroke, if it is hanging open. When it backfires, it could be blowing the oil out the exhaust port.

This engine 2 cyl. up right. remove the valve cover will tell if a push rod is bent or lifter is off the proper point. when engine back fires push rods bend.

Possible the float is allowing to much gas to be sucked into engine
Remember the engine is timed a bit different than standred 2 cyl. locate TDC of front cyl. check timing. then 1/2 turn of flywheel and check timing on @2 cyl.
Is this magneto or distributor type of ignition.
ken

Norm W · Dec 20, 2016

Check to see if you have good fuel flow to the carb. Back firing and low power is usually a lean mixture. Pull the fuel bowl drain on the bottom of the carb, fuel should flow at a good clip. If you did the overhaul did you replace the ignition wires with solid core copper or resistance wire? Resistance wire and plugs will take out a mag. They cause a higher voltage and can cause arcing inside the windings, thus shorting out the coils.

ndgregor · Dec 21, 2016

This tractor has a points and condenser with distributor. I replaced the condenser and rotor when restoring it. I tore the whole bottom end of the engine apart to get new bearings in. I did not re-ring it, as the top end appeared to be in pretty good condition and had been bored to .040 over at some point. I re-timed the tractor using the method mentioned above. It had no problem, and would fire off right away for the past 6 months.

Sounds to me like I will check the points, then move to the valve cover and see what it looks like there. How to I ensure the points/condenser are in good shape? The lifters are at the bottom end (camshaft end) of the pushrods correct? How do you inspect them? Other than a bent pushrod, broken valve spring, is there anything else I should be looking for to have failed related to valves?

Thanks!

Egon · Dec 21, 2016

It may be retarded timing.

bigdeano · Dec 21, 2016

If you have another condenser that you know is good I'd try it. I've seen new ones fail with only a little use.

DJ54 · Dec 21, 2016

I wouldn't worry to much about the lifters, but do check the push rods, and valve springs. It should be pretty easy to spot.

And as others have mentioned, maybe the timing slipped, due to the distributor not being tightened down snug, and moved. Points gap changed, due to slippage, or rub block worn. Even though new, depending on the quality, I've seen the less expensive sets fiber rub block actually bend closing some, and burning the contacts.

Then there's the fuel issue. If it sat for while, or gas has been in it a while, could be some gunk in the fuel system clogging things up, making it run lean as mentioned above. Check flow to the carb. first. Then there should be a strainer attached to the elbow going into the carb. where the fuel line attaches. If that is clean, then the next likely place would be the discharge nozzle cross drilled holes being clogged. You can always open the drain valve on the bottom of the carb., catching fuel in a small container, or shop rag, and check for sediment.

It's lots easier to trouble shoot if you're there, and actually heard what was going on. Such as only 1 cylinder backfiring, or both. A sputter on both cylinders, or one hitting strong, and somewhat of a "poof" on the other. I had an MT 25 years ago, neat little tractor. Let it go to someone who made me an offer I couldn't refuse.