John Deere 140 Engine Issue

My 1968 JD 140, SN3457, 12hp Kohler, starts and runs fine for about 20-30 minutes of mowing and then sputters/sometimes backfires and then dies unless I back off the throttle. It will continue to run and mow at about half throttle, but will sputter and die if throttle is increased. It has new head gasket, spark plug and condenser. Any suggestions?
 
When the engine quits check for fire to your sparkplug before the engine cools off. If it still has hot spark to the plug open the main jet adjusting screw on the carburetor. This will enrich the fuel mixture. Also make sure
you have a full stream of gas from the gas tank
by disconnecting the gas at the carburetor. Use a clean container to catch the gas. Hal
 
I had a JD 140 with a 14hp Kohler. I bought it with a blown engine and I installed a new short block. Hal
 
Apparently, heat is changing something and that is causing your problem.

Air cooled engines need a constant flow of air across the finned areas to stay within the proper temperature range. If critters have invaded your engine and built nests, then that could be restricting the flow of air.

Coils will often begin to drop output when they reach a critical temperature but this usually results in the engine quitting outright and not restarting until everything has cooled down.

Heat can also affect the fuel pump flow. It could be that your engine is starving for fuel when it is hot and that is why it will only run at half-throttle. Have you tried advancing the choke to make incoming air/fuel mixture richer? If not, then try it and see what happens.

Modern fuels suck. They are subject to boiling at a lower temp and causing vapour lock. Have you looked at your fuel lines in relation to your exhaust system?

Check the vent hole in your fuel cap. If it is not open, then that can cause fuel starvation. Loose clamps on the fuel line can also let in air, making the fuel pump work hard to pull fuel from the tank.

There can be other reasons. All too often, owners do NOTHING to these engines until they stop working. Just saying. LOL

When was the last time you set the valve lash?

The last time you removed the cylinder head, did you de-carbonize the combustion chamber and underside of the valves?

How about the points? Have they been checked, filed flat and the gap properly set?

Did you check to see if the carb is tightened down properly?
 
Had a 10 hp Kohler do something similar to me years ago. Thought it was ignition related at first, but after changing every component with no luck, I started to remove the covers and shields around the cooling fins and found a wad of oily grass and crud was partially blocking the cooling fins around the head and valves. Cleaned it up, put everything back, and its run great the past 10 years. Evidently, after 20 minutes or so of running, the engine was overheating just enough the valves were sticking in the guides, causing backfiring out the muffler, even some flame out the muffler until it shut down. after sitting a minute or 2, it would start back up, run good for a bit, then do it again.

Mice love to build nests behind the shrouds on these engines, and they are fast workers.
 
When I was plowing with my Wards tractor 5 or 6 years ago it seem to smell hot. I had to remove the cylinder head to get inside the shroud and here's what I found. The engine didn't seem to lose power. I'm still running the tractor. Hal
a159548.jpg
 
I've had 2 similar problems on an old 10 horse Kohler, one turned out to be the coil and a couple years later one of the valves or whatever in the fuel pump came loose in the plastic pump body.
 
gab, you probably have hit on something there. I have seen 3 of those old fuel pumps fail recently. One of them could and would sometimes mow for hours before quitting.

I made an adapter plate to go over the fuel pump hole in the block, tapped it for a hose barb and installed one of the late type pulse fuel pumps on it. Its on it"s second year running like that.
 

I recently brought home a Ford LGT 10hp was acting very similar, seemed like the timing was off so cleaned and adjusted the points, got about 12 hrs on it since without missing a beat... I never use anything more than 400 paper on a set of points.
 

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