John Deere 727A Stalling once hot

[Bfavro]

I have been trying to diagnose an issue with a John Deere Ztrack 727A for quite some time now with little luck. The JD 727A has a Kawasaki 23HP V-Twin. The tractor starts up and runs really well after being choked for the initial start. You can engage the PTO and start cutting grass and all runs well until the motor gets to a certain temperature. It stalls once the engine gets to a high temp. On cooler days you can get "some" grass cut because it will run for about 15 minutes. On hot days it could last about 5 minutes before stalling. The tractor always starts right back up but then runs with a loss of power, and acts like it is running lean, fading in and out / revving up and down. Originally this lead me to suspect a fuel issue. Engaging the PTO at this point just stalls the motor and irritates the driver . Once at this point I can only run the tractor to return it to the shed or wait until the motor cools to start from scratch and hope to get more than 5 minutes of mowing done.

What I have tried so far:
I initially replaced the PTO clutch because the clutch was emitting a grinding sound, which I thought the drag was causing the stalling. The new clutch sounds great but did not solve the issue. I have drained all the fuel and put sea foam into the tank. I have replaced all the fuel lines from the tank to the carb. I have replaced the fuel pump. Plenty of fuel flows through the gas line when the ignition is turned on. I have removed and cleaned the carburetor in carb cleaner. I let it soak for a couple days. All jets seem fine. I reinstalled the carb with all new gaskets. The tractor ran great when I restarted after this but after 15+ minutes stalled and the symptoms repeated. I feel pretty confident that the issue is not fuel related. My focus is now shifting to ignition. I have put in new plugs. They are the exact plugs called for by John Deere in the manual. I have confirmed the gap is correct. I have tested spark using an inline tester, both before and after the motor stalls to see if the spark weakens. Spark appears fine but I am wondering if perhaps too much resistance, due to faulty coils, is causing a weakening of spark.

Hoping to get some solid advice on this issue. The ignition coils on this tractor are covered but my next plan was to check Ohms on the coils.

 

[Spanner]

Consider checking the valve clearances.

:wrench:

 

[Bfavro]

I will have to research how to approach that. Any good primers on this topic you recommend?

 

[tmajor]

If it was electrical, I don't believe, it would be inclined to start right up, after stalling. On the other hand, I would consider that it might be vapor locking. If the fuel lines are rubber, I wouldn't jump on them, but maybe. I'd try to determine, if the carb, was getting particularly hot. Maybe, throw a bag of ice on it or near it, to see what happens. Also, try running it with the gas cap off or a little loose .. the vent may be plugged.

I don't believe, you mentioned fuel filter ... does it have one?

 

[tmajor]

Another thing, which might tell you something: spray some WD-40 or other flammable around the intake manifold, if it straightens up, the intake manifold gasket could be leaking. Plus, you could spray into the carb. intake, which might give you a clue that it's starving.

 

[ericm979]

If the gas tank vent is blocked it'll develop a vacuum and gas won't feed to the carb properly. Try running it until it develops symptoms and then open the gas cap. If it runs ok, that's your problem.

Another thing to check is that the choke is not stuck on. With integrated throttle and choke controls the choke plate is operated by a linkage, and it could be stuck. If the choke's on it'll run poorly when it gets hot. Sometimes it acts similar to running lean.

 

[4get gto]

I remember some of the old Kohler motors were prone for the exhaust valve hanging up when hot. As they cooled off you could actually hear the valve click back sometimes. Of course that depended on where the valve and cam are at the time. They would seize in the valve guide. Then cool off and good to go again.

 

[Spanner]

I will have to research how to approach that. Any good primers on this topic you recommend?

Probably You Tube is a good place to start.

:wrench: