Kohler runs well for 2 mins, misses for 3 mins, then runs well again

[namegreb]

Hi --

I have sought advice from a number of sources and will appreciate any thoughts anyone might have.

I have a Kohler ZT740 with a Keihin one-barrel carburetor on an Exmark Pioneer S zero turn mower. It is 7 years old with around The engine starts promptly and runs well for 2 or 3 minutes before starting to miss badly and losing power. It usually runs rough for 3 or 4 minutes and then suddenly resumes full power. Occasionally it backfires while it is missing but not always. Sometimes immediately after a backfire it returns to smooth running. Usually, once it returns to full power, it runs perfectly for another 2 hours.

As I recall, this happened even early last spring after getting a new fuel filter and it has continued since then. Once or twice I've mowed with no problems.

I've tried

+ Replacing the plugs (and they show a bit of carbon fouling)
+ Using Heat and other treatment for water in the gas
+ Using Sta-Bil
+ Spraying carb cleaning in the intake before and during episodes of missing
+ Thinking that the choke may be sticking, I also tried starting the engine without choking it by using starter fluid.
+ For what it's worth, I tapped on the fuel shut-off solenoid while it was running poorly.
+ Siphoning fuel out from around the in-tank filter.

Nothing has helped. As I noted, I think this happened even after the fuel filter was changed last winter.

The thing that baffles me and others I've talked to is the fact that it goes through its few minutes of rough running and then returns to perfectly smooth operation. Only very rarely does it go through another brief rough stretch. It seems like a fouled fuel line or something like that would not allow the recovery I am seeing. Are the carbon fouled plugs a clue? But why do things resolve on their own after a few minutes?

Thanks very much for any wisdom you can share!

George B

 

[MParr]

Remove your carburetor bowl and clean it.
What’s the condition of your air filter?
Have you adjusted your valves?

http://www.kohlerengines.com/engines/onlinecatalog/pdf/32_690_03_ENA.pdf

 

[Rivets]

Does it blow white or black smoke when it starts to run rough? White smoke would indicate water in the gas and black smoke would indicate a rich running condition. Another thing to try is getting rid of the Stabil and other gas treatments and try running a tank or two of 91 octane, ethanol free fuel. If we can eliminate a fuel problem, I would be looking for an electrical problem, like a chaft kill wire rubbing somew.

 

[ILENGINE]

I would throw a couple spark testers on it and see what happens when it is running rough. Loosing fire on one or both cylinders intermittently possible.

 

[Rivets]

I agree with IL. Both these tests can be done at the same time. This would tell us a lot about how to proceed.

 

[slomo]

7 years old now. I would:

1.Clean the cooling fins and engine block. This is a yearly task in your engine manual. Neglect this and you are looking at engine damage.
2.Pull the head/s and de-carbonize the cylinder/s, head/s and valves. Neglect this and you will get cylinder bore scoring eventually making her use oil and smoke. Check your engine manual for more info.
3.Load test the coil/s.

4.Clean and flush out the fuel tank and install new fuel line/s.
5.Carb is probably getting dirty, needle might be randomly sticking open/closed. Clean and rebuild the OEM carb.
6.Delete the carb mounted fuel solenoid.
7.Polish the coil mounting area to provide a clean ground for the coil. Sand off the rust.

8.Valve adjustment.

 

[nacl9163]

I had a similar problem. Turned out the tiny vent hole in the gas cap was clogged, so the thing would run, but as it had trouble equalizing the air pressure, it would stall. If it sat for a minute, the pressures would equalize, and then it would run again for a short while. Don't remember how I discovered this (probably dumb luck), but once I blasted the cap with compressed air, the problem vanished. Had this problem more than once. Good luck!

 

[namegreb]

Thanks very much to all of you for your suggestions.

I don't see smoke of any sort when it is missing. I've tried removing the gas cap to check for a plugged vent. The air filter is clear and it misses even with it removed. .The cooling fins are clean.

Going forward... I've ordered a PET-4000 spark tester, and think I know where I can find ethanol-free gasoline. I'll try those suggestions and then move on to others.

Many thanks!

 

[Fish]

Sounds like a little water in the tank/carb.

 

[namegreb]

Thanks once again to everyone for your suggestions. Here is a follow-up report . . .

I picked up a PET 4000 spark tester (very nifty) and tried it while the engine was missing. Sure enough, the spark in one cylinder was deficient. I ordered a couple of ignition modules, and, happily, all is now well with the engine.

I appreciate the consults very much!

Best wishes,

George B