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

[hlw49]

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

How did it react when you sprayed carb cleaner in the intake? Did it effect it at all? If not check the ign modules.

 

[slomo]

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

Still have some homework ahead of you. My step 1 and 2 should be attacked next. Glad to hear she is sparking now.

 

[slomo]

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

Also before ordering coils, sand/polish the block to coil mounting area. And the bolts and where the bolt head touches the coil. Need a good clean ground for the coils to fire proper.

Test the above with a multi-meter on Ohms. Should be roughly 0 Ohms from coil mounting bolts to engine block.

Those PET-4000 testers are kinda pricey. Great tools for this purpose.

 

[bertsmobile1]

Also before ordering coils, sand/polish the block to coil mounting area. And the bolts and where the bolt head touches the coil. Need a good clean ground for the coils to fire proper.

Test the above with a multi-meter on Ohms. Should be roughly 0 Ohms from coil mounting bolts to engine block.

Those PET-4000 testers are kinda pricey. Great tools for this purpose.

Just an expensive variation of two pieces of HT cable in a tube with a gap in the middle

 

[slomo]

Just an expensive variation of two pieces of HT cable in a tube with a gap in the middle

Also the gap is adjustable to load her down.