Kohler Courage SV720 missing and backfiring at speed

It is a Kohler Courage SV720 twin cyl on a Cub Cadet LT1050 rider. The engine idles fine however at full throttle the engine starts to miss and backfire. I have installed new spark plugs and fuel filter. I disconnected the oil switch to test the engine. I have added sea foam to the gas and marvel mystery oil to the oil thinking maybe a valve was hanging up or there was some water in the gas. It seemed to run ok last night but then started missing again this morning. Before I start pulling parts and throwing parts at it I wanted to get your opinion on where to go first. The carb looks like a pain to pull. I already have the blower housing off for testing.

 

Sort of sounds like a carb problem to me. Does applying partial choke help smooth it out?

 

Pulling the choke out a little does not make it run any better. I did remove the carb and soaked it in my ultrasonic cleaner with heat for two cycles and then blew out all the passages. There was not any crud in the bowl. Didn't have time to re-install it last night so will do today. I am also going to hook up an auxilliary tank and test it off of that. It is sure acting like there is water in the gas.

 

Re-installed the carb this morning and hooked up an auxilliary gas tank in case gas was bad. Same thing missing an backfiring at full throttle. I am where to go next. Since it is backfiring and the fuel pump picked up the auxiliary tank with no problem, I am going to try one coil and then switch them in case there is no improvement. What do you think ????

 

Try disconnecting the kill wires to the coils first and see if it smooths out.

 

Will do - makes sense - had not thought of that - could be a safety switch ??

 

I believe that engine has a diode between the coils incorporated in the kill wire. If so, and if it is bad, it can cause missing like you described.

 

OK - I pulled both kill wires off the coils. It still missed and backfired after a while. I then put some plastic shielding around the spark plug wire on the left side and that seemed to help some. It runs good then every once in a while it misfires and backfires. This time I did see a small puff of black smoke when it backfired. The new coil is due here this afternoon. I did not see any diodes in the kill wires. Are they integrated into the magnet on the flywheel ????

 

They're in the kill wire harness.

 

Adjusted the valves yet?
That's what got mine running smooth again.

 

Here is some new information. I had replaced the spark plugs just before I dove into this mess. I pulled the plugs this morning and found the driver side plug to be clean and tan however the passenger side plug is now completely sooty and black. What is this telling me ???

Also I tried the new coil on the driver side and t did not make any difference.

I am really at a loss on this one.

 

Is it the #2 cylinder (lowest cylinder)? If so, I'd suspect the float needle in the carb is letting it fill up too much.

 

It is the cylinder on the passenger side which is also the starter side.

Looks like it was the coil on the passenger/starter side. I left the new coil on the driver side and installed the coil previously on the driver side on the passenger/starter side. The engine seemed to run fine - no missing or backfire. I drove it around at full throttle both with no load and with load and it ran fine.

Thanks everyone for your help.