Problem: Severe Backfiring in 26 hp Kohler


New User
History: Did repairs to tractor and engine. Started and drove around for 20 minutes. No problem. Went back following morning and battery was drained. Change he d battery at 20A automatic. Then removed charger, tried to start, nothing at all. Checked for blown fuse, blown and replac d. Tried to start and immediately blew fuse. Then disconnected carb solenoid and voltage regulator. Replaced fuse starter turn and dash lights worked. Discovered when disconnecting that solenoid wire was pinched between to metal engine shrouds - I adjusted wire. Then attached regulator and starter turned. Then reconnected carb solenoid and starter turned. I then tried to start mower. Engine turned, but backfired so strongly and the engine would not start that I stopped trying to start.

Can the starter solenoid create this kind of response: heavy backfiring and no start engine? Perhaps voltage regulator? I m seeking recommendations as to potential causes. I tend to not believe it is valves because it ran well before and only carb solenoid and regulator were touched since it ran well. Thanks in advance for your help.
did repairs to tractor and engine. Are you saying you did other repairs (other or before) everything you did after you started and drove
20 min?

Starter turned did it turn briskly like it shoud or was it pretty slow? If it is turning slow (you didn't say which engine you were
working on) it is possible for the ignition to fire the intake charge back through the carburetor because the intake valve is too slow to
What work was done before the 20 minute
drive? Anything to do with the valves or
ignition system? Possible carburetor work?
Could there be more wires pinched

The tractor is a Craftsman 917.986450. Engine is a Kohler KT745-3012. Mower was missing periodically run on 1 cylinder, then after 2-10 minutes might go back to 2 cylinders. When running on 2 cylinders ran well. Took to repair they rebuilt carb. Took to a different repair shop the changed plugs, gaps, wires. In both cases ran OK for a few hours then back to one cylinder. The first repair said could be valves but if that s what it is it s not worth it and I should buy new tractor. So I took heads off. Gapped the coils. All gaps were checked multiple times and all bolts were torqued as per repair manual. One head showed more carbon inside that head. The other was pretty clean. All were cleaned up. Cylinders were very smooth with no signs of etching. I disassembled valves, pushrods, rockers. All were cleaned and straight. Did surface set the valves and seats using recommended grinding paste with manual and drill set at slow to medium speed. Cleaned all again. Installed new valve guides and new gaskets with small amounts of black gasket builder. Re assembled with specs and digital torque wrench. At that point I decided to start and drive the mower. Did so for 20 minutes with no issue. Great power and no missing. Then onto the first post I made. I bought a ebay carb, but choose to cleanup and go with the original carb.
Thanks for the additional info.

I am thining you have a coil that is 'playing tricks': (meaning it only works properly part time.) The problem is it probably only
malfunctions when it is really hot.

If you don't have an in-line spark tester, get one. Or two if you want to test quicker, connect them to your sparkplugs and connect the
other end to your sparkplug wires. Start the mower-assuming it will start- and observe the light from each tester. If there is a
noticeable difference-bright and weak-, swap testers and see if the difference stays with that coil. If the weaker light stays with one
coil, replace that coil. If the run on one cylinder does not immeditaely happen, try mowing with it and see if it happens then.

Harbor freight has those in-line testers and they used to be cheaper than anybody else.

You said you changed head gaskets and torqued them to manual.

You said that you lapped the valves and adjusted them according to the manual. [Aside: At the last in-person Kohler Service School I
attended (pre-nnalert) the service representative told us to no longer lap Kohler valves. He said it damaged the stelite coating on the
valve. But you said that lapping info was in their manual. I personally have always lapped used valves before reinstalling them.]

Another thing to try if your engine is running on one cylinder: disconnect the spark plug wires one at a time and see which cylinder it
runs on. The other cylinder would be the problem cylinder.
I use a heavily insulated spark plug pliers and do that with the engine running, but you can disconnect one spark plug wire and then start
the engine so you won't get shocked.

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