Kohler M18 skip
 

I recently purchased a 782 that has a M18 transplant from a 1811. The engine starts easily and runs fairly well although it seems to misfire intermittently at all rpms. Not know any history other than what the PO told me "runs good" I started down the diagnose and tune road. First, a compression test. Both cylinders are between 95-98 pounds cold. New plugs of correct heat range, gapped and installed. Adjusting the carb has no effect on the misfire but I can get lean rpm drop or too rich when turning the screw. Fuel pressure is steady at 3 pounds. Old plugs show right bank is lean. Pulled manifold and replaced bad gaskets between the head and manifold. Very slight improvement - less misfire. Thinking it is fuel related I swapped carbs with a known to be good carb from the M20 in my 2072. The 782 still misfires. Put 782 carb on 2072 and it runs fine, so it's not a carb issue. I did look for cracks on manifold when I had it off. It looks good.
I have begun to suspect the ignition module/coil. I disconnected the kill wire at the plug. No help. Listening to the motor it sounds as though the timing is off. If this was a small block Chevy with a distributor, I'd turn it a little. The motor just sounds like it is laboring, and of course skipping. Can anyone think of something I've missed? In order to replace the coil, I will need to pull the motor from the chassis so as to remove the flywheel shroud. I have no problem doing this but would like to eliminate everything else. Has anyone ever experienced anything like this?

You may try removing one spark plug wire at a time while the engine is running. It should just miss, but if it quits when one is removed that may indicate a problem with the spark plug, wire and/or coil. I use a an insulated fuse puller to remove the wire...i don't like getting "tickled"

When you say you removed kill wire at plug, do you mean at the plug on coil or ignition switch? To isolate that circuit, probably best to remove it at the coil to eliminate possibility of grounding out through the wire.
While you’re in there, best check the air gap too.

Beth I forgot to mention that I pulled each of the plug wires one at a time while running and both make the same difference in rpm loss while pulled. And yes, insulated pliers is a must when doing this test!

I unplugged the three wire lead out at the top of the engine near the left spark plug. True there is still a length of kill wire from the top in the engine down to the coil itself, but all of what I can see looks perfect. To check the last couple inches, I'll need to pull the engine to get the shrouds off.

If I remember right, both plugs fire at the same time. Maybe you can extend one of the wires so you can reroute and switch them to see if it makes a difference.
I went to heck and back trying to get an M18 in my 1811 run correctly. The final answer was to put an adjustable jet carb on it. I checked compression, swapped wires, changed ignition coil, New pistons and valve job, used a temp gun to see if one cylinder was hotter than the other---but an adjustable jet carb made the biggest difference.

This is a LONG thread but a LOT of knowledgeable folks chimed in with suggestions and helped me think this one through. I suggest glancing through as a lot of good information was posted by fellow OCC members.
http://www.onlycubcadets.net/forum/s...ad.php?t=47254

Years ago, there was a little plate that could be installed between the carb and intake. It was supposed to help "swirl" the fuel/air charge. I have installed them but it did not always help. According to the Kohler engine rep at that time (early-mid nineties) the intake was longer on one side and the fuel could condense and dribble into the engine and cause this odd firing situation. Lew may have a copy of this service bulletin.

Yeah that intake does not look symmetrical. I don't remember seeing any "additions" that can be installed in the service manual. I wondered the same thing--please someone post any findings you run across on this. There are 2 groups of carb rebuilders here on OCC. Those of US that can't get one of those non-adjustable carbs "working" regardless of what we do. Then there is the group that says we are idiots and that they can--but only 1/4 of those guys have ever opened up one of those particular carbs.:rolleyes: FWIW, I tried the same "bad carb" on a 782 this summer (it already had one of those to start with). $30 for a used adjustable carb of Ebay solved that issue.

You are right John, adjustable jet carbs do solve a great deal of sins.:beerchug:

Stan stated in the first post that he adjusted the carb rich to lean and can get a change in the engine. A fixed jet carb doesn't have any adjustment, so it is a safe assumption that he has an adjustable jet carb. Doesn't sound like a fuel issue to me.

Swapping plug wires doesn't help diagnose the issue.... he already stated that pulling the wires one at a time yields no change. The problem is across both cylinders.

Yes, this is a wasted spark engine, so both plugs fire at the same time.

I know the "swirl" plate you are talking about Beth. Some engines have them, some don't. I'm sure they are still available, but I don't know the number. I honestly don't know if they help much or not.


Stan,
A failing coil is possible.

Simple test:
Hook up a timing light and point it at the open hood. Watch the flashes and see if you have a misfire. Check both wires. If you do see a "misfire", plan on replacing the coil. If you don't, I'd go checking to see if you have a sheared flywheel key. No, it isn't likely, but who knows if the flywheel has been removed before and was correctly tightened on install. I'd also check the magnet to make sure it is still intact. (Ignition trigger magnet.)

Use the timing light and see what you find.....