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.....

Good to have you back Jon!:beerchug:

The needs of the many, outweigh the needs of the few. Or the one. :beerchug:

Thanks Jon. Welcome home.
I'll check with a timing light tomorrow. I had't thought of a sheared flywheel key, but that's exactly what it sounds like. Timing. Hopefully I'll have something to report tomorrow night. Stay tuned.

Glad yer back Jon, ---you cantankerous young whipper snapper :biggrin2:
Do those mag coils have a built in electronic advance like some do?
I have not tried checking with a timing light, but the trigger "pellet" just don't seem large enough to contain the electronic's needed.:bigthink:

Sorry to hijack your thread Stan, welcome back Jon.

Thanks guys. :beerchug:

No George. The Mag coils do not have an advance in them. :ThumbsUp:

Well, I dug out my old inductive pick-up timing light and didn't really see any distinct pattern in the flash that it was skipping on either cylinder. If I had to describe the sound of the skip in writing, it would be run run run run run PAH run run run run etc. Just every so often it drops a cylinder once and then fires correctly, but as I said previously it sounds like it is laboring. I pulled the motor to inspect the coil and flywheel key. The flywheel key is ok, the flywheel is tight, the magnets are tight and in place. The kill wire to the coil is fine. No noticeable burn marks or cracks in the coil itself. The air gap measures around 10 thousandths. Since I'm this far into it, I'm going to throw a coil on it. If I had one to test with I'd try that first but none in stock here in the shop. Who knows what little gremlins might live in the old coil. Other than throwing a coil on it, I'm tapped out. Are all the flywheels the same for the Magnums? I have no history so it may have been changed at one time. Any guesses?

Mags all use the same flywheel.

If you don't see it missing with the timing light, it probably isn't, but that doesn't mean the spark isn't weak. Or that the plug wires are good. They aren't replaceable, (which I hate, but hey, they didn't ask me how to make it) so it could even have a wire that has high impedance. Oh.... what brand spark plugs did you use?

You have no history of this engine..... Hows the muffler? Could be plugged up.

I suggest running it while it is about of the tractor. Could be something in the rear end causing it to labor. Again, not likely, but possible.

There are still several other things that I can think of..... but I don't want to send you on a goose chase as I'm just guessing. Stan, could you record a video of it running and link it up for me to hear it?

Next time I have it assembled enough to run it, I'll make a recording of it. I may need some help posting the recording though. I hope to have the new coil on Thursday. The new spark plugs are Autolites. What ever is occurring is happening on both sides I think. I have changed plugs around from one tractor to another as well. There doesn't seem to be a weak and a strong cylinder.

I had a single cylinder command a while back that had similar issues with intermittent missing... it turned out to be a weak coil... I hope this works out the same for you... sounds like you have eliminated most of the other possibilities.. keep us posted!!:beerchug:

Thanks Jay. How can I miss? I've got a great team behind me here. When we beat this thing, and we will, I'll be sure to post the solution so everyone can learn.

Well, the coil came a day early so I got that installed. No improvement, still sounds the same. On the lighter side, now I have a spare coil! Jon mentioned muffler back pressure so I pulled it off and inspected it. No loose baffles and I could easily blow air thru it in every direction with a blow gun on the shop airline. Since I had the exhaust out of the way, I thought I'd check valve adjustment. All four where in spec so no joy there either. While running the engine out of the tractor and clamped to the bench I eliminated any driveline problem causing this. One new symptom I did notice the engine won't rev very high. I ran the engine while holding the throttle shaft to control engine speed. I wanted to check governor action so I held the throttle open and felt the gov pull the throttle back -- good. As I held the engine at full throttle OVER COMING THE GOV, the engine failed to rev up much past what I'm guessing would be about 3000 or so. That's a guess. The air cleaner was not on during the run. I am now wondering if someone had this engine apart and did not time the cam to crank correctly. It acts like maybe it's valve timing. Jon - what do you think? Sorry I didn't get a video of it running for you yet.

Have you tried swapping the carb AND intake from the 2072 to see if the engine performance improves?:bigthink:

I swapped carbs but not the intake itself. The carb made no difference either way. The 2072 ran good with the 782 carb and the 782 skips with the know to be good carb from the 2072. I have had the intake off the 782. The PO must have been playing around because the intake to head gaskets were shot and had some silicone smeared on them. When I first spotted the loose intake and bad gaskets I thought I had found the problem. Nope!
Since replacing the manifold gaskets I have spayed ether around the joints to check for a vacuum leak. None found.

I read your earlier post regarding the carb swap. It cannot hurt and it does not cost anything to try the intake swap. You may find the problem or not...but it is easier than splitting the engine and checking the cam timing!

I agree Beth, there is a possibility that there could be something lodged inside the manifold blocking flow. I'll check it out tomorrow.

IIRR,
While watching the valve action in the tappet boxes:
At TDC on the exhaust stroke, the exhaust is just closing and the intake is just opening with appropriate equal overlap of both valves.

You can look in the spark plug hole with a pea light and see the piston top, @ TDC, as well as feel it as you turn the crank slightly in direction of travel as it gets easier a bit as you achieve TDC, as both pistons are @ TDC,and have stopped, before they start to change direction.
I hope I haven't lost you?:bigthink:

This should tell you if the cam is timed correctly.

Nope you haven't lost me. I had one head off and watched piston TDC and valve action during all 4 strokes. As best I can tell it looks ok visually but still, who knows, one tooth off?

I don't know...... I'm thinking.

I agree with George, on how to look the simple way to see if the cam is close. That's a good way..... but if it's just a tooth off, it may be hard to see.

The deal with it not being able to rev up..... I don't know. Stan, as many motors as you have worked on I think you should be able to judge RPM, so I trust your guess. I agree that timing is possible. So is lack of fuel, but since you have switched carbs with no change, then it would have to be a supply issue, which is possible, but unlikely with the other symptoms. Cam timing seems to be the only next logical step with the diagnosis you have done. Just not much more could be wrong.

Vacuum leak is possible, and would cause the same symptoms, but it seems unlikely as there are only 3 gaskets to leak, and they are all new. I have seen warped parts though that wouldn't seal. Your leak check with brake cleaner should have yielded a result if there was an issue though.

I'd still like to hear this thing running.

If I have a chance tomorrow, I plan to pull the intake manifold off and check to see if something is lodged in one of the runners, a washer or something that might disrupt the flow to a cylinder. So much seems equal though. I will say one thing, I got time. I don't care if I have to take this hunk of aluminum down to every last nut and bolt, inspect and reassemble. I will beat this thing, I'm just trying to do it in a logical manner. I'll try and get some video footage for you Jon.

I would like to hear it too. You’ve ruled out everything I would have tried.
Been awhile since we’ve had a good puzzle.

The fact that it sounds like a SBC running retarded timing, gnaws @ me,
and it don't want to rev up.
Did you notice if the valve springs seem to have the same pressure/lift when checking clearance?
Also you mentioned silly cone on the intake, does the block look like it has been split in the near past??
thinking someone mistakenly missed the cam index mark.

Saw a 3/8 nut setting in an intake port on an old stove bolt 6 once,
it had an intermittent miss, drove me crazy till I found it.
The nut was was worn round and the port/valve were as shiny as a chrome bumper Ha,LOL!

Well, it ain't the fuel system. Today I took the entire manifold, carb, air cleaner, fuel lines, fuel pump, the whole shebang off of the perfect running 2072 and installed it on the 782. The skip and the laboring are still there. Took it off the 782 put back on the 2072 and started it 2072 purrs like a kitten. It is either mechanical, like cam timing, or electrical, and I've changed the coil out and put a new one on. I've run it with the kill wire totally unhooked from the coil. No difference.
To answer George. The manifold had silicone on the gaskets as did the valve boxes, but none on the case halve as far as I can see.
In a previous post I threatened to tear it down into little bitty pieces and inspect each one. Here's what it looks like now! If I have time tomorrow I'll split the case.

I wish the KT/Mags were made like the CH motors. They would have been worth rebuilding if they were. I hate split case motors. Honda engines (ATV/motorcycle) with split cases suck too, but at least they have ball bearings. Point of that comment is, on a CH you could just pull the front cover and have a look.

Glad to see you back Jon! :beerchug:

I'm really curious to see if all magnets inside the flywheel of this engine are where they belong and what kind of "Gremlin" Stan may find in this one. Pretty intriguing to say the least.

:popcorn::popcorn::popcorn:

Agreed... the KT/Mags are good engines... just not designed to work on very easy.. definitely not like the old K series singles or the CH engines.

They really aren't hard to work on.. just very time consuming and tedious... and the parts are not cheap for them:bash2::bash2:

Back when KT's were new, the shop dad worked for had a "encounter" with the Kohler rep over warranty repair times. They had to go into the same KT 3 times for being blown--I think rods were thrown from over-revving. It wasn't a piece of equipment they sold but they were a Kohler dealer. The company owner (dad's boss) told Kohler to pay up or take their contract and shove it. Kohler settled with them---they sold way too many parts not to. The last time they put that exploding KT together the boss pretty much told the equipment owner he refused to work on it again as they suspicioned the engine was being ran too fast. It was on a 2 paddle concrete trowel, or whatever you call those upside down helicopter looking things--it was big enough to ride on.

To the OP, this is a great thread, keep posting what you try and what you find.

Today I thought I would find the answer to this puzzle. Nope, this M18 is still fighting back. I carefully split the crankcase keeping the crank and cam gears engaged to see what the timing between cam and crank were. There is only one mark on the cam, a hole adjacent to a tooth, and one mark on the crank, a cast line at the base of the counterweight. THEY ARE LINED UP. Crap! So------ Now begins the "identify each piece", clean it, measure it, check reference manuals etc. The cam lobes have some funny marks where it seems as the lifters have never touched. Next step is to pull the valves out of the jugs and take a good close look. At a glance, the springs look to be the same coil on intake and exhaust. I thought the exhaust used a heavier coil. The marks on the cam make me think the valves are floating because the lifters are not staying in contact with the cam. I did discover a notation scratched into one of the tins -- valve job 8/9/16. Somebody has been in the motor at least that far. Crankcase had a thin seal of something like black RTV, but very little of it.
A question - since the charging stator creates AC voltage, is there any way by a short or whatever that a possible bad stator could effect primary voltage in the mag? AC going to ground thru the coil winding? I'm grabbing at straws here, I know. This tractor never really showed good charge on the ammeter.
I've changed out the ignition coil and Jon mentioned all the Kohler Magnum flywheels are the same. Magnets look good and are tight. Stator magnets are in place and tight as well. Stay tuned.

I just checked Parts Tree and it lists the exhaust and intake valves as taking the same spring part number, so that's not it. I also checked the flywheel - only one part number listed, no serial number break or anything. Is there a possibility of more than one cam or crank for these M18's (besides the obvious of crank PTO length for other applications) that would change a timing mark or something?