After reading through this thread I'll grasp at a straw and suggest a sticky valve (or valves). It's the only thing that hasn't been apart or checked so far. I'm thinking just sticky enough that sometimes it can't close fast enough. I know it's unlikely but it's something to check.:Huh:

I'll second that. At this point anything is possible.

I stripped down the heads today for inspection. Definitely not a sticky valve, in fact the guides are quite worn on the exhaust valves. The valves and seats are ground at 45 degrees and had a pretty nice seat pattern in the middle of the face. This is correct for the cam without the orange markings as per Lew's service bulletin. Margin is good on three valves, they look new. One exhaust valve looks like it got forgotten and wasn't refaced either. Even so, the valve has a good seat mark. I believe I may have mentioned that on the fan shroud there is markings stating valve job and a date two years ago. So it needs guides and one exhaust valve, BUT the valves all show they were seated and the engine had decent compression. I've honed the jugs and am waiting for a set of pistons and rings. Didn't really need pistons, but It's opened up and I'm not going to skimp.
The more I look at this engine and all the pieces, I feel that the stator may be the fault. Time will tell. I'll run this thing out of the chassis with and without the stator when I get it back together. Stay tuned.

:popcorn::popcorn::popcorn:

Where did you get pistons from? The 17 I just pulled open has both pistons right at minimum wear spec for piston thrust face. Considering replacing them, but they look ok aside from that. Been shopping around and trying to figure out if it’s worth it.

Really enjoying this mystery thread.
:popcorn:

Have dealt with this vendor many times. Their prices are fair and the parts quality good.

https://www.ebay.com/itm/Piston-and-...ss!14845!US!-1

Well, things are moving slowly, but I am getting the M18 back together. The only thing I have found so far is that the valve guides were worn, one exhaust quite badly. Perhaps that valve was not seating well at running rpm causing the skip. Not having found anything else at this point and I'm guessing of course. The new valve guides are installed and reamed. I also decided to replace that one exhaust valve because the face margin was getting a little thin. The next item of business will be a trip to the machine shop to have the valve and seats cut and lapped in. The new standard ring sets are here and the end gaps were checked after the cylinders were honed to break the glaze. The bores are really good, no wear to speak of and no scuff marks. I also took a few minutes to clean and inspect the stator. After a good bath, I used my DVOM to check continuity and insulation. I have good continuity on the winding and it is insulated from the iron core. The lead out wires looked good and without damage, so I believe the stator to be ok.
Stay tuned.

Sounds like you are covering all your bases.. really the only way to do it right! Hopefully the valve was wobbling around enough at certain RPM's to cause your skip... I believe that is entirely possible.. I have my fingers crossed for you!:beerchug::biggrin2::biggrin2:

I got the jugs back from the machine shop today. Nice fresh valve job. We put in one new valve, all new guides, and new stem seals. Now I know that the valves are as good as humanly possible.
After getting back home I loaded the pistons and started things back together. Rings gaps are all in spec. I had to grind the stems a little to adjust the lash on the valves, .003-.006 on the intakes and .016-.019 on the exhaust. Using a great deal of patience and only grinding a touch at a time, I was able to get the lash in the middle of the range for all four. If I get a chance, I'll snap a couple of pictures tomorrow before the heads and tin work go back on. One piece at a time, and when done, hopefully - NO SKIP!

We are all pulling for you on this! Hope you got it solved.