Stall, then less compression 86 400XC

I had the bike out for an afternoon ride in the snow the other day and just as I was coming down the last straight stretch before it was all down hill to the house the bike stalled on me and would not start again. There is a noticeable loss of compression (as measured by how easy it is to kick it over now). I checked the spark plug to make sure it was tight and I took a look into the exhaust and the cylinder and ring look fine from what I can see.

I am guessing I just need to pull the top-end for a full on inspection but thought I would check here to see if I am missing something obvious.

Thank you!

RUT-RO!! Chris

had that but worse after a very dusty meeting in dorset on my silver streak , finished race .... checking it over later in pits ... no compression at all ???? left it for 10 mins full compression back and good for rest of season . Can only think ring got stuck like in the castrol r days , but i was running on normal 2 t oil 25:1 !

put this with his riding in the snow and a good point to check if the ring was gummed and stayed in the land

Thanks for the advice so far! It looks like I could have some time to work on the bike tomorrow. What sort of readings should I be getting when I run my compression test?

Didn’t get a chance to check compression today but I did check the key. Is this what it should look like?

the key is fine, but that Motoplat looks a bit dicey

As 2premo mentioned ....

I freaking hate Moto plats I'm having nothing but trouble with the one on my 500 XC

Lucky for me the bike came with some spare ignition parts. I’m guessing though that there is a reason the current ignition was on the bike since none of the other “spare parts” have been very functional.

I recall seeing some threads on how to test these systems so will give that a go. Are there still folks repairing these units?

did the outer part of the magneto, the flywheel, come with that?
the two parts should have the same number, the were matched at the factory, then drilled to index timing

Vance Smith - https://motoplatrepair.tripod.com/

It did come with the external flywheel and the backing plate, but I don’t see anything that resembles any matching numbers. The flywheel has a six digit number and the only thing I can identify on the other is a “361”, which doesn’t appear on the flywheel anywhere. The plastic is pretty scuffed over though and it’s hard to make out much. I have the ignition that’s on the bike apart and will check that for numbers.

Thank you Vinskord for the link!

Thank you for the

Did a compression test today and only getting about 60 psi. Nothing looks out of the ordinary from the exhaust port. Will pull the top end and have a look.

Looks like the cylinder and piston got too comfortable with each other and welded the piston ring in place, hence the lack of compression. It was previously bored over twice (83.5). Looking for advice on whether I should go another bore over or try and find a cylinder that has not been bored and start with that. Any thoughts are appreciated.

That's pretty rough, get a machine shop to bore it out some and see where it cleans up.
There is larger pistons, you will end up with, close to a 430.
I have a pile of cylinders if you like less vibration from a 400, small bore again.

is that aluminum on the cylinder wall or gouges?if aluminum see if it can be removed
been down this road years ago and my cylinder was usable