Sudden loss of compression?


Well-Known Member
Local time
8:28 PM
Jul 31, 2023
Port Townsend, WA, USA
So my California Motorbikes "PK80 Wildcat", finally broke down to the point it won't run. It definitely lost most of it's compression. I took the head off to see if there's any piston or ring damage an there isn't, as in none at all that I can see, other than running slightly rich. There's no bad score marks on the bore and the rings seem to have held up too?

Where would it have lost compression? I have weeping around the base gasket area, but I attribute that to crappy exhaust gasket, and it ran fine like that until today?

Is it down on the crank seals perhaps? If so it's something I will NOT be fixing until I have a replacement engine, as I really rely on this silly thing and I'm not confident in my ability to quickly and easily get that far into it and get it fixed quickly.

I'm VERY low budget, so, I wonder does Bicycle, actually sell anything different? Do they offer a payment plan like CMB? Do they sell "engine only"? Cuz I don't need a whole kit to get back running, and it's a waste to buy one when I can barely afford an engine only at CMB.

I guess I was thinking I wanted more power anyway, but it sure sucks having my only mode of transpo, besides the local free busses (free is nice, but service ends at 6:30PM, and doesn't start till 7am again.)
Did you try retorqueing the head before you pulled it apart? to my knowledge crank seals shouldn't mess with compression.
Did you try retorqueing the head before you pulled it apart?
No, but it had been done several times to where it was staying tight. I may be able to just get new gaskets and do that, but just to clarify, it rolls over super easy now, and that was all of a sudden. You can almost pedal with the clutch engaged and barely notice the compression fighting you.

The radical and quick loss of so much compression had me thinking there had to be piston damage or something, but there doesn't seem to be.
I saw no sign of a head gasket leak either. So can a base gasket or crank seal fail so badly it stops running? I wonder if just a gasket kit will fix it?

Do you need new studs every time you remove the head?
If your base gasket or seals blew out, you would lose most of the pressure that drives the intake charge into the combustion chamber. No air/fuel in combustion chamber to really compress. It will feel like you have no rings because the volume of intake charge is too small