Super low compression on one cylinder


XS400 Addict
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Edmonton Alberta
So I picked up another project bike, it's an XS, but an 1100, not a 400, 1979 XS1100 Special. Completely original, bought from the original owner with 23,000 kilometers on it, that's just over 14,000 miles for those south of the border, and paid $400 for it, so I'm quite happy with my purchase so far, even from a parting out standpoint I could end up ahead. It sat outside for a lot of years, it was "covered" by a plywood sheet, but still exposed to the elements.
Anyways, I pulled the carbs for cleaning and I hooked up a battery after pulling the plugs and squirting a shot of WD-40 in each hole, and went to do a compression test. 150-150- 30 -150. My first thought is, gotta be a stuck valve, or maybe a rusty valve seat, or a chunk of something on the valve seat or face, but could it be piston rings stuck in the grooves? Would it be that low? It's a DOHC engine, and it uses the shim style valve adjustment, and when I popped the cam covers off and checked valve lash they all seem within spec. I figured that if a valve was stuck or had a piece of garbage in it the lash would be excessive on that cylinder on one of the valves.
Next I got the bike as level as possible on the center stand, so that the cylinders were as straight up and down as possible, then I put a couple of squirts of 10W-30 into the spark plug hole and let it sit for 20 minutes, I spun it over and checked compression. 65 PSI. Couple more squirts, wait another half hour or so, 125 PSI. I waited another hour while I fiddled with a few other things, reinstalled the Cam covers, pulled the cover off of the brake reservoir, broke one of those rotted screws, but i got the cap off anyway.
Finally, after a couple of cranks, 150 PSI!!! I'm pretty excited.