Low compression cylinder 2

Cyl 1 = 145
Cyl 2 = 90
Cyl 3 = 140
Cyl 4 = 150
My first thought was Valve clearances, but the PO claims to have had it done reciently. The bike sat for a few months, but the carbs seem alright. I will probably pull the valve cover to eliminate the valve clearances, but what else could cause poor compression in only 1 cylinder?

 

Yeh, don't take any ones word for the valve specs, check them for sure, low comp' can be warped valve, carbon deposit on the seats, bad rings or bore, test the comp' again with a cap full of oil down the plug hole.

 

i envy you're compression numbers, ha. i'm sitting at 123-129 on all cylinders, hoping the new 6 shims i've been scrounging up will fix that up.

 

I will try a cap full of oil. If this increases my compression what does that tell me? If there are carbon deposits on the seats, will that affect the clearances? and is there a way to clean the seats of carbon without lapping the valves?

 

If it drastically increases, it indicates a ring or cylinder/piston/ring problem of some sort.

If it doesn't change much, it means a valve or valves aren't closed/not sealing properly, for the reasons Wiz indicated.

Or you could have a hole in the piston. But it probably wouldn't even make 90 then.

 

I didn't try using oil yet but my original results were taken before I started the bike after storage. I had the bike running for about 15 mins in the yard yesterday, and sounded great. This morning before I started the bike I decided to retest cyl 2 and the result was 130. It must be a bit of carbon or something, all other cylinders were the same as original results. I will go for a ride before checking clearances to try to clear any remaining deposits so I wont get a poor reading.