Very low compression in rear pot

[MikeScoot]

   Fired up the XZ for the first time today. Got pretty good life out of the front pot, but the rear pot isn't firing.  I tested the compression a while back and it was very low, so I put some ThreeBond stuff that disolves just about anything in there a few times and it seemed to get the pressure up considerably - but still way short of specs.
  I had expected that after the front pot fired-up (which it did) it might help to free-up the rings in the rear pot, or bed the valves in again. But it wasn't to be.
  I'll try again tomorrow, but if it doesn't come good I plan to wire-up the motor in the spare bike and check it's compression.
The motor with the low comp. is a 16R (Australasian model) with 70,00K's on the clock whereas the other bike is an 11H (Canada and USA model?) with 17,000K's.

  I might get lucky! If not, I suppose I'll be looking at a tear-down.

  Off to read the workshop manual, about top-end work :-)

[WGuaire]

Have you tried pouring oil into the cylinder? If compression goes up, it's the rings.

[MikeScoot]

Yeah, I did that and the compression does come up from 10PSI but only to 50PSI. What do you think?

[MikeScoot]

I've tried a ring-freeing agent a few times, but it's not the Yamaha one. Guess I'll try some more before I get it off the stand and get the spare bike up on the stand to check its compression.

[cvincer]

A  'leak down'  tester will indicate if its the inlet valves, exhaust vales, or rings/bore (assuming the cylinder head gasket is good).

You will need a compressor (air).

[MikeScoot]

I guess the engine would have to come out to do the exhaust valve test, eh.

[cvincer]

No....compressor connects to the leak down tester....leak down tester connects by hose (screw in) to the spark plug hole.  You are basically filling up the cylinder with high pressure air & observing (hearing) where the high pressure air leaks from.  The leak down tester has a gauge which  (red/yellow/green sections) indicates how bad the leak is & whether or not a repair is needed & where (inlet valves / exhaust valves / rings/bore)  With only 50psi, a repair is indicated.

[MikeScoot]

Thanks for the info. I was thinking a leakdown test was where you pumped air into the intake or exhaust port (via a blanking plate) to check each valve individually.
I've whacked some more Threebond decarbon agent in twice today - once and then turned the motor to get it onto the valve seats, and then filled the pot up and put the plug back in to let it sit in there overnight.
If there's no difference I'll order some Yamaha Ring Free, get the bike off the stand, get the other bike up and check its compression while I wait for the Ring Free to come in.
The other bike only has 17,000 on the clock and was last registered in 2003. I might get lucky! :-) Then again, I still haven't quite given up on the one I'm working on now.

But it really is looking likely that this motor will need a top-end job.