Pressure Build up in engine! Help

[natemoen]

Have you ever checked your valve shims? Could have one on #4 being held open.

[petejw]

if ur getting a lot of pressure build up and little compression then uve probably have a hole or a crack in the piston, might have 2 pull the head and hav a look.

[GLoweVA]

Update

So, I finally got the compression tester on it.
1- 150#
2- 145#
3- 130#
..... drum roll please....
4- 20#

Before anyone asks, i pulled the carbs off, so no restriction to flow there.
I had checked the valve clarences a little while ago and while they were tight, they were just a little under the minimum spec.

So, I had some 'mystery oil' and put about 2-3 tbsp in #4 and tried to get the compression up, NO JOY
So, I took the exhaust off to look in the hole to see if the valve might be leaking by, didn't see anything there.
Ran the starter again and looked/felt the intake and exhuast sides to see if it was leaking there, NOPE. Didn't see anything around the head gasket either.
So, with #4 down, I put about 1/2 qt of oil into the spark plug hole and let it sit for a while. Used a homemade dipstick to see how much as in there, checked it a few minuites later, and the level dropped a LOT. Once my dipstick was dry, I noticed the 'oil level sightglass' oil level was up. So, it looks like the oil on top of #4 drained right into the sump!
Tried cycling the engine a few more times.... pressure still at 20#
NO MAS!!! HELP.
I guess I hate to ask, but what are my options here?
Cylinder/piston rebuild?
Maybe just one set of rings?
Anyone in the Virginia area got a spare engine?
Part this sucker out and try a again with another XS?

[cywelchjr]

Either really really bad rings, or a holed piston. Your cheapest and easiest bet is probably to look for a good replacement engine. If the rest of the bike is in good shape, then finding a running bike without a title for the engine may be the best bet, or just an engine. You should be able to get a good engine for around $100 right about. At least that's what I paid for mine, and prices on a lot of parts are higher here on the left coast, and it was far enough away that it cost me shipping which added about $50 on top of it, plus stupid sales tax cause it was in state. But I've put almost 20,000 miles on that engine and it's still going just fine, just a little rattle from 3k to about 4.5k under light to medium accel that matches the description in the TSB for the vacuum advance issue, and it's not changed a bit that I can tell in 20,000 miles and it DEFINITELY timing related, if I retard the timing it quiets down, if I advance it it's worse, but the TSB part for the problem is NLA so I'll live with it (if I didn't have a fairing I probably wouldn't even notice the sound).