Some info on my EX500 and low compression?

[My name is Jonas]

I have a '91 Kaw. EX 500 which I JUST bought and took into the mechanic after it died on me three blocks from where I picked it up. The bike had been sitting for months and months, bla bla bla, tune up --> carb clean. It's still in the shop but after the tune up I was informed of low compression in one of the cylinders. The guy just said that it would always run "a little" crappy but didn't say it wouldn't run. I've been reading everywhere that having low compression in a cylinder is pretty serious and needs new piston rings or an engine rebuild. What the hell? Who is right? Can I have slightly low compression but still have a decent running bike? I remember before it died it had a nice kick to it and when the mechanic rode it around after the tune up it was pretty zippy and actually impressed him. Mechanic also informed me that if I keep riding it and put in thicker oil that it's possible for compression to gradually increase again. Anyone?????

[SuperTech]

Low compression in the cylinder can be caused ** many factors, and you will need a compression AND a vacuum guage to find out what's happened likely.
The most common problems is actually a leaky intake or exhaust valve. The easiest way to look for this is to remove the crank cover on the side and use a wrench to rotate the cylinder under test to top dead center, and blow into the cylinder from an air compressor using a spark plug hole fitting (lock the crank in place with that wrench, the compressed air will try to rotate the crank!!!). If you hear air at the exhaust, it's the exhaust valve...at the carbs...inlet valve...at the crank case breather, it's the rings, pistons, or both.
Since most people don't have compressors, buy a compression/vacuum gauge, they're not expensive.
Since your engine runs, check the vacuum on the carb of the cylinder in question. If it fluctuates between vacuum and compression (if you see any pressure at all really) then your inlet valve is leaking. No pressure should ever be seen from the carb side.
place the guage into the spark plug hole, and rotate the cylinder to top dead centre (you may need to go round again if you hit the exhaust stroke). If the guage goes up and then slowly falls, note how high it went. rotate it a couple more times till it's at top dead on the compression stroke again.
Also note how slowly the needle drops. Now put some oil into the cylinder. Not a lot, but enough to coat the top of the piston area. If you have no idea how much that is, I suggest 1/2 a "shot" at least, but no more than a "shot" (liquor shot glass of oil).
Run the piston around to top dead on compression again.
if there is a DRAMATIC change in either the compression or speed of loss, it's the rings and or pistons. If there is little or no change, it's one of the valves.

If it's a valve, they aren't too hard to fix.
Once the head is off, spray a solvent (I use carb cleaner) into an upturned port....meaning point either the carb intake port or the exhaust port streight up into the air.
if there's a leak, the solvent will find it, and leak out the valve.
If the valve isn't bent, it will at least need to be lapped, or you may need to have the seat re-cut.
Most cases you can just lap the valve, and it'll be fine.
mfgsupply.com sells a wonderful course and fine in one container lapping solution and lapping tools (each around ten bucks). If the valve is damaged (bent), you'll need to replace it. Bikebandit.com will have valves for your bike.
If it's not your vlaves, then I suggest replacing the rings. Make sure the cylinder isn't scored. If that's happened, it's a trip to the shop for sure.