Bike in question is a 97 gsxr 750. Cylinder number 4 has 0 compression, it doesn't even move the needle. The other 3 are within specs. My buddy thinks it's the rings or head gasket. I would think that it would have something if it were either of those. I'm thinking it is a bent valve.

With the weather getting colder I'm thinking about pulling it apart and getting it fixed back up.

Any other ideas of what it could be?

yeah, head gasket or rings would give you at least five psi.  you may have ventilated a piston.  

First post.

Hole in piston (which is not good).

if you can get the engine oriented so that the piston is travelling straight up and down, get the piston near the top of the bore, and see if it will hold oil or wd-40 (try to fill it so that it overflows the plug hole).  if it just drains down, you know what your problem is.

I didn't remember hearing anything out of the ordinary. It was giving me problems so I took it to have the carbs set and forks rebuilt after I picked it up it ran fine for about 3 miles then died at a light. I was able to get it started by pushing it but it wouldn't stay running so my wife came and met me with the trailer. Since then its just been sitting.

If it was a hole would I get gas in my oil?

Do a leak down test.

Remove the oil fill cap.
If you hear air leaking from the oil fill port then you have a holed piston or some really worn piston rings.

If you hear air leaking from the exhaust then you have a bent / burnt exhaust valve.

If you hear air leaking from the airbox then you have a bent / burnt intake valve.

I'm going with a bent valve.

A compression test is a fairly poor indicator of such problems. It's good to use as a comparative tool - which is what you've done - but past that the results are essentially meaningless. Trying to use it to pinpoint the issue is generally a fruitless endeavor.

A leakdown test, which uses compressed air to pressurize the cylinders individually, will not only tell you the health of the cylinder, but usually where the leak is coming from. With the piston @ TDC on the power stroke, if your leakdown number is excessive and air is heard in the intake tract, it's likely an intake valve. If the sound of air is heard in the exhaust - exhaust valves. If air is heard rushing out of the crankcase (oil fill), it's likely the rings. And if it's the head gasket you'll usually see bubbles rising in the radiator.

This isn't a 2-stroke. I'd personally find it very surprising if you holed a piston. Not impossible, but unlikely. Especially when the other 3 cylinders are GTG.

Regardless of what the issue is, if your compression test is being performed correctly with properly-functioning equipment, then you've got a problem that's going to necessitate, at a minimum, pulling the head. No amount of conjecture is going to change that. That is, of course, assuming the performance decrease matches up with your finding of zero compression. If your comp is really that low (with all other cylinders being good), then you should notice a very significant difference in power & response.

Oh I noticed a decrease, it's at 0hp

The leak down test was my next task. I ended up trading for a sportster soon after it happened so the gixxer has been on the back burner. I'm thinking about dropping the motor out over the winter and going through it.

I would still think that would give some compression

a hole in piston or all valves are bent

Yes it would.

If you want to add to your tool chest, then a leakdown tester is a great thing to have.

That being said, you'll likely find the issue upon visual inspection / measurement when you have the engine apart. So it's not absolutely necessity.

Where I really find a leakdown tester useful is in ruling out cylinder/head/valve/head gasket problems. Case in point - assuming your problem existed before the bike was worked on - it would have been quick & easy to test the condition of the cylinders first before just jumping into the carbs hoping they were the issue.

Good luck. If you have any questions PM me & I'll be happy to help if I can.

<- motorcycle mechanic specializing in Japanese sport bikes.

oh shit.  if they set your carbs lean, it probably burned a hole in your piston.  document everything.  how did your spark plug look?  white residue?

How hard is it to get the cam cover off that particular bike ?
That's the first thing I would take off to have a peek at the valve stem lengths.

Plus, OP, did the work done on the bike include a valve adjustment ?
( have seen some instances where a valve keeper was left out causing the valve to drop )

No valve adjustment just a carb sync.

These bikes have an auto cam chain tensioner that is know to give out. If the cam chain jumped could it give me these problems?

I would think the problem would duplicate exactly on one other cylinder.  possibly not though. I dont imagine it would run at all if it skipped.

Its just too coincidental that it happens 3 miles after a carb change. IMO.  Im really curious to see what the plugs look like.

Go rent or buy a bore scope, insert thru spark plug hole and take a look at the cyl surface. I am voting for holed piston or stuck valve. How does the plug look, is it all there, any pieces missing, broken off.
I wonder if something was hosed up at the service, it sounds like it. Call the shop and talk to the mgr and tell him what happened, get his take on it.

Totally agree. The head will be coming off to determine the amount of internal damage.

Leak down test is completly neccesary to.accurately diagnose what's wrong with you bike while its still together.  Also, 0 psi means major problem.

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My first thoughts as well. I'm not a big believer in coincidences. However, the OP said the bike was having problems before which necessitated the trip to the shop. So it's possible the underlying issue was always there, remained unaddressed (they worked on the carbs when it was in fact something else), and then worsened in a big way once back on the road.

IMO the shop either dropped the ball & caused the problem, or they didn't find a pre-existing problem.

Shit like this comes up when im invited over for dinner, someone will start telling a story like this, and at this point right here, I insist on getting up from the table and investigating, because I just absolutely HAVE to know what happened.  Then the wife hates me, and I stop getting invited over for dinner.