#2 Cylinder compression is ZERO!

What could cause zero compression in one of the cylinders? If there was a problem with the valves or gasket, I would assume to get just really low compression - not zero?

Thoughts are appreciated.
Rob

 

You would get zero compression if there was no piston...lol

You probably are getting a small amount of compression but your tester is unable to register it. You have to realize the typical compression tester is not made for sub 15 psig testing.

If you are registering zero compression, it probably means your valve is stuck open.

 

+1 on valve stuck open

 

a valve could be stuck open, bent, broken, cupped or stretched, or you could have a hole in the piston. bring the piston to TDC on "compression" stroke (both valves shut) and take the plug out and try to pressurize the cylinder with a compressor. see where the air is going.....you may need to take the header and carb off, also you might want to pop off the hose from the CCV. if you take the cam cover off you can see and be 100% sure when the valves should be closing, and also you can check the clearance.

 

Double check your compression gauge is sealing. I've got one of the $25 Harbor Freight ones and if I go past just lightly sealed the o-ring rolls out and it leaks enough to register 0.

Otherwise I'm betting on a valve problem.

 

If you have access to shop air remove the spark plug, with piston at TDC (both inlet & exhaust valves closed) use a duster gun to pump some air into the combustion chamber thru the spark plug hole. You may need to assist sealing the hole with a rag. Check the tailpipe, carb and oil filler plug hole in the crankcase. I'm betting one of these will have a lot of air rushing out. You can isolate rings by squirting oil down the spark plug hole and winding the engine over one complete revolution as well, if the rings aren't too bad the oil will assist sealing briefly.

 

Okay. I pressurized the cylinder and I have air venting out into the airbox. I tried this throughout all cylinder positions and air vented out of the airbox. I also heard it vent out the exhaust as well when appropriate.

Is it safe to assume that my #2 cylinder intake valve is stuck open? That would make sense to me.

Looks like I'll be ripping into it again this weekend.

 

your on the right track,#2 intake valve stuck open,bent,ect

 

Um, was #2 at TDC when you pressurized it? I have to ask, but if you did it during the intake stroke, you're going to get air in the air box with good valves. The best way to determine TDC IMO is to remove the spark plug and turn the engine over with a socket on the crankshaft in normal operating direction. While turning over, seal the plug hole with a finger, and during the compression stroke (if the valves are good) your finger will be pushed off the hole letting the air out. I guess this won't work that great with a bent valve, but you need to be at TDC to do the pressure test.

 

sounds like he did it right. take a look at that valve and post up what you found.

 

I did confirm looking through the spark plug hole that the piston was at TDC. I then rotated the engine manually and brough the piston to TDC again and checked again. I repeated at least 6 times just to be sure. Air vents out the airbox regardless of the piston position. I'll tear into it and report back.

Thanks for the advice.

 

You did it right.

Post what you find when you pop her open. I'm curious what is keeping your valve stuck open. I'm also curious how long it takes you to fix it.

 

On these XJ-Engines, it's going to be highly unlikely that you have a Valve "Stuck Open"
Highly unlikely. Not impossible.
The Valve springs are double and very strong.

Pull the cover and measure your valve clearances.
If you find a tight one on the low cylinder ... it's likely you burned a valve.

Do a wet Cylinder Compression test and determine if its Valves or Piston Rings/Piston causing the Problem.

Add a tablespoon of Marvel Mystery Oil ... or, Motor Oil to the Cylinder and recheck the Compression.
If it Pumps-up with the Oil in there, you have a Piston-PistonRing-Cylinder Wall issue ... not Valves.

If it doesn't Pump-up with the Oil in there ... its probably a Valve.
What ever it is; I hope its not anything more than can be fixed by pulling the head and having some valve work done.

The season is upon us.
Not a good time to have the bike go down!

 

#2 intake valve clearance is .004" between the .004" - .006"spec.

Wet compression test still yields zero compression.

I'm pulling the head on the weekend to see what's up.

 

Its actually the Exhaust Valve that is more likely to be burned.

Sorry you have to pull the Head so close to the start of riding season.
I know there's nothing else you can do when the compression is lost.

I'll hope its a Valve or a Seat so you don't have to get into an extensive amount of work yanking the block and recovering from damage to Rings or a Piston Crown.

I think if you burned through the Piston you's have known it. It ain't much longer after welding a hole through the top of a Piston that some really bad things happen and the Oil Level drops-off pretty fast; too.

 

Funny. The valve clearance is in spec but still leaking... That has to suck. Good luck to you man and like Rick said, keep us posted.

 

So #2 intake valve was in fact not seating. I took the valve out and cleaned out the carb on deposits. After using some soapy water and compressed air, the valve was still not sealing.

Looks like I will need to invest in some valve lappers and lapping compound. In the meantime, I'm running another head that I have. Bike is running.

 

Quick work, Fraps.

Sounds to me like you should take your time and invest it into reconditioning the Head you took-off.

If that Head needs a major de-carbon, then remove the valves and get into the Ports as far as you can with a suitable rotating wire brush.
Do the valve stems and faces; too.

Bring the Combustion Chambers down to bare aluminum and sand-out imperfections to the casting where carbon collects whilst the Engine is running.

Jockey the Pistons up and sand away the Carbon deposits on them as well.
Finish-up the job by polishing the Piston Crowns and Combustion Chambers.

Then, after you have installed some new Valve Seals and had the Valve Faces and Seats re-cut ... you'll be the proud owner of a better than new Cylinder Head ready for some performance tuning.