81 GS650L - Compression Low on Cylinder 3? New Issue.

[DaveP]

Well, my fun with the 81 GS650L appears to be short-lived. Last weekend I completed my coil relay mod and R/R replacement and then went for a fun 40 mile ride. Was ready to just jump on and ride today, but only made it about 8 miles before I lost power and the engine was running rough. Luckily I wasn't far from home and limped it back home.

It won't stay running now without giving it some throttle. Every time I had to pull in the clutch, the engine would die.

Figured I would check the basics to see what was going on. First, I can confirm that it should be getting fuel. I removed the gas line from the tank, but not the vacuum line. Quickly learned that cranking the engine will open the vacuum petcock and fuel will drain from the tank. :grey:

All four spark plugs look good and each has a good spark.

So I did a compression test. 1, 2, and 4 were all 175 psi. Good numbers. Cylinder 3 is only at 105 psi. Not so good. Well below the 142 psi low level and 70 psi separated from the other cylinders.

When I first got the bike I did a compression test. This was with a cold engine and carbs completely removed from the bike. Cylinder 3 was better then with a 150 psi reading and when I sprayed a bit of oil in the cylinder it came up to 165 psi. Something has changed with cylinder 3 which can't be good.

Getting beyond my engine repair skills probably, bu where should I look next to see what my problem is?

The odometer is now exactly 27,000 miles as I got the bike back into the shed.

Thanks,
Dave

 

[JTGS850GL]

Check your valve clearances first before you get too out of wack over it. Are you seeing any significant smoke? Strange that it would happen suddenly like that.

 

[Steve]

I agree with the valve clearance check.

.

 

[Nessism]

This is what happens when valve clearance maintenance is ignored.

 

[DaveP]

Valves were adjusted when I first started working on the bike. Most valves required adjustment, but should all be to the high end of spec now. Just went back to my valve adjustment notes. Cylinder 3 has 0.1mm on both the exhaust and intake valves after adjustment. Both measured 0.05mm at the start. All other valves range from 0.07 to 0.1 mm after adjustment using standard shim thicknesses.

I did have some smoke on the right side while finishing my electrical work last week, but it has not been seen since in the last 50 miles of riding. Still no smoke now.

Dave

 

[tom203]

Recheck valve clearances- something might have shifted/moved after a few hours of running and now you have no clearance on a #3 valve. This thing should run ok on a weak cylinder, not die. A bad petcock on mine knocked out #2 at idle but it didn't die.

 

[DaveP]

Just had enough time to take the valve cover off tonight and poke around a bit. If anything the the #3 intake loosened up a bit, but everything else stayed the same.

The timing marks are still aligned with 20 chain pins between the 2 and 3 marks, but it is much harder to rotate the engine with a 19mm wrench than it was previously. As it hits the hard point, the timing chain develops slack between the two cam shafts until getting past that hard point. No lights in the shed, so I can't see what the common point is as it tightens up (spark plugs are removed...)

Only checked the #3 cylinder for compression tonight. With a cold engine it is 90 psi and does not improve with some oil added to the cylinder. 1+ year ago, this cylinder cold was 150 psi and 165 psi with oil.

 

[Bortasqu]

Is the cam chain in spec, and is the automatic tensioner ok? Otherwise it looks slightly dark from my perspective.