Confusion.

I drained the case and added a measured 800ml. This took it to near the top of the hatchmarks on the unscrewed dipstick. Nothing foreign in the strainer or on drain plug.

Installed the catchcan on a dipstick adapter for testing; if it worked I'd move it to the valve cover breather.

At idle (and with the valve cover still vented with a 2' tube) there is so much pressure that oil was pushed up about 1.5' up the new dipstick-attached tube, just short of the catch can itself. I revved it and oil blew out of the top of the catch can like a water hose. I would say at least 400cc was ejected in a couple of seconds.

Could blow-by really be causing this much pressure? Maybe there is a blocked oil passage and the oil pump is forcing oil out some other path of less resistance?

If blow-could cause monster pressure like this I guess I need a compression tester to see how the piston is doing.

You could also try a fitting that screws into your spark plug hole, and accepts an air chuck. When you pressurize the cylinder, any leakage will be evident. You need to turn the crank to TDC to perform the test. Air rushing out the muffler is a leaking exhaust valve, air blowing back through the carb is a leaking intake valve, air coming out of the dipstick hole is rings. Expect some air to come out of the dipstick hole, because of the ring gaps.


You can make your own adapter by breaking the ceramic off of an old plug, and then welding or epoxying an air chuck fitting to the threaded body of the plug.

Some bikes do generate quite a bit of crankcase pressure. I recall someone forgetting to put their oil cap back on a KLR only to get a leg full of oil... Unless that's because the clutch is right there on a KLR?

I ordered a compression tester and will let everyone know what I find out.

Ok, I got a compression tester with 10mm adapter and an attachment that will allow me to blow air into the sparkplug hole. So some new pieces of information.

1. Cold engine has 125psi of compression.

2. adding .5ml of oil manually bumped it to 128psi or so. Certainly not a big jump.

3. blowing into the spark plug hole with 60psi caused the engine to turn over

4. when it came to rest from #3 the air came out the exhaust

5. when I put my hand over the exhaust while injecting air I got some oil to rise up the catch can line. Releasing my hand dropped the oil again.

Am I any closer to a diagnosis?

Try turning the motor to TDC and holding it there with a socket and breaker bar; repeat the compressed air test, and see where the air goes. My guess is out of the dipstick hole.

Correct, out the dipstick hole which is currently holding the catch can tube.

Does this narrow it down to a compression issue?

I ran a little camera-on-a-wire thing down into the cylinder. It looked like there was still crosshatching on the walls and no other scratches. Piston was carboned up, probably from the oil dumped into the airbox before I figured out there was overpressure.

Save me from going back through the thread; could there be a blockage in the vent tube?

No, because it was originally blowing oil out that vent tube. That's why I got the catch can and mounted it on the dipstick side.

I haven't pulled off the valve cover since this started happening. Maybe I should do that and look for broken parts and repeat the pressurization test.

I expect pressure to leak past the rings and blow into the crankcase, but a minor amount. Maybe a ring was broken on assembly, which explains the decent crosshatch.