strange compression test

[zed]

Last night I did a compression test on my L18, which has now done 10 000km/6000 miles since the rebuild. Test done with cold engine - 200 200 195 203 psi.

The strange thing happened with #3 cylinder - as I cranked the motor, the compression gauge needle would rise steadily until about 145psi. Then it would stop rising (as the motor continued to crank), then suddenly shoot up to 195psi. Is this a sticking valve? What causes a sticking valve - does the valve stem bind against the valve guide, or something stops the valve from seating? Anything I can do to check further? If it is a sticking valve, I suppose I must take the head off?

[datzenmike]

If the stem was bent it might hang up but I would expect it to be more consistent. If the guide is worn the valve may close up on the edge of the valve face and then drop down properly onto the seat. The exhaust valve could be 'burnt' and leak sporadically.

 

Check the lash carefully on #3, maybe too tight. Is the spring broken?

[Icehouse]

How many times did you compression test that cylinder? One data point is kinda low, I would do it a few times could just be the gauge. Rule out the tools before jumping to conclusions, just my 2 cents :)

[datzenmike]

Do a couple of times and see if it's repeatible.

[zed]

I originally repeated the test on each cylinder twice - both times it 'stuck' on #3. None of the other cylinders. This morning I tested #3 again - and, wouldn't you know it - the 'sticking' is gone... I'll re-check the valve clearances tonight, and see if I can see anything wrong with the springs. thanks for the guidance everyone

[Icehouse]

Cool! Maybe it was just a fluke!