Cylinder compression test

[Mark]

Prepping a 3.5K1 with a view to getting it up and running. Carbs clean, new jets etc, and thought I’d do a compression test as a matter of interest as I have little idea of its mileage although it has done a few miles since I got it.
The first time both came out around 10 bar but the second time around the results were lower and sometimes varied by as much as 20 - 30% between cylinders. Curious, I’ve since tested them a few more times and the results are always different, that is to say sometimes the front cylinder has better compression than the rear, sometimes not. Sometimes a cylinder will show 10 bar and an hour later will record 6 bar. I’ve turned the engine over with both the kickstart and the starter. I think the tester hose seal to the plug socket is ok. - any ideas?

[3potjohn]

How can pressure drop? Past a valve or past the rings, or the gauge is misleading. I should be inclined to run it up and get oil around. After all if you haven’t got a compression tester you would not know. I take it you have 0.1 mm valve clearances. Someone may have better advice. A small drop of oil in the plug hole first won’t go amiss if the bores are dry.
After a bit of a run you can aways retest.

[Mark]

Cheers John.
Sounds like good advice. I’ll double check the valve clearances, bolt it back together and take it for a spin.

[BumbleBee]

As John says, a compression test is usually done by first running the engine for a while. I was always taught to then quickly drain the float bowls and make sure the incoming airway wasn't obstructed. In other words wind the throttle wide open then do the test
Just kicking it over with the meter in place is often sufficient though.