Cylinder compression test

The 3 1/2 forum
Post Reply
Mark
Posts: 152
Joined: 28 Jul 2012 15:57
Location: Cannes, France.
Location: Cannes, France.

Cylinder compression test

Post by 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
Posts: 1398
Joined: 02 Jun 2007 13:58
Location: Devon

Re: Cylinder compression test

Post by 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
Posts: 152
Joined: 28 Jul 2012 15:57
Location: Cannes, France.
Location: Cannes, France.

Re: Cylinder compression test

Post by 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
Posts: 203
Joined: 13 Jun 2017 21:10
Location: Reading

Re: Cylinder compression test

Post by 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 :D
Just kicking it over with the meter in place is often sufficient though.
Post Reply