Hi, Today i ran a compression test on my R1 engined MNR. I used a Laser compression tester but can someone confirm my results. When we cranked the engine the needle ran between around 110psi and 160psi. The gauge constantly moved between these points not sticking to what i presume is max value like id seen in some you tube videos. So have i done this wrong or is my value of 160 correct?
Thanks
Adam

Youre looking for consistency across all cylinders. The one i have has a telltale needle for the max compression. This is the figure youre looking for consistently, so as long as all yours are 160 its fine.

It's your highest reading then isn't it. This will be at TDC.

Schrader valve is not working in the the gauge.

Gauge should rise over several turns of the engine then stop a max pressure, if it drops then the valve is unwell.
I crank 5 times with wide open throttle.

Definitely could be a duff gauge given its from halfords. But given that all cylinders jump backwards and forwards all topping momentarily at 160 suggests my rings, valves, headgasket are ok right?

Just a duff gauge

IIRC mine has a little knob on it that needs to be tightened when taking a reading then released to relieve the pressure before the next test. if yours is the same and it is open then it would explain why the needle doesnt stick....

Try another gauge ! The pressure recorded on the gauge shouldn't fluctuate , it should record the pressure built up in the cylinder not ant drop in pressure

I was always taught to keep the throttle wide open, and found the following from my old course work......

The reason the throttle must be wide open is to ensure that each cylinder fills completely. If the throttle is partially open, a vacuum will be generated in the inlet manifold and the pressures indicated will be lower than expected.

There is another reason why the throttle must be jammed open and that is Valve Overlap. As each piston reaches the end of its exhaust stroke there is a point where both the inlet and exhaust valves are slightly open. This point is called "Valve Overlap". If a compression test is done without the throttle wide open, Valve Overlap will cause some cylinders to read higher than others because they will draw air back through the exhaust valve and exhaust manifold into the inlet manifold which worsen the compression readings.

Not sure this applies to bike engines......hope this helps

Oh and disconnect the injectors so you dont flood cyls

A

[Edited on 30/1/11 by AndyW]

Hi again, just thinking about this and could it be that although throttle was wide open the black plastic sliders(sorry for crap description) in the throttle bodies werent held open?
As my halfords is on the way to my girlfriends i might aswell try another gauge at the same time.

£25 from Halfords or £9.50 inc p&p off ebay

Halfords know how to charge.

it'll be the gauge valve not holding pressure, it should hold the peak pressure on each compression stroke, which should build up with successive strokes. Not having the throttle open will just slow the gauge reaching full pressure.

Cheap £5 ebay one was even worse with a male to female air adapter connection which started hissing out psi instantly it just didnt want to seal.

I tried again today with the same gauge. This time i made sure the plastic sliders on the throttle bodies were kept open along with the throttle. This resulted in the guage working and recording 160psi flat across all cylinders. Now i have this reading do i compare to the haynes manual reading of 200 or ignore and be happy in the fact they are all even.
Cheers
Adam

You can check for worn rings etc by putting a few squirts of oil through each plug hole and repeating the compression check ,If compressions rise by any large amount then rings etc are worn . Known as a wet compression test