Pressure testing two stroke crank case

[DangarStu]

Hi there. I have a Yamaha 15HP two-stroke circa 2000 that isn't running to well (idles fine but will never achieve full power, long story), and another Mercury 90HP two-stroke that is hard to start. One thing I've heard with two strokes is that bad reed valves or main bearing oil seals can prevent good compression being achieved in the crank case and cause these type of problems. I guess the question I have is a bit generic, but how do you best go about testing compression (and vacuum) in an outboard crankcase? When I watch videos on motorbikes they seem to have a set of plugs that can be inserted into the perfectly round and easily accessibly intake and exhaust manifolds, but this isn't really the case with outboards, particularly the exhaust. Anyone got any experience with conducting this type of test?

Thanks,
Stuart

 

[racerone]

What model # on your big Mercury, inline 6 or 3 cylinder ?-----If a motor idles well then likely the reeds are OK.

 

[DangarStu]

The Merc is a 3 cylinder, and although it may not be the problem, I'm interested to hear how people conduct this test on an outboard.

 

[99yam40]

I never heard of anyone doing it on an outboard motor

 

[gm280]

Personally I've never hear or seen anybody talk about or doing that on any marine engine that I've ever seen... If folks are doing that when rebuilding, then I didn't do my rebuild correctly. And my Johnson specific shop manual forgot to post it in their engine manual as well.

 

[DangarStu]

I'm not thinking of it so much as a final test after rebuild as a diagnostic tool. It would be nice to have some confirmation it is the cause of a problem before going to be trouble of replacing the main bearing seals. If there isn't an established technique for outboards I'll figure something out that replicates the techniques used in other types motors.

 

[99yam40]

remember you have the top and bottom seals along with seals between cylinders on crank.
rings,and reed valves
Probably some recirculation check valves, and possibly fuel pump pulse port
The crankcase seams
they all can cause leaks in crankcase vacuum

good luck in you venture