1999 Mercury 90 elpto compression

[badbird]

ok guys I have a 1999 mercury 90 2 stroke serial 0G910530, restoring boat and figured I would address engine while at it.. I need to know what is the correct way to check compression on OB and what should it be, cold cranking I am getting about 65 across all 3 cylinders..I have no way to warm the engine it is in barn with no running water

Thanks
Ron

 

[mikeneal]

Re: 1999 Mercury 90 elpto compression

65psi is pretty low. I also have a 90 ELPO and it is around 110 psi cold. Compression is checked like anyhting else, kill ignition (lanyard kill switch works good) and open throttle some and turn over several times until pressure stabizes. People talk about de-carbonizign 2 strokes and helping compression, could try that I suppose.

 

[fishnsled]

Re: 1999 Mercury 90 elpto compression

Just did my 115 elpto tonight. I was told to crank it to get 4 compression strokes out of the motor. I cranked for about 5 seconds. The kill switch is a good way to prevent the motor from firing up, I guess you could pull the wires off of the plugs if you do not have one. I got 120 out of each cylinder. I did have the garden hose and muffs hooked onto the motor as I didn't want to risk damaging the new impeller I just had installed. I would think you would have comparable psi but I'm not sure. Maybe you didn't crank it long enough. I read on one site for car that you should crank it for about 10 seconds, found that after I had done mine.

 

[lncoop]

Re: 1999 Mercury 90 elpto compression

Pulling the plug wires is sufficient to prevent accidental starting. 120 is very good. 100 to 120 is fine as long as there's no more than ten percent variance between cylinders. 65 across the board almost sounds too bad to be true. It's possible of course, but let's hope you just got an anomalous reading.