compression

[caspari]

does nyone know what the compression is suppose to be on 91 merc 135 outboads. motor wont start until i take one sparkplug out and ground it to the motor. found this out while checking for spark, don't understand, at a loss

 

[KCLOST]

Re: compression

The actual compression in psi is not as important as the difference in compression between each cylinder. No more than 20psi difference (according to a seloc manual) or 10% differnce according to others that are experienced posters on this forum.<br /><br />I would expect, if everything is in order that you should see somewhere around 100-130 psi in each cylinder, but the consistency between all of them is the key.....<br /><br />You're actually helping the motor turn over by removing one spark plug. So that may help it to start if you have spark in only some of the cylinders. Do you have spark in all cylinders? Is this a sudden problem? A little more info would be helpful, if you have any...<br />Good luck.

 

[caspari]

Re: compression

I bought a gauge but dropped it taking it out of the package so it's stuck around 25 psi at rest. the check showed about 120 on the gauge but I'm not sure if I need to deduct the 25 from that reading. My manual doesn't give the specifics, believe that.

 

[caspari]

Re: compression

Oh yea, motor seems to run ok after it cranks. It takes alot of spinning to kick off without removing the plug. I checked the choke and it seemes to function ok.

 

[dogsdad]

Re: compression

Caspari, I have a 1991 135-horse Merc also. My compression checks ran from 105 to 110 on all six cylinders.<br /><br />I doubt you need to add or deduct 25 to those readings. If you can borrow someone else's gauge, check it against theirs. Maybe it's still useable.<br /><br />I thought I was the only person around who could have something like that happen with a new gauge or tool! I know exactly how that feels. <br /><br />We must be kindred spirits.<br /><br />-dd-

 

[dogsdad]

Re: compression

After reading your post more carefully, I am curious about the sequence of events required to make your motor start. Would you mind describing it in a little more detail?<br /><br />-dd-

 

[caspari]

Re: compression

It takes alot of spinning to get #2 motor to kick off. Motor #1 kicks off quicker but still needs to spin a bit itself. On #2 I have to slowly adjust the throttle setting until the motor starts to spin over faster and faster and then it will crank, it seems to like a certain spot on the throttle. We checked the timing and at Idle showed 4 degrees then advances(?) to 4 degrees the other way when you gas on it. Both motors seem to perform identical timing wise but the readings I get aren't what I interpreted from the book.