Will a 4.5hp mercury run with 65lbs. of compression?

jbjennings · #1 July 3rd, 2008, 07:05 PM

A friend got himself a junkpile 4.5hp mercury and brought it to me to see if I could fix it for him. It had a bad coil but I replaced it and it has good spark, I've cleaned the carb and tried squirting fuel into the carb. throat and cylinder. It won't even try to fire. I finally tested compression and it had about 65lbs. after several pulls with a rope wrapped around the flywheel. I've had johnsons that would crank with that low compression but I'm thinking the mercury won't. It should have about 125lbs., right???
Any ideas? The reeds looked good, too.
thanks,
JBjennings

JB · #2 July 3rd, 2008, 07:57 PM

65psi is okay for a rope start engine, JBJ.

Since you have provided fuel, spark and compression I suspect either timing or scavenging trouble.

Check for a sheared flywheel key or a leaky gasket preventing scavenging vacuum or pressure in the crankcase.

Let us know what you find.

jbjennings · #3 July 4th, 2008, 03:38 AM

Thanks JB,
I'll check that flywheel key. I'm not sure how to check for a darned leaky gasket without the thing running, though. My expertise level is not there yet. I was thinking it ought to at least huff and puff a little. That timing sounds reasonable......Will let you know if I find anything out. I sure thought that 65lbs was too low----My 9.8 merc has about 125 on each cylinder.
BTW, nice beard

, wish I could've read your article but this dial-up isn't worth a poot.
Later,
JBJ