1956 evinrude 7.5...compression?

[heloman]

Does anyone know what is the acceptable psi you should get during a compression check? Thank you

 

[IndySpecialty]

Re: 1956 evinrude 7.5...compression?

heloman:-- well it depends on the elevation your at, but, lets say you are at sea-level about a buck twenty would be in the ball park, Thats 120psi sorry about the slang, give or take alittle depending on your gage. IMO anyhting under 100psi apart she would come if it was up to me.<br /><br /> happy cranking<br /><br />indydan

 

[Chinewalker]

Re: 1956 evinrude 7.5...compression?

Hi Heloman,<br /> Those older small OMCs can get away with slightly lower compression numbers. If you are over 90psi and more or less even between the cylinders, it ought to run fine. Because of the richer oil mix needed for those motors, there's a good chance you've got carbon buildup in the rings and in the exhaust passages, so some Seafoam or OMC engine tuner might be in order to help clean up the innards...<br /> When you do get it running well, make sure you keep the oil mix on the rich side. I run 24:1 in my '55 7.5...<br />- Scott

 

[heloman]

Re: 1956 evinrude 7.5...compression?

I tested it right after I bought the motor, and I got 75psi exactly in both cylinders, now this was after the motor had sat all winter and surely there was not a drop of oil in the cylinder. It actually does run, and really nice at that, however it does seem like it never revs out all the way (or maybe it's supposed to run like that?). Its very quiet and smooth, its on a 14 foot (row) boat now and it seems reliable but I am suspecting it may need a top end rebuild. Thanks for the reply guys and any other info you have about this motor is appreciated.

 

[Mark R Groves]

Re: 1956 evinrude 7.5...compression?

I would suspect that if it is running good at low and mid speeds, compression probably isn't your problem. Have you tried "tweaking" the adjustable high speed jet? Proper adjustment of this is critical. If this doesn't improve matters, a cleaning or rebuild of the carb may be in order.

 

[heloman]

Re: 1956 evinrude 7.5...compression?

I think it's time I get a manual for this, any recommendations which one and/or where to buy?