Hi WR,<br /> They're a good, strong engine. They do quite well in the fuel department, too. I'd like to see the compression well in excess of 130psi in each hole, with no more than 5% variation between the three. My 1980 75hp with 600 hours on a rebuild still has 145-48 across the board.<br />- Scott
Hi WR,<br /> 130 is kind of the benchmark I use on those 3-cylinders. Above that and the motor is really healthy, below that and the motor might be getting tired. Keep in mind that there are variations between gauges, and the across the board readings being close is more important than peak readings. If your motor is running well and only reading 120 in each hole, then it's probably not a bad thing - I just prefer them to be higher. A simple decarboning treatment might even help bump that back up.<br />- Scott
Hi WR,<br /> Decarboning is using a spray (OMC's engine tuner or SeaFoam to name a couple) that loosens up any carbon that may have been deposited in the combustion chamber or in the rings. It can help free up sticky rings which can help boost compression. It should be done once a season if you're using the motor regularly...<br />- Scott
Wow, I had never heard of such a product! Do I just spray it through the carbs. while it's running or do I have to spray it through the spark plug holes?<br />Thanks for all your advise.
Thanks to I boats forum,and help from Clanton and others, I wasn't comfortable with the way my 70 johnson (89) was running, found low compression on one cyl and did a teardown. This motor is a pleasure to work on. When running right, it gets great gas mileage and has lots of power. I would take off the vro and raise the tattle tale outlet to the top of the block.See previous posts on the 70. Cant wait to get back out fishing
