Re: Johnson 7.5 hp compression
Well, the compression value should be within 10% of each other. 14 point spread is a bit much. Couple of things you can do, if you can get the motor running, is to decarb the motor. The piston rings may be coked up with carb and stuck and may benefit from a decarbing to free the rings up. Seafoam is only $6 to $8 a can. Cheap enough to try. (Prior to decarbing, remove both sparks and pour and ounce of Seafoam in each cylinder. Replace the spark plugs and let it sit a day or two with the spark plugs facing the sky. If there is carbon causing the rings to remain stuck, the Seafoam may dissolve the carbon some and free the rings for better compression.) But I would still do the full decarb as per the instruction below.
Link for a decarbing instructions.
http://forums.iboats.com/showthread.php?p=2648076#post2648076
If you can not get it to run, then inspect your cylinder head cover. The head gasket may be damaged. You would have to replace the head gasket and re-torque the head bolts.
Torque range is 60 to 84 inch pounds with a torque wrench. Tighten 50% torque value first pass and then 100% torque value the second past. After you run the engine a while, stop and let it cool a bit and then retorque at 100% torque value.
I will provide a torque pattern diagram that is similar to your motor as an attachment.
You can inspect the cylinders and the rings by removing the bypass covers to check and see if the rings are damaged and see if the piston is damage on the lower cylinder. Replace the cover if the gasket is not damaged. If the gasket on the bypass cover is damage order a new one, clean the cover and the bypass mating surface and retorque the bypass cover back on.
Fuel ratio for your motor is 16:1
Here is a link for a parts diagram for your motor. Well it for a 1958 motor, because marineengine.com does not have diagram for the 1957. You can order parts from them as well.
http://www.marineengine.com/parts/v...ntage-evinrude-johnson/377512/37751200001.htm
