Getting my mom's 78 Sea Raider boat back in the water. (35 horse Johnson) I picked up some new plugs and bought a carb rebuild kit. I was wondering if anyone knew where I could find instructions on line for the rebuild? I also want to do a compression check. Does anyone know what the compression should be on these engines? Also, the plugs are L78V, does anyone know if they are 14 or 18mm threads, or something else? The local harbor freight has a basic compression tester for $8 but it only fits those thread sizes. Large kits are in the $25+ range.

Last, What else should I check in the engine to make sure I minimize any problems at start up and when I take out out on the water for the first time?

Thanks

It might be wise to buy a service manual for your engine. All(or most) of your questions will be answered and you will have a resource for any future problems.

If the boat has not been used in a while, replace the fuel hose and bulb with a OMC hose(no aftermarket)and inspect the inside of the gas tank for rust or contamination. Fill with fresh premix gas.

It might be wise to change the lower gear case oil.

The compression #'s should be close for each cylinder. If you have more than a 20-25% difference between any of the cylinders than you have a bad cylinder which would need further diagnostics to find out the problem.

Like Glades said, you should pick up a service manual.  I can't count how many times over the years I've used mine to answer a question about my engine, fix something, etc etc.  There is no better tool than a good book with some good diagrams.