1990 Johnson 100 HP Won't Start In Water

[Dickiex]

My Johnson outboard was running fine on the driveway with the muffs but when I put my boat in the water I had trouble starting the motor. It ran for a while but at low rpms the motor stalled and I couldn't restart it unless I raised the prop out of the water, start it and then lowered it again into the water. I performed a compression test on it. The port side upper and lower both read 95. The starboard upper reads 95 and the lower cylinder reads 80. I read that adding yamalube with ring free to clean the carb might help with compression. Does anyone have any suggestions on what else I could do to get my outboard to start in the water and do I need to worry about the compression test difference on my one cylinder?
Thanks in advance.

[emdsapmgr]

The 95 compression is close to normal. The factory would like to see all the cylinders have a compression range within 10% of each other. The compression on the 80 cylinder is considered low. Doing a decarb on the powerhead may help and is worth your effort. Yamaha's Ringfree is similar to the Bombardier product called Carbon Guard. For the decarb, I'd probably try a can of Bombarier's "Engine Tuner" first-it provides a single process for cleaning carbon and works well. If your compression does not come up after the decarb, you should probably pull the head and visually check for a leaky head gasket-a relatively easy fix. You should also be able to visually determine if the top (outer) ring is still in one piece. Unlikely that low compression is the actual reason that the engine won't run in the water. It is possible that the powerhead is not running on all 4 cyls. Any engine will seem to run fine on the garden hose, when only firing on 2 or 3 cyls. You need all 4 firing when the exhaust outlet is 20" under water.

[Dickiex]

Thanks for the reply.

I did the de-carb with Seafoam and it brought the compression up to 92. I ran it in a barrel of water and it started fine. I launched it in the water and it would not start. I will do the next step you recommended.