'76 Johnson 9.9 low rpm only

[Pat A]

I recently picked up a non-running 9.9 short shaft and got it running. The compression is low but equal, around 55psi in each cylinder. I've got it running, but not idling smooth, but with a little throttle, it will run no problem.

I put it on the water today and realized that I can get the RPMs up very high at all. I have it on a 12' very light boat and it would not even get close to plane.

I assume it's the plugs, so I picked up some new ones, but the old ones don't look bad. They are NGK B7HS. It will be a few days before I can get back to it, and wanted to know any tips that may help. I've had boats with low compression that ran ok in the past. What is your opinion on the 55psi, is that a major problem?

 

[racerone]

That compression is way too low.------Motor might be running on only 1 cylinder.----Trouble shooting needs to be done.

 

[Pat A]

I had time to troubleshoot it tonight and it was only running on one cylinder. The top cylinder had good spark and the lower one had very weak but very intermittent. Turns out it just needed the points cleaned. I cleaned them and it fired up and ran great.

The way to test to see if both cylinders are firing is to pull the plug wire off one at a time. If it's the non-working cylinder, then there will be no change it the way it runs. If it's the only working one, then it will die immediately.

Hopefully tomorrow, I'll head to the lake to see how it runs at high speed.

 

[boobie]

You just figured out what a cylinder drop test is. Give us a report back after you test it. You could also try some OMC Engine Tuner for that low compression.

 

[oldboat1]

Have you changed out the impeller on your new-to-you motor? Too hot to touch (top of the cylinders) is too hot to run.