1994 120HP Johnson runs bad when warmed up

[sjoefl01]

I am just getting started with this problem. I don't trust this motor.
It cranks right up and sounds beautiful I can run for a mile or two then it loses power and starts to shake likes it running on a couple of cylinders. It will do troll speeds all day. I haven't had the boat very long. It's never done really well. I am just testing the water here in case this is a known problem with a module or something. I also think that maybe the waterstream is not what it shoud be but I have nothing to compare it with.

 

[rothfm]

Re: 1994 120HP Johnson runs bad when warmed up

I am just getting started with this problem. I don't trust this motor.
It cranks right up and sounds beautiful I can run for a mile or two then it loses power and starts to shake likes it running on a couple of cylinders. It will do troll speeds all day. I haven't had the boat very long. It's never done really well. I am just testing the water here in case this is a known problem with a module or something. I also think that maybe the waterstream is not what it shoud be but I have nothing to compare it with.

You say its new to you...Have you done any maintenance on it? If not you should "baseline" it to rule out some issues it may have and be comfortable with it. There could be allot of possibilities.

First, is compression. Check it. Next ensure your fuel/water separator and inline filters are fresh. Then I would use a thermal temp-gun to read head temps at slow and fast speeds to ensure you dont have a cooling issue, and verify the overtemp switches on the heads.

While its not running well, you could pump the fuel bulb to see if that has any affect, as in a fuel delivery issue.

You can use the forum to see about checking spark under different conditions. Ignition items tend to break down when warmed up.

you can carefully remove spark leads to see which has no affect on the motor when the issue arrises. This should narrow down what cylinder is not firing, or use an inductive timing light to monitor the spark.

Attack the issue methodically and carefully, and you should be able to narrow it down to an area.

Post some info/findings and the good folks here will jump in.