1990 40 hp johnson surging

[nathansr06]

I have a 1990 Johnson 40 hp pushing an 18 foot pontoon. I was first having an issue with it only getting up to about 3500 rpm and then dropping back down to 2400 pms or so. it would run fine until I gave it more than half throttle. I checked compression and it was 131 in the top cylinder and 135 in the bottom. I have rebuilt both carbs, replaced every fuel line, new fuel tank, new plugs, I am getting good spark on both plugs. I have tried pumping the fuel ball when it acts up with no change. I have tried choking it when it acts up with no change. I have pulled the plug wires one at a time and when I pull either one it dies. when I took it out yesterday I would give it full throttle and it would jump to 4500 rpms and then surge down to around 3000 and go back and forth a few times before stalling out. it always starts right back up and idles great and runs good until I would get past 1500 rpms or so. im at a loss right no for what the problem is. any help is appreciated. thanks!

 

[boobie]

Try squirting some fuel in the carb throats when this happens and see what it does. You may have missed something in your carb repair.

 

[nathansr06]

Ok. I didn't think of that! I will give that a try tomorrow. That would really narrow down my culprit!

 

[boobie]

Let us know what happens.

 

[racerone]

When you try choking / priming the motor when it is running you should hear a change in operation of the motor.-----This indicates that the fuel pump may not be working correctly.---Easy to inspect.

 

[boobie]

Or you got some crap in one of the carbs.

 

[Tulsa_Tank]

I had a surging problem on a 70hp and i found material from the old fuel lines clogged up the connection at the motor didn't see it after i replaced the lines. I could pump the bulb when it was acting up and nothing would happen because of that.