'51 Johnson does not run smoothly

[gmarshall43]

I have a 1951 5hp Johnson. It starts up fine and will run fine for a few minutes then it won't keep the same speed. That is it won't keep the constant RPM's no matter where I put the throttle. It will slow down then back up on it's own, run well for a minute or so and repeat. I have cleaned the carb and the speed adjust needle and adjust that for max performance which is about 1 turn out. I've tried adjusting that knob on top of the gas cap and no change in the issue. I'm wondering if the coils may act up after it warms up or maybe an internal issue? Any suggestions?

Gary

 

[flyingscott]

What about the lower needle. If you look under the shroud in front there should be an L shaped handle. That is another adjustment for the carb turn it lightly to closed then 1.5 turns out. Another thing is does it have the original float and is the tank clean there is a screen in there that can get plugged and restrict fuel flow. Is the fuel valve clean.

 

[F_R]

Are you running that thing in a barrel? If you are, get it out of there and on a boat on the lake, then adjust the carb properly.

 

[HighTrim]

The knob on the gas cap is a vent, it gets left wide open when running.

The low speed needle, the L shaped one is set at about 5/8 from seated for best performance on these, all the ones I have done anyways seem to end up there.

The surging you hear is likely a fuel restricition, as mentioned check the filter in the tank (unscrew petcock) and ensure fuel line is free from debris. Have you cleaned carb out?

 

[gmarshall43]

Thanks for the responses. here is answers to you questions:

I am running it in a barrel but I had it on a boat early this year and it was doing the same thing.
I did clean out the carb and I believe that it still has it's original float. Would that be an issue?
That L shaped needle is the one I had out and cleaned. I have it turned out about 1 to 1 1/2 turns..
The fuel valve is wide open but I have the vent on the gas cap about half open. I will open it up all the way.
I did not know about the screen in the fuel tank. I will look into that.

Thanks

 

[flyingscott]

What about the high speed jet on top of the motor dial with all the numbers.

 

[HighTrim]

1 to 1.5 on the low speed needle is too rich for your model. Try 5/8 out from seated. High speed about 3/4 out from seated.