1975 Johnson 25 Hp

[khanderton]

I currently have a 1975 Johnson 25 hp (25R75B) which I had purchased about a year ago. The PO had installed new water pump/impeller and rebuilt the carburetor before I purchased. Since that time, I have only had to replace the fuel pump. Until recently, the motor had been running quite well. Unfortunately now, when at WOT, the motor is bogging down and will not reach top rpm. I had the carburetor rebuilt again, and added new spark plugs. It improved some, but I'm still not reaching full rpm. The motor idles very smooth. What would you suggest to troubleshoot next with these symptoms?

Thanks in advance,

Keith


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[Johnny]

Keith - Welcome Aboard !!

there could be a number of issues concerning the top speed.
personally, I would check the fuel delivery system from the
tank, through the hose, through the squeeze bulb all the way
to the fuel pump. then, the hose from the fuel pump to the carb.
ensure all hose clamps are tight and not leaking air.
aged hoses and squeeze bulbs are often the culprits for performance issues.
oh, and do you have the vent screw on the gas can cap ?
good luck !

other members more experienced than myself can walk you through
the float adjustment and jets

 

[ADIBOO]

Look at the fuel system like posted above, are you running a newer plastic tank?

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[khanderton]

Look at the fuel system like posted above, are you running a newer plastic tank?

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Yes, the tank is a new. I purchased about 1 year ago. All fuel lines to motor are less than 1 year old. All fuel lines inside motor were all replaced 1 year ago as well. I use a quick connect from tank to line and another quick connect from line to motor. I always use ethanol free gasoline.


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[Pappy]

Does the engine seem to reach top or near top RPM then slow a bit? If this is the case is it repeatable?
If this is the case look up your carburetor and purchase a new high speed jet for it.
We have solved quite a few of these issues by installing new HS jets. Over time the brass jets will erode internally and become enlarged.
I frequently find this issue in the shop. Over the last several years I have probably seen upward of a dozen or so.

Also, on that engine, the "plastic" ring on the outer diameter of your mag plate could be worn and creating timing/clearance issues with your points. A new delrin ring solves that quickly and you should notice a much smoother running engine.