motor hi - lows in upper throttle range

[kurtatiboats]

Reworking a boat with a mid 70's 40 hp evinrude norseman outboard motor
Got it starting at the touch of the key, it idles well, while not under load it throttles to high power well under load it idles well and performs fine up to roughly 40 % throttle After that it occasionally surges, also frequent hi - lows (fast and slow) but for the most part does not throttle up when propeler is engaged

 

[HighTrim]

Re: motor hi - lows in upper throttle range

Welcome to iboats.

I would always start with a spark and compression test, on any outboard I am working on to know that my base is good. However, sounds like you have a fuel delivery issue. Are the fuel line original or aged? They may need to be replaced and tightened with updated hose. The carb likely needs a bath/rebuild as well. Will spraying premixed fuel into the carb throat pick her back up again? Choking it? What about pumping the fuel primer bulb? The fuel pump may need a rebuild as well.

BTW, what is the condition of the coils? If they are original, they will need to be replaced, as they will give you a headache troubleshooting spark issues.