1978 Evinrude 2hp Mate Runs Rough at High Speeds (4-strokes, no power)

[oldboat1]

...a puzzle.

look for five lead shots (according to the parts diagram for the carb) -- more, and a vent might be blocked (less, and the carb would be sucking too much air, or maybe leaking fuel).

The nozzle gasket is a thick one (usually cork) that fits over the bottom of the jet where it seats in the bowl. (In your opening post, you had referred to the top of the nozzle/h.s. jet -- don't know if that is significant.) The gasket goes on the bowl end (where your thumb is in the pic). One of the little openings in the nozzle appears to be peeking out just above your thumb. That's where blockages can occur -- not sure how many there are in that nozzle.

Probably all old news.

Let's see -- should be a plastic washer between the jam nut and the needle washers.

 

[RustyGull]

This is an old thread but the symptoms and attempted fixes are basically identical to my situation. My 82 Johnson 2hp runs like crap when the throttle is opened beyond than the start position. That boss gasket sure seems like the culprit but the problem persists. If this was ever resolved I sure would like to know how.

 

[RustyGull]

It turned out to be the boss gasket in the bowl of the carb. It wasn't sealing. Although it looked just fine it was too thin.

The fix was this: I stacked an o-ring on top of the boss gasket. Assembled, it formed a sandwich of: upper carb body, o-ring, boss gasket, lower carb body).

The o-ring was an '010'. If you go to Napa etc and ask for an 010 size o-ring they should know what you're looking for.
If you want to use just an o-ring without the boss gasket a 108 size o-ring should work, it roughly the same diameter but fatter than the 010.

And while you're buying o-rings....the o-ring for the lower unit (the one in the gear case head) is number 226.