1972 Evinrude Lark 50hp 50273C with electronic shift. Motor is a little difficult to start when cold, and immediately stalls when advancing past about 3/4 throttle. Those are the only two symptoms. It stalls immediately at the exact same engine speed every time. If I quickly pull back, it’ll sputter for just a sec and then run smoothly again.
Carbs were drained and have been using brand new ethanol-free gas. The fuel pump/filter screen are not leaking or clogged. The motor does not lose prime. I have no issues at idle. In neutral, it will rev to full throttle without issue. This is one half of a twin setup, and the other motor has no issues.
Any help on where I should be looking for the problem is much appreciated.
Check the carb main jet, it could be partially plugged. You can pull the float bowl plug at the front bottom of the float bowl, and the jet is within. Don't lose the gasket! If the jet looks all shiny and new, it's likely ok. If you pull the carb and then remove the screws holding the float bowl to the body of the carb, you can get a better look inside. Merely flushing a carb doesn't ensure it's clean and the jets clear.
If you see Green Stuff or any kind of debris in the main jet, you'll have to pull the carb and clean things out. Use caution if removing the main jet, it's easily stripped using the wrong kind of screwdriver. You don't have to pull the jet to clean, just use a wire and carb cleaner to get the debris out of there.
Another thing to check is your "mag" plate, which rotates to advance spark timing as you increase throttle. Check that the plate is moving with throttle and at full throttle it goes fully into the rubber "timing stop". If the plate isn't moving, you won't get full power even if the carb opens up all the way.
Edit: forgot to post the carb diagram. #35 is the main jet, #26 and #26 are the gasket and plug, respectively.
Are you keeping fuel pressure at the higher rpms?
A simple test would be having someone keep pumping the primer bulb lightly while running at rpms where it usually cuts out.
If you know you have good fuel pressure at higher rpms, could you be loosing spark at higher rpms?
A simple test would be using a timing light to check your timing at these rpms you have issues. And or use a proper spark tester to see if you are getting strong enough spark.