I recently purchased a 2002 mercury 50hp 2 stroke. Motor runs great at just more then idle but starts hard and runs rough at idle. I've had the carbs synced and adjusted. It's better but still not right.
2002 mercury 50hp very rough idle when trimmed up
A carb re-build and a link and sync will likely solve the issue.
You may even have a few plugged orifices.
Carbureted motors can be a real pain in the rear!
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Sounds kinda like the issue I had with the 2000 135hp Merc I had. I found the only real way to get it to idle smooth was to have the motor level. When I mentioned it when I had it in for a tune up, the mechanic said something about the carb floats being set for when its level and when it's trimmed out too far it would cause it to run lean(?) and idle bad. If I sped it up a little it would smooth out.
My 50 does the same with no fuel injected motor. Can only start it trimmed almost all the way down. Let it warm up for a minute before trying to idle away. Once motor is warm no choke needed to start again and runs fine... I never use reformulated gas and premium grade only. Fuel stabilizer in every tank of gas also.
Might try a fuel pump? Any other ideas?
Should have mentioned...problem is MUCH worse with motor trimmed up.
Also...when I say starts hard I mean I have to choke it almost everytime even when warm.