Misfire at specific RPM with bigger carb

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alfaguy

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After fitting a larger carb, I took my Evinrude 25/30 to the river and it ran really well except for a severe misfire at 2200 rpm. It is fine at 2000 and 2500 but totally breaks up at 2200. I installed a new power pack, coils and plugs (but not wires) two years ago.

The "new" carb is a choke type pre-84 and the engine is a 1988 model, but I blocked off the primer feed hoses with plugs. I rebuilt the carb and the transfer holes under the large plug were clear. I wonder if crud could have gotten back in there.

Any suggestions on where to start looking for the cure?
 
Your "misfire" may be nothing more than the transition area from low to high speed circuits in the carburetor that are being aggravated by improper sync and link of your carb to ignition. This would be very typical for that engine. These settings are critical to smooth running. The other issue may be not enough propeller pitch to load the engine enough to keep it in the 2-cycle mode while running in the lower RPM range. Again, typical for an engine that has been modified to increase horsepower without going up in pitch to accommodate the new power.
Am taking for granted that you are running in the water, on a boat. Not in a test tank.
 
Thanks for the response Pappy. I suspect it is the transfer point but did not explore timing yet. It is set with the roller between the marks when the butterfly starts to open, but I will play around with it. My testing will be done in the water. A #53 drill fits the main jet, so it should be 0.067 and the plugs looked fairly clean after running.
I did not change the prop but it is a 10x13 pitch.
 

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