Evinrude 50 hp backfire problem

[Crawfish Dundee]

Two years ago, I purchased a used 1995 Evinrude, 50 hp twin cyc. The seller was the original owner from day one, this motor ran flawless for him for many years, then one day while under power it started to backfire continuously. He had the carbs cleaned and installed new carb kits and it still backfired. The day I bought it from him, it fired right up but backfired, more of a cough out the carb, not a bang out the exhaust. We tested the compression and both cylinders are 150 lbs. It baffled the seller so he just bought a new motor and made me a deal. Now I haven't installed the motor as of yet as I am rehabbing the hull as I type. I have purchased a Seloc engine manual from Iboats here, and I'm pondering on replacing the reeds as a first stab at this issue, or is there another fix that someone knows about that I may might want to focus at?

 

[Joe Reeves]

I strongly suspect that the servicing mechanic overlooked something within one of the carburetors. While the engine is running and this problems present, stick two fingers into the carburetor throat(s), acting as a manual choke. When/if the engine smooths out somewhat, you've found the offending carburetor.

I retired in 1991 but I am assuming that your engine's design incorporates two single barrel carburetors. Is this correct? If so, your problem is usually that the extremely small off center vertical brass tubing (slow speed fuel passageway) within the center high speed nozzle has become slightly fouled, resulting in a fuel restriction at a low rpm (running lean). Clean it with a piece of fine single strand steel wire. Also double check the cleanliness of the brass high speed jet that is located horizontally in the bottom center portion of the float chamber, way in back of the drain screw as solvent and air pressure alone just doesn't do that job properly

Let us know what you find.

 

[Crawfish Dundee]

I retired in 1991 but I am assuming that your engine's design incorporates two single barrel carburetors. Is this correct?

Yes, it is. Thanks for the tips, I think I can do that. It will be awhile before I get around to it, hopefully sooner than later.