40 Johnson, running rough... Using to much fuel?

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75MIRRO16

Member
Joined
May 30, 2011
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Location
Mastic Beach, NY
1974 Johnson 40 Sea Horse, pull start.
Model = 40RL 74S
Seriel = J4032440

Got this on a boat I just purchased, came with new coils and a "rebuilt" carburetor. I had to pull the carb and drain all the old nasty fuel from it and the fuel tank. The carburetor is very clean, and seems to have a new needle and bowl gasket. Also the motor has newer coils.

Turns out the flywheel key was sheered, after replacing it and refueling it fired up after only a few pulls. I've been out with it on two short trips, and it definitely gets up and goes.

Unfortunately the issues I have is the motor seems very rough at lower RPM and it seems to be using a lot of fuel. I had 5 gallons in it, I probably used 4 of them in the two short trips. I have no rod on my air fuel screw so I can't adjust it without taking the cover off. Is that important? Also, I had the screw at about 2.5 turns out and the carb was popping at low RPM, and still rough. I turned the screw in (maybe 1 turn out) and the popping stopped, but the motor is still very rough down low.

Any help would be appreciated!! Thanks!

Here it is running yesterday.
2035492482129_ORIG.jpg
 
First, make sure the engine is coming up to temperature at idle. Lots of those old engines have had the thermostats removed by people thinking they were not needed. Second, make sure you have the correct plugs in the engine, a colder than normal plug will not handle excess fuel as easily as the correct plug will. Third, go ahead and adjust your idle mixture until the engine smoothes out, speeds up, and either coughs lean or slows down. Once this happens back the adjustment screw up about 1/4 - 3/8 turn and leave it there.
If your prop is worn that really kill the mileage so check it.
 
Pappy said:
First, make sure the engine is coming up to temperature at idle. Lots of those old engines have had the thermostats removed by people thinking they were not needed. Second, make sure you have the correct plugs in the engine, a colder than normal plug will not handle excess fuel as easily as the correct plug will. Third, go ahead and adjust your idle mixture until the engine smoothes out, speeds up, and either coughs lean or slows down. Once this happens back the adjustment screw up about 1/4 - 3/8 turn and leave it there.
If your prop is worn that really kill the mileage so check it.

All great tips! Do you happen to know the correct plug for this motor?

Thanks! I will report back after checking these things.
 

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