Re: 1974 25 johnson
i don't believe 74 25 had a tale tell, sounds like the recirc hose from bottom of crankcase to intake manifold, that recircs unburn fuel. if so yes it will effect the the motor. but less start at the beginning.
check compression and spark on both cylinders, i would like to see atleast 110 psi, and with in 10% each other. then check spark, it should jump 1/2 inch. if you don't have these, you have other problems. if all is up to this, then buy a carb kit and rebuild the carb, replace all hoses with alcohol resistant hoses. ethanol degrades the old hoses, and messes up the carb. . your carb has adjustable low speed jet and fixed high speed jet, follow these directions.
http://forums.iboats.com/showthread.php?t=244013
after the carb is cleaned. motor reassembled,
(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)
Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.
Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.
Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.
Note: As a final double check setting of the slow speed valve(s), if the engine has more than one carburetor, do not attempt to gradually adjust all of the valves/carburetors at the same time. Do one at a time until you hit the above response (die out or spit back), then go on to the next valve/carburetor. It may be necessary to back out "all" of the slow speed adjustable needle valves 1/8 turn before doing this final adjustment due to the fact that one of the valves might be initially set ever so slightly lean.
When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
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