Regardless of the problem, always start with the basic trouble shooting procedure. Without this information, you are simply doing a guess procedure and performing the actions of a parts replacer.
Preferably after the engine is having the problem, warmed up, etc...................
Remove both spark plugs. Check the compression. What is the psi readings of the two individual cylinders?
With the spark plugs still removed, check the spark. Rig a spark tester whereas you can set a 3/8" gap to do a spark test. The spark should jump that 3/8" gap on both cylinders with a strong blue lightning like flame... a real SNAP! Does it?
Please don't come back with a answer giving generalities such as... The spark and compression seems okay or some such reply as that means nothing to us. Many boater do just that and it simply drags the problem out.
The spark plugs should be Champion J6C plugs gaped at .030 .
No spark tester? Build the following.
(Spark Tester - Home Made)
(J. Reeves)
You can use a medium size philips screwdriver (#2 I believe) inserted into the spark plug boot spring connector, then hold the screwdriver shank approximately 7/16" away from the block to check the spark or build the following:
A spark tester can be made with a piece of 1x4 or 1x6, drive a few finishing nails through it, then bend the pointed ends at a right angle. You can then adjust the gap by simply twisting the nail(s). Solder a spark plug wire to one which you can connect to the spark plug boots, and a ground wire of some kind to the other to connect to the powerhead somewhere. Use small alligator clips on the other end of the wires to connect to ground and to the spark plug connector that exists inside of the rubber plug boot.
Using the above, one could easily build a spark tester whereas they could connect 2, 4, 6, or 8 cylinders all at one time. The ground nail being straight up, the others being bent, aimed at the ground nail. A typical 4 cylinder tester follows:
..........X1..........X2
.................X..(grd)
..........X3..........X4
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