Help with Johnson 48hp

[Destroyer]

Originally Posted by Curapa

My buddy has an early 90's Johnson 48hp that is having some problems. He purchased it several months back and hasn't ran right since. It seems to run well on the hose but not so in the water.

Past two trips symtoms:

Runs decent for the first couple of minutes. At WOT the motor cuts in and out, you can drop back the RPMs and it will straighten out. After a few times the RPMs drop and stay down until the engine shuts off. Then the only way to get the boat to run is by holding the choke in. We had to limp back to the boat ramp both trips by playing with both the throttle and choke. It will intermittently run good and then back to crappy.

We took it to the shop:

Mechanic replaced broken reed valve
Rebuilt carbs
new plugs and wires
Tightened some bolts he said was previously loose around the intake
Told us to ditch the aluminum tank for a plastic one due to condensation

We bought a new plastic tank and took it to the river. The motor runs in the same condition as when we dropped it off with the mechanic.

Any thoughts?

My first guess would be to look at your coils. You've ruled out fuel as a problem by rebuilding/replacing just about everything, so it seems to me you have an electrical problem. Since the engine runs good while it's cool and then starts to act up when it warms up I think you have a cracked/bad coil somewhere. Of course, that's just an opinion and I don't have nearly the expertise of some of the other guys here, so wait and see what they have to say before you pull anything apart. (But if it were my engine that's where I would look first) In all cases I think your problem is electrical, not fuel related.