50 hp Yamaha 4 stroke Runs rough

[Greg Thomas]

I have a 50 hp Yamaha 4 stroke that only runs rough from idle to 3,000 rpm after 3,000 rpm it runs fine. The mechanic says it may need a coil pack? This engine only has 65 hours on it? Anybody have a clue on this?

[99yam40]

compression, spark, timing, and fuel pressure/vacuum are all easily tested without throwing parts at a motor.
if spark, & timing are good while problem is happening then you know it is fuel related.
Most motor problems are fuel related, but it is too simple to test and eliminate spark and timing.

Find a Yamaha shop with good service techs if you cannot do it your self

[TownsendsFJR1300]

If the motor has sat for any lenth of time without fuel additive or a fuel stabilizer in it, the low speed jets (which are extremly small) clog up easily. The running rough at low speeds is likely due to one or more jets being clogged. After 3,000 RPM's you running on the main jets, which are apparently clean.

If its not real bad, you may luck out by running a heavy dose of "Seafoam" thru a couple of tanks. If it doesn't help, the carb jets (accessable after removing the float bowls) likely need manual cleaning.

I have seen (on a 500cc Kawasaki motorcycle engine with clogged jets) where we put straight Seafoam in the carbs (after draining them) and let them sit overnight. Drain the Seafoam, prime as usual and crank her up. That particular bike, after some revving, surprisingly, ran like a top without ever pulling a carb..

[Greg Thomas]

Thanks Scott good Idea I will try that first!