Johnson 50hp rpm loss Question

[SFRacing]

my son has a little fishing boat with a 99 johnson 50hp on it. Recently it started dropping rpms between 1200 and about 2500rpms. it will just start falling off at lower rpms as he is runnning along. Runs fine up around 4000-5000, however it will occasionally accelerate to about 6000 rpms on its own while running between 4-5000 rpms.

it runs fine on the hose, but under load it struggles on the low end. Gas is fresh as he uses the boat a few times per week to fish during the summer. any suggestions where to start? thanks guys:thumbsup:

[SFRacing]

back ttt, anyone?

[Jeff_G]

Check the fuel pump for leakage.
Put a timing light on each cyl and see if you lose or have intermittent spark while running. Just look for a spark, you don't need to look for any timing.

[Mark Poole ModVP]

This is the first time I have heard of anyone having that problem since it happened to me with a 1990 Johnson 50 (In 1990). This motor was on a pontoon boat and would only do it in the water not on the dyno. If you tried to run it at 1500 rpm it would drop to 900 on its own. If you tried to run it at 2000 rpm it would go up to 3000 on its own. I remember trying everything except a wholesale swap of the ignition parts. Since then I have learned a few things.

OMC made a retro fit throttle cam that replaced the ones that came on the motors from the factory. This changes the timing advance/fuel relationship at certain throttle settings. This would be the FIRST thing to look into.

If problem persists after that make sure that none of the large gauge red or black battery cables are routed close to the flywheel. They have discovered that if the cable is too close to the flywheel the big wire can cause a "trace" charge in the flywheel magnets. If the problem persists I would try another timer base (sensor coils).

[SFRacing]

thank you Jeff and Mark, i have been going nuts trying to figure this out.:thumbsup::thumbsup::thumbsup: i will give it a shot.

[Bruster]

I'm thinking you have ignition issues. If your checks verify this, I have complete ignition systems for this motor. $100 delivered to your door.
Bruce