1999 75HP Mercury Slowly Lose Top End RPM

[pshelfo]

Over the past two years my top end RPMs have gone from the norm of 5000 rpm at WOT to now sitting around 4500 rpm WOT. I have had the compression checked, tuned the motor, had a de-carb treatment, adjusted the sync and link and changed the primer bulb. At WOT the boat runs fine but it almost seems like it should really have more to the top end engine sound. I even tried changing props but that have no impact. What else can I be checking as the spec for that motor is to run at 5000 WOT. At first it dropped about 200pm and then over time it has made its way to 4500. Thanks!

 

[Texasmark]

What about the boat? If the boat is heavier than it used to be it can be loading up the engine and dropping your rpms. Since you have done all the engine checking, seems you need to be looking elsewhere. Besides additional load in the boat and possible waterlogging flotation, what is the condition of the hull? A dirty hull can drag you down and it doesn't take much slime or barnacles.

Mark

 

[enginepower]

Try pumping the bulb when this happens. If rpm's pick up, your fuel pump needs repair or replacement. Could also be carbs have a partially clogged jet or ignition system problem.

 

[pshelfo]

Thanks for the replies! The hull condition is very good with no build up or barnacles. The boat is stored in the garage so very rarely gets wet. One point I racheted up the trailer as high as it could go and left it like that for couple of days to drain any water build up - very little came out. I like the idea of pumping my primer bulb when driving but will have to be careful . Might have to take it back down to a boat shop and ask them to look at the carbs closely.