225 Merc RPM go up and down at WOT

[Luis1127]

Hello all,

I have a 96 Merc 225 carb that seems to run fine at cruising speeds, but when I get it at WOT the engine starts to act up. It starts fine, and gets up to about 5300-5400RPM but for a short while, then it drops down to aproximately 4400-4500RPM and then goes back up. It does this over and over again with nothing that I can notice to provoke it, I have checked the fuel for water, have put new fuel/water separator, inline fuel filter, run the engine with seafoam, new spark plugs and cant seem to find the problem.

I also notice that when I have it out of the water on muffs when I try accelerate it and keep it at aprox 1500 RPMS it jumps around between 1200-1800 without touching the throttle. I was wondering if this could be a need of a carb job or maybe fuel pump or if I can get pointed in the right direction.

Forgot to mention, the oil injection was removed by the previous owner, don't know if it makes any diference towards this problem.

Would appreciate and help

 

[achris]

Re: 225 Merc RPM go up and down at WOT

Next time you're out and running at WOT, grab the fuel primer and give it a squeeze. If that solves the problem, you need to rebuild the fuel pump. Diaphragm replacement is a service item and should be done annually. The part number for the kit is 21-857005A1 and it's about a 20 minute job. You will also need the special ties that Merc use (don't be fooled into using normal ties on the fuel lines, they will leak) and their part number is 54-816311T.

Chris.....

 

[Luis1127]

Re: 225 Merc RPM go up and down at WOT

Thanks Chris,

I had already tried that last time I was out, made no diference... I am thinking and leaning more toward the high speed jet. Do you have any idea of anything else it might be?

Luis

 

[achris]

Re: 225 Merc RPM go up and down at WOT

If you still have electronic spark controls, like idle stabilizers and rev limiters connected, I'd be disconnecting them and trying again.... Take the ignition system back to basic. Stator, trigger, switchboxes and coils....

C......