Fuel delivery issue. Merc 90hp

[buckeyeBEN]

Frustrated!
2004 mercury 65jet which has a 90hp 3 cylinder head.
Idles fine. Starts fine.
When I throttle up, I'm losing power and rpms.
Had the motor checked out by mechanic who says it runs fine. Well, he ran it in a tub and not on the open water.
Compression and spark fine. It has nothing to do with the jet.

Everybody has an opinion...
Fuel pump
Tear down, clean and rebuild all 3 carbs.

I'm not a mechanic.
How to troubleshoot the easier items with minimal outlay of cash?
Any suggestions are appreciated.

 

[kofkorn]

Fuel pump can be diagnosed by having someone manually pump the bulb while you're driving. If the condition goes away with someone pumping, it is likely a problem with the fuel pump.

To diagnose the carbs is a little more difficult. A couple of things to help determine if it is the carbs:

When the motor is stumbling, bump the "Enricher" on your key (AKA: choke) If you feel the motor pickup, then you've got a problem with the carbs and they need to be cleaned.

Another way would be to use a spray bottle filled with pre-mix fuel. Again when the motor is stumbling, spray a small shot of fuel directly into each carburetor. If the motor continues running the same or even drops a little, that carb is ok. If the motor performance picks up, then that carb is probably running lean and needs to be cleaned.

If I had to put money on it, I'd say you probably need a carb cleaning. A fuel pump problem would show up after about 30 sec - 1 min, and would affect both idle and high speed. Even if it only affected High speed, it still would probably accelerate normally for a little bit before beginning to stumble.

I would do a carb cleaning and focus on the two or three tiny holes that are in the throat of the carb next to where the butterfly contacts the body. It is possible for only one of the two or three holes to be blocked and would cause poor performance from off idle until the motor speed picks up at WOT.

Good luck and keep us informed!