sounds like air is getting into the system, dbl check all fittings and make sure no air can get in them.

I would say not enough fuel pressure. Either from your fuel pump or possibly an air leak. The anti-siphon valve could cause poor fuel supply if it's old. Replace the anti=siphon valve, it's about $15. Them make sure no air is getting into your system. If you have a quick connect fitting that connects the fuel line to the engine, check the little o-ring on it. Ethanol eats those things up. Did you ever rebuild your fuel pump? Also have you checked the reeds to make sure one isn't chipped or broken?

Fuel pump was redone the last time I took it in. Reeds checked out ok when the mechanic looked at them Where is the anti siphon valve located?

Also, can I get a replacement valve at an Auto store or hardware store or will I need to order one?

Is you prop vented? Also, my mechanic added some little brass relief things on the upper part of the mid-section on mine. This carppy gas might have something to do with it too. Mine can be erractic at times. Most of the time it runs good but sometimes it just like someone shut off a couple of cylinders. I have dual tanks and when I switch to the fresher one it will run fine.

have you tried your vent tube to your gas tank to make sure a critter hasnt built a nest in it?I've had that happen and it can cause a problem just like this!!!

I can't believe you paid a mechanic to look at it and he let it leave not running right.... AND he's got you tracking something he should have fixed.... With what he THINKS is the problem... Sounds like a hell of a mechanic....

To be fair to the mechanic, it does not show anything as an issue when he works on it. Only on the water does it have this issue. Im not sure if the prop is vented. I will have to take a look.

Umm thats when they need to lake test it, put it on a dyno or at the very least put it in a tank of water.... I know my mechanic will be happy to jump in the boat with the customer to try and duplicate a problem. It is almost impossible to repair a problem if you can't duplicate it! You could have 2 dead cylinders and it would seem to run ok on muffs!

Boat motors are great deceivers. They can run great on a hose but sometimes you just have to put them in the water. Dynos can really help to put the motor under a load put sometimes there is just no substitute for the water.