1989 Merc5 runs - full speed, then gently slows down

[BigSkyMercGuy]

New Forumite here.
The story: Warmed up, I can take it at full speed across a lake, then in about a minute it slowly decreases in RPMs, down to a putt-putt troll speed, but throttle handle is wide open. (Not sure, but I think it'll eventually stall out). Spark plug is new and gapped. New gas. It gives me enough warning to head toward the shore. Fuel pump? Jets?

 

[enginepower]

Sounds like a fuel issue. Restriction in fuel pick up? Does the bulb go flat when it does this? Tank not vented? Possible torn diaphragm in the fuel pump.

 

[frustratedboater]

I would agree in the tank venting... This reminds me of my camaro years ago, to which the main vacuums hose off the carb was a thin-walled hose, which collapsed when accelerated. You don't have vacuums hoses, but the tank venting may contribute...

I'd also rule out the jets, due to the fact that you have a couple of minutes of 'good' throttle.

 

[BigSkyMercGuy]

Got it figured out; inside the portable gas tank, the rubber pick-up tube was crunchy and cracked - vacuum leak. Thanks all.