I have a Merc 40 2 cycle. I think it has an air leak in the fuel line. Portable tank, line, bulb, connections are all new. Pump bulb, start, runs fine for a few min then it will only rev to bout 2800 rpm's. I can see air pocket in fuel filter. Would a small air leak cause this type problem?
Check the fuel outlet on the tank, the black plastic outlets tend to crack at the threads where the brass fuel line connector screws in.
I will thread tape and double hose clamp all connections to be sure. I'm guessing an air leak would cause this type of issue, or is a small leak no big deal?
Once upon a time I had a motor that acted the same way. After I replaced all of the hoses, priming bulb, etc. it still acted up at higher rpms. Ran great at low rpms. Mine had a mechanical fuel pump attached to the engine (a small box looking thing) that the fuel line went into and then a line into the carb. Inside that box thing there was a plenum (thin rubber gizmo) that created a vacuum to pull gas from the tank as well as a pump to keep the carb full of fuel. After a guy showed me this and I replaced it the motor was perfect. I was not able to see the fine deteriation of this with my eye, I had to use a magnifying glass. At high speed there was not enough rigidity to it for it to work properly and caused the pump to create turbulence to the fuel. I think the part was less than $5.
