[WallyMaster]

I have a 1981 9.9 Mariner (Japanese Mariner). Starts well (firts pull always), idles perfect, but when I go into full throttle, it will have full high-speed power for about the first 15 seconds, then gradually loses power until it is going maybe only 70% power at full throttle. Never comepletely dies, though, just continues as if it were an 8hp. After I stop and troll around a while, then again throttle up to high speed, same issue: full power initially, then begins dragging down to about 70%. First thing I did was replace the fuel pump and cleaned out the carb...that didn't help. Then, thought it might be an overheating issue, so checked the telltale but there is a good strong stream of water coming out. Ideas? Thanks.

[Vlad D Impeller]

Make sure that the fuel tank is venting and check for air leaks in the fuel lines.

[WallyMaster]

The fuel tank, fuel lines, and all fuel connectors are brand new. Vent screw verified open.

[Vlad D Impeller]

There could still be an air leak somewhere in the fuel lines, tighten all of the zip ties on the connections.