15hp Mariner power under load

[triplehooked]

Hey all,

I recently sold my mud boat and picked up a Sears Game Fisher (15' fiberglass semi-v/tri-hull design?) with a 1989 Mariner 15 hp (Serial # OC117061) for duck hunting.

I'm suspecting a fuel problem as the previous owner stated that the gas was a year old. I've replaced the gas with rec fuel and 50:1 oil mix, and I currently have the carb stripped down and dipping the main body (berry's carb dip) and will be replacing the fuel lines upon reassembly. (The fuel filter had sludge in the bottom, inside and out, as well as the infamous rubber chunks.)

The issue I'm having is that the motor starts and idles great, and will go into reverse at full throttle with no issues. In fact it goes fast enough to flood the boat if I let it. But.. if I try to go into forward gear the engine will not plane and will bog down, then pick back up, bog down, etc... but it will not stall. If I choke it, it will not bog down, but either way, it's only giving me about a quarter throttle choked or not, low rpms, no plane, and a max speed of 7mph.

Everything points to a dirty carb with exception of the reverse full throttle scenario... I'm a little confused as to why it would rev so freely in reverse but not forward. I'll post a prop thread in that forum as I want to pick up a back up prop since they are so cheap.

Finally, should I change out the grey fuel line and bulb at the tank? It doesn't look like any residue is coming from there. Does it go bad and flake like the black fuel line at the carb?

 

[triplehooked]

Pic for reference.

 

[racerone]

Make sure you have spark on both cylinders !----In reverse there is lots of exhaust in the water.----Hence it is less load to turn the prop.

 

[triplehooked]

I thought I would post what I did for future reference for others. The spark was good (thanks for the suggestion racerone), compression was 110psi in each cylinder (I forgot to check it with wide open throttle so it was probably closer to 120). I dipped the carb and cleaned everything up. I re used all gaskets, diaphragms, needle, etc.. and removed the original filter and housing and replaced it with two of the inline see through designs, one just after the tank, and one just before entering the carb. I also replaced all of the hoses. Once primed, the engine started on first pull and after a small adjustment, it idled and ran perfectly. I averaged 18mph with a hunt load (decoys, blind, chairs, etc) and 70lb lab, which I think is respectable for a fiberglass boat. I will be installing an actual rebuild kit in the off season due to the enriching valve diaphragm showing slight deterioration. Best of luck everyone!