15 HP Won't Idle Unless Primed

[bigboat606]

My 1987 2-stroke 15 HP Mariner starts fine cold or warm but 30 seconds after start, it begins to run a little rough and the idle speed begins to slow until it finally stops. If I pull the primer level (inject more fuel) the idle speed picks right back up to where it was and then a few seconds later the idle speed slows back down until I pull the primer level again. This can go on indefinitely. I have not had a chance to run the motor at high speed (it?s on the flusher), but if I move the throttle plate on the carburetor the engine revs up nicely. I did take the carburetor completely apart and inspected all gaskets and passages (including the integral fuel pump) and all looked good - no dirt. I squirted aerosol carb cleaner through all the small passages including the low speed adjustment needle seat. I then blew all the passages out with compressed air. After reassembly the engine ran the same. I am stumped and looking for some good advice. By the way, this engine is run on the flusher once a month and it always starts on the first pull and idles perfect (including last month)?until today.

 

[Maxz695]

Re: 15 HP Won't Idle Unless Primed

Be sure the vent on the tank is fully open and has air flow second check all hoses for leaks crack or pinholes. It might be best to hook the tank to the bulb and straight to the fuel pump for testing. Check the filter or by pass it temporarilly just to see if it helps. The bulb must get hard if not there is a leak or blockage somewhere between the tank and the needle seat or a poor float level

 

[bigboat606]

Re: 15 HP Won't Idle Unless Primed

Thanks for the ideas. The tank vent is fully open and the bulb gets hard as a rock. Looking for air leaks in fuel hoses is a good idea, I'll try it. By the way, I tried squeezing the (hard) fuel line bulb while the idle speed was slowing - no change - the engine slowed down and stopped.

 

[Maxz695]

Re: 15 HP Won't Idle Unless Primed

It definitly sounds as if it is lacking fuel. You could try disconnecting the fuel line from the carb after primimg and turn the engine over to see if the fuel pump is pushing the fuel at least to the carb. If so the problem lies within the carb float level, needle seats or a bloccked passage within the carburetor, loose carb or a bad carb gasket.

 

[bigboat606]

Re: 15 HP Won't Idle Unless Primed

What I don't understand is if it is lacking fuel how can the engine rev-up nicely when I manually open the throttle (butterfly valve in carb throat)? The engine responds quickly as if nothing is wrong. It appears to get fuel very well except at idle. Or am I missing something?

 

[Maxz695]

Re: 15 HP Won't Idle Unless Primed

The engine pulls fuel through the main jet when suction occurs from the intake of the cylinders. There are ports in the carb that allow fuel to enter the carb at idol with minimum suction. That port is more than likely obstructed. Is the engine responding to the mixture screw? start it from a light seat out to 1 1/2 turns and then adjust it clockwise to where it starts to faulter. turn back counter clockwise until it sounds best add another 1/4 turn and it should be close to where it should be.

 

[bigboat606]

Re: 15 HP Won't Idle Unless Primed

Thanks Maxz695. It will be this weekend until I can try your suggestion. I'll let you know how it goes.