1985 Mariner 115 Acceleration Issue

[swmissourian]

Hello, long time lurker, first time poster. I'm having an issue with my 1985 Mariner 115 inline 6. She starts up and idles great, but frequently struggles to get on plane. When I push the throttle forward, it will sometimes reach around 1500 RPM, then just sputter out. I can always keep the motor from dying by pulling back on the throttle. This problem has just started within the past week, right after I had to drop suddenly from WOT to idle in order to avoid an obstacle. I used to start it up, let it warm, push the throttle forward and it would jump right up on plane. Also, when I get it on plane now, it will only hit around 4900 RPM, when it used to hit 5500.

I've changed the spark plugs (old ones appeared normal with no odd discoloration). I've mixed Seafoam in the ethanol-free fuel. I've used Seafoam spray to clean the carbs while the engine was running at the dock. After the spray treatment, it jumped right up on plane, but the problem returned on subsequent starts. Engine compression is normal and all 6 cylinders have spark. No evidence of water in the fuel.

Any help would be greatly appreciated! Thank you!

 

[Chris1956]

Sounds like the idle mixtures are too lean. Maybe the idle passages are partially gunked up? Maybe the floats are sticking?

 

[racerone]

If you keep running / hoping for a magic fix it can damage the engine.----Say it is a plugged carburetor then little or no fuel is going in to 2 cylinders and no oil.------Always confirm that carburetors are clean.

 

[swmissourian]

Thanks for the replies. Could you tell me where the idle mixture screws are located? Would you advise a Seafoam "shock treatment?"

 

[Chris1956]

Sure, that motor should have sidebowl carbs. The idle mixture is the only screw, about in the 2PM position, on the face of the carb throat. I will assume your idle passages are a bit gunked, the float is gunked or there is some other issue, as carb idle mixtures do not get out of adjustment on their own.

 

[Mi duckdown]

^+1... Seafoam is not a magic fix. If I were you I would purchase a service manual, and go thru trouble shooting.???

 

[swmissourian]

Thanks everyone! I'll give the carbs a very good cleaning and tweak the idle mixture and report back.