25HP Mercury Idle & Low Throttle Issue

[blindjbe]

My 1986 25HP Mercury all of a sudden will not idle and run at low throttle. It will crank up fine with Full Choke & idle great, but a little rough. When I go to Half Choke or No Choke, the engine will die. It will crank again fine with Full Choke & idle again fine.

If I crank it at Full Choke and quickly push the choke into No Choke and then Full Throttle, the engine will take off and run like a champ, but if I let off the throttle to halfway or lower, the engine will die.

I have checked all fuel connections, hoses, & fittings and primer bulb stays solid. Not sure if the next step is to clean the carb since that hasn't been done in a while, but wanted to see if anyone had any additional ideas for me to try.

Thanks!

 

[mcdgary]

Re: 25HP Mercury Idle & Low Throttle Issue

Can you specify the carburetor type? For the two stroke 1983 25 HP Mercurys there are 4 different carbs: BCIA, BCIB, BCIC and WMC. Mine is stamped WMC right above the air intake throat. The idle adjust screw is near the stamp and the fast idle is part of the linkage. Different for the other carbs.

 

[blindjbe]

Re: 25HP Mercury Idle & Low Throttle Issue

Just went and looked above the air intake throat and noted the following id, WMC113135.

Going to start taking some small steps tomorrow and take the carb out and clean the jets to see if that will resolve anything. If not, at least I'll have the peace of mind knowing it is clean.

 

[mogfisher]

Re: 25HP Mercury Idle & Low Throttle Issue

Let me know how this turns out for you. My 83 18Horse acts the same way. I'm taking the carb apart today for inspection but I'm curious to see how yours turns out.

 

[blindjbe]

Re: 25HP Mercury Idle & Low Throttle Issue

Took the carb apart today and thoroughly cleaned everything. The lower fuel jet appeared to have a small piece of trash blocking it and once all was cleaned and reassembled, the motor fired up great. Idle and low throttle are back to normal. Glad something as simple as cleaning the carb fixed the issue.

 

[mcdgary]

Re: 25HP Mercury Idle & Low Throttle Issue

Congratulations. Keep that carb and motor clean.

 

[madanorak]

Re: 25HP Mercury Idle & Low Throttle Issue

There's a small plate on the top of the carb,held on by 2 screws, did you take that off and clean inside. This is where you find the idle needle and 2 very small holes into the venturi. Any muck in here and it will destroy the ability for the engine to idle. If you haven't, then I suggest you have a look, as you may have disturbed some muck in there, which will let the engine run fine until it gets in the way again. Before removing the idle needle, count how many 1/4 turns it takes for it to be fully seated, and then when you put it back in after cleaning, you know how far to unscrew it from the fully seated position, for it to be set correctly( usually about 1 & 1/4 turns).

Kev