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Motor problem


Farley

Question

I have a 18 H.P. Merc older one that has problems keeping an idle. I have to choke the heck out of it to get it started, as in full choke, even after it has been ran a good deal on the lake. Then I have to put it in gear while choked and then I can finally take it off of choke when I get the RPM's up a little. Does anyone have a guess as to what this might be. I just had it in the shop before the season and it ran great then. It will idle for maybee 10 seconds before it dies and it doesnt run very smooth when I'm that low on RPM's.

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Farley, is the primer bulb good and hard when you run, or soft? Sound like a fuel supply issue to me - check the bulb or just replace it and the fittings at the tank and to the motor, pretty cheap and then you'll know for sure what you DONT have a problem with. Fuel pump diaphram would be the next stop in my mind. Maybe BigD or ST can chime in with some other ideas.

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Farley,

Sounds like the diaphram. I had the same thing happen to my 1980 50hp merc. I took it to the shop for a tune up before labor day last year and didn't have a chance to test run it before the big weekend. Sure enouph, ran like crap and couldn't use it the whole weekend. Brought it back to the shop and they said it was the diaphram. Runs great now.

Good luck.

Mike

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  • 'we have more fun' FishingMN Creators

If it runs fine at higher RPMs with no choke then your lines, bulb and fuel pump should be fine. I'd start with riching the low speed. A set of new plugs wouldn't hurt either.

Is there gas running out the throut of the carb when you idle with the choke on?

What year is it?

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I dont get what you mean by riching the low speed? As far as gas running out the throut of the carb, I'll check that tonite too when I get it out there. Do I want to see gas there? It is a late 80's model. Black and gold paint. The bulb is hard when it's run, that is the first thing I checked. But I'll pick up some new spark plugs tonite and run it to see if that helps, I can't believe I didnt do that last weekend. Thanks for the help guys, I just spent a couple hundred on it before the season, the last thing I want is to bring it back in.

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  • 'we have more fun' FishingMN Creators

Don't know which carb you have. You'll have two places to adjust the carb, Idle speed and Idle mixture. You'll be turning out the idle mixture screw.(counter clockwise)

Turn it out past to were it runs smooth then back in till it just starts to smooth out again. Once you get to that point turn the mixture out 1/4 turn. You'll be running a hair rich and that what you want.

If you have a Mikuni side draft carb the the mixture screws adjust the air and not the gas so to richen the mixture you'd do the opposite, instead of turning out you'd turn in.

If you can't get it run run smooth and your adjustments don't change how the engine runs its time to clean the carb.

Make sure you soak it in a carb cleaner and use a very fine wire to clean out

No you don't want gas running out the carb when its choked. I just wanted to know for diagnostic reasons.

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