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Cough at idle 1988 90 hp

D

donaldboyer

Contributing Member
Mercury 90hp 1988
90hp 0b312461
Compressions 110lb on all 3 cyl.

Had our boat out today for first run since being at the shop for carb rebuild and clean, and new stator and switch box. Ran for about 20 mins at 4000rpm, then were looking at some fishing boats at idle speed and she had the occasional cough (which would cause drop in revs, then recovered. Never stalled though).

Sounds to me like the idle mixture screws could do with a 1/4 turn CCW? Might be a bit lean do you think??

Thanks,

Donald.
 
Re: Cough at idle

Agreed, but don't just dial away.

1. Warm the motor thoroughly with the boat on the trailer or tied to a pier.

2. Put it in gear and SLOWLY mess with one carb's idle screw at a time.

3. Move it about 1/4 turn, then WAIT at least 10 seconds to see how it reacts. That's the trick: waiting!

4. Repeat for the other carbs.

Jeff
 
Re: Cough at idle

OK, I hear you about the waiting.

So if I start with the top carb, and no improvement (after waiting), move screw back to it's original position, and try the next? As I am guessing it may only be one carb that's too lean?
 
Re: Cough at idle

It could be one, or all three of them. Get EACH one to idle the best you can, then shift to the next. And then go back to the first one and check it again.

Jeff

PS: Think Ferrari mechanic at work!
 
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