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2003 Mercury 15hp 4 stroke stalls when trying to slow down after a long run at 3/4 throttle

J

JanS48

New member
Greetings everyone, this is an odd one. After running for 2+ hours at 3/4 throttle, if I attempt to slow the motor is sputters and stalls.
This is what I've tried so far:
Replacement carburetor
Replaced fuel hose (tank to engine) + bulb - as a test: if I dis-connect the tank end while running the motor it sucks the bulb flat (proves that no air is leaking)
Put a re-build kit in the fuel pump
Replace the fuel filter
Changed out the fuel lines from the pump to the carb.
The problem persists.
Get this: Once it stalls it 'always' restarts but will not run at low speed - 1/2 throttle or above it runs fine - sounds great.
If repeating the attempt to run slow with many re-starts it will 'eventually' run at lower throttle settings - can take a number of tries (5).
I've worked on MANY engines over the years but this issue has me stopped cold.
If anyone has seen this behavior and has a cure I'd be forever grateful.
Thanks
Jan
 
Have you checked compression just to be sure?
Is there a fuel mixture adjustment screw hidden back there that you could adjust? On my merc factory put a brass cap over it so the operatorcwould not be able to adjust. Drilling cap out and tweaking settings slightly fixed my idle issue.
 
The engine is in great shape power wise, I've not checked the compression but plenty of power spinning a 4 blade prop and pushing a 25ft sailboat.

Regarding the idle adjustment screw, my original carb had the brass plug removed, and the replacement carb had the idle screw available. Does screwing it out (counter clockwise) richen the mixture? About how much throttle does the idle adjustment have control over? 1/8, to 1/4 throttle perhaps?

The behavior of this motor after a long run is very odd to say the least. Since I have a spare my next step is to carefully ream out the idle jet very slightly with welding tip cleaners/files.

Any other suggestions would be most welcome - I'm sure someone else has seen this behavior.

My guess is it's a flaw in the carb design, given that I've replaced the carb, I suspect the Chinese replica carb also replicated the flaw.

Thanks to all that have reviewed this thread.

Jan
 
In talking with another owner of a smaller 8 hp Mercury 4 stroke outboard of a similar year, it experienced the exact same issue. This person went on to say that these engines have this flaw that shows up with age. His local Merc dealer did not have a fix. He also said it helped a lot if he ran gas without ethanol (tough to get unless spending $ 10. per gal for racing fuel). So is there anyone out there that is a Mercury mechanic that has a fix for this anomaly?
Anyone know of another msg board that may have Mercury techs?
Thanks in advance
Jan
 
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