2003 Mercury 15 hp 4 stroke - dies after a 30 min run at 1/2 throttle

[JanS48]

Greetings
I have a 2003 Mercury 15 hp 4 stroke on my sailboat. After running at 1/2 to 3/4 throttle and attempting to slow the rpm drops suddenly and will eventually stall.
I have replaced the carb with no change in outcome.
Anyone seen a problem like this?
I see a check valve in the carb parts diagram - anyone know of it's function? Also the plunger mechanism on the side of the carb - it has an adjustment - anyone know what that does exactly?
This is a strange problem for sure any suggestions are much appreciated.
Thanks
Jan

 

[tpenfield]

:welcome: to iBoats . . .

Fuel tank vent :noidea: Symptoms sound like the tank may not be venting.

 

[QBhoy]

The vent is the obvious one. Few others would be crap in the carb or bad fuel currently or previously. Is the fuel line with the primer bulb the right direction ?

 

[JanS48]

Thanks but I have checked all the obvious things, vent is fine. Also if I pump the primer bulb just before or after the drop in rpm there is no effect either way. I'm sure plenty of fuel is getting to the carb. If I attempt to speed up after the rpm drop it goes to speed just fine. It just will not slow gradually. If I let it stall and re-start it runs perfectly. It has to be something in the carb.
Thanks for they reply
Jan