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Jeff G.
Jeff G., Marine Mechanic
Category: Boat
Satisfied Customers: 4724
Experience:  42 years experience, Certified Master Technician, All Major Outboards, Sterndrives, Hi-Performance
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I have a 40hp, older vintage. Runs very well accept when i

Customer Question

I have a 40hp, older vintage. Runs very well accept when i gets warm it wants to bog
down when I'm running at slow speeds. Seems to run better if I speed up
Submitted: 1 year ago.
Category: Boat
Expert:  Jeff G. replied 1 year ago.
Welcome,this question may need more than a single answer. Please post back with additional questions on this at any time.
What engine are we referencing here? Make, year, model?
Customer: replied 1 year ago.
1998 Mercury 40
Expert:  Jeff G. replied 1 year ago.
Please get me the serial number as that is how Mercury catalogs the engine as to 2 stroke, 4 stroke, fuel injected,etc.
Customer: replied 1 year ago.
sn: 0G660731
1998
40 ELPT0
Expert:  Jeff G. replied 1 year ago.
Slower speeds, do you know what RPM that is? Idle, fast idle?
Are you familiar with the carbs on that engine? On the top port side of the carbs is an idle mixture adjustment screw. It has a slot and a spring around it.
Slowly turn it in while counting the number of turns, then back it out about 2 turns.
Post back how many turns until lightly seated.
Strt the engine and let it wrm up a few minutes. The slowly turn the screw in 1/8 turn waiting 20 seconds between adjustments. Do this until the engine begins to want to die, pops, backfires. Before the engine stalls turn back out 1/4 turn. Repeat on the other carburetors.
Now set the idle speed to about 900 to 1000 in neutral.
Put the engine in gear and accelerate, boat has to be running in the water, not in a barrel or flushing cups.
If the engine idles OK, not stalling and accelerates, you are done. If it bogs then open the idle mixture 1/8 turn on each carb and repeat.
Do this and post back what you get.
I would also replace the spark plugs if over a year old or blackened.

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