Engine dies at low rpm...why?

[steve forsythe]

2000 Merc. 200hp carbed outboard, runs really well at higher rpms, and really well at idle speed. At first the engine will troll great just in gear at the lowest rpms possible (maybe 1500). After a little while it seems to run rougher with a harder and louder shake to it. This even happens after getting out and running to a new spot for 30 minutes at 5500rpms. I would think that would 'blow it out' or something. Is the engine 'loading' up with carbon? The plugs look ok, and cost $11 apiece to change. Could i buy a decarbing product, and is it a do it yourself job? The motor does not always die, it might run rough but will only conk out once or twice in a day,(for now) and restarts without a problem (for now). One more piece of info that could help....my primer bulb is 'never' hard. While the engine is running, rough or fine, the bulb is very easily squeezed. I changed out but new one does the same thing. Could this be the cause of the rough running and conk outs?? Thank you for the help.

 

[oldboat1]

Re: Engine dies at low rpm...why?

guess I would look to enriching the idle adjustment when the engine is hot (runs lean?). I don't think the bulbs indicate anything. In my experience, they are usually really firm only when initially pumping gas to the motor at start up. When running, the gas is just flowing on through.

 

[steve forsythe]

Re: Engine dies at low rpm...why?

To tell you the truth, i dont think the bulb gets very hard at the begining either. When you say runs lean, what does that mean? I dont think i really want it to run lean, do i? The temp looks fine (175 running full out, and 150 at idle). Do you mean you think the fuel mixture is to lean now? And if so how is that fixed?

 

[oldboat1]

Re: Engine dies at low rpm...why?

too "lean" means not enough gas -- causes engine to cough and sputter. Engines tend to run leaner when they are hot, so an idle adjustment made in the tank (certainly on muffs) might need to be changed when the engine is at temperature. I'm not familiar with your motor, but typically you would set the idle screw a little richer (maybe an eighth of a turn counterclockwise) and see if the motor idles better -- a little more if needed, but remember what the original setting was. If you reset the idle adjustment, you may have to tweak the high speed adjustment a little too when you are underway.<br /><br />Yup. That stuff about pumping until the bulb is hard can often mean spraying gas around the carburetor or the inlet.