Small rpm fluctuation

[beachnut]

I have a 2000 410 dancer with twin 454 mpi 380 hp gassers. The starboard engine has a slight rpm intermitant drop of maybe 100 rpm or a little more. Just enough to make my sync gauge jump around, and you can feel it if you are paying attention. When I throttle up real slowly it is not as bad or doesn't do it at all. When I am first leaving for the day it is most likely to happen. Does this sound like an electrical problem or a fuel problem? I've had a mechanic tell me I need a new wireing harness. Is their a fuel filter other than the fuel separator filter? Any help on this matter would be appreciated.

Thanks again
Gus

 

[Larry]

These motors are notorious to have an RPM "hunting" effect at just above idle to about 1600 RPM then it should smooth out. If this is what your experiencing there is no complete fix, the only thing that we have found is that it seems to get worse when the spark plugs get older, if you havn't changed plugs in a couple years, chage them and it will get better if not 100% it will be close. Make sure you use the correct platinum plugs that are specified, yes they are expensive!

 

[beachnut]

Thanks Larry for replying to my post. I have just changed the plugs with those plugs that are about $10 per plug from Marine Max and it still does it. I know what you mean by hunting,but it's not that. It's more of a drop off of power for a split second about every 10 to 30 seconds. But once again thanks for your reply. Gus

 

[Ruff Life]

My old boat had a 502 MAg MPI with a similar problem. I finaly ended up running a data logging program, Scannerpro, and ended up finding that the motor would pick up an occasion knock on the knock sensor. This will then cause a slight mixture enriching along with a timing retard of a couple degrees causing a 100 rpm change. It took about 30 seconds to slowly return to normal. Never did find the cause as the sensor can trigger from other than an actual engine knock. Spark plug wire too close to the knock sensor and wiring is one common false trigger. Flapper valve metal to metal (rubber worn off). Rocker arm noise, etc. I only had a single engine so it was just an annoyance and I new there was no actual damage being done.

 

[beachnut]

Thanks for your response . It sounds like a scan is in my future. I will write back with my findings.

Gus