Revs fluctuating at WOT - 1999 Force 120hp

[Onegazza]

Normally my engine runs faultlessly and, the last time I used it back in June was absolutely fine.

When I used it last weekend, the engine started fine as normal and, ran fine as normal, up to about 4,700RPM. It was happy sitting there steady, with no issues at all.

However, when I pushed it to WOT, it sat at 5250RPM as it normally would but, only for 3 seconds or so, then, it started stuttering. The revs were fluctuating between 4,800 and 5,250.

When returning below WOT, it was fine again, happy holding the revs constant and, running as smooth as it normally does.

Any ideas what's caused this all of a sudden?

My initial thoughts are a quick tune of the idle mixture screws on the carbs.

My next port of call is the timing, which may be ever so slightly off.

Any other ideas to help me pinpoint?

 

[Redbarron%%]

Do you have a water separator? If so have you checked if there is any water in it?

 

[Onegazza]

Indeed I do. One of the pre-filters with the clear bowl water separator.

If it's full, could it be a cause?

 

[jerryjerry05]

Water should be a problem at all rpm's.
Stuttering at high speed, not getting enough fuel.

 

[Redbarron%%]

The reason I ask is the separator, if nearly full of water, might feed some water at high fuel flow where it might not a lower flow. Just guessing, but I lost a piston for this reason (I think)

 

[Onegazza]

There was only a few droplets in the water separator.

I think I've traced it to the fuel pump. Will fit a new diaphragm kit over the weekend and see if it solves it.

 

[tg3690]

Mine did the same thing last year....a new fuel diaphragm was the fix. It didn't have any holes or tears in it but was very stiff.