Nautica Rib Mercury Sportjet idles perfectly, bogs over 3K rpm; one plug dry, clean

[iraqvet70]

Hello,

I have a 90 HP Mercury SportJet in my Nautica Rib that won't go over 3K rpm without bogging or stalling. I pulled the plugs and the top two look normal (light oil/gas mix, not dripping or fouled) and the bottom plug looks brand new and dry, like it's fresh out of the box. The plugs are new NGK B9HS-10's.

The boat is setup to run on premix (40:1) and has one carb feeding all three jugs. It runs beautifully below 3K rpm -- fires right up, idles smoothly. It just won't kick into high gear.

I plan to change the plug wires (as soon as I can find some) and change the fuel filter. I thought I'd also try squeezing the primer bulb while it's running to rule out the fuel pump.

Here are my questions:

1. If the #3 plug is dry and looks new, is it starving for fuel (and therefore lubricant) and if so, is it at risk for a piston meltdown? If so, I won't try anything else until a mechanic looks at it.

2. Could it instead indicate a lack of spark in #3? I noticed that there seem to be issues with triggers/stators on these.


Any assistance you could provide would be great. Thanks!

 

[kend301]

Re: Nautica Rib Mercury Sportjet idles perfectly, bogs over 3K rpm; one plug dry, cle

One Carb ???? Not on a 90 hp Sport Jet , Is this a conversion ???

 

[iraqvet70]

Re: Nautica Rib Mercury Sportjet idles perfectly, bogs over 3K rpm; one plug dry, cle

One Carb ???? Not on a 90 hp Sport Jet , Is this a conversion ???

It seems to have been converted, and the carb seems to work fine. It's also converteid to premix. Is there a way to check for spark in the bottom cylinder?