2001-90hp low speed miss.

Engine misses when accelerating from about 1200 rpm to 3000 rpm. This only happens after boat is is driven at low speed [1200 rpm or lower] for 5 to 10 minutes. Miss clears up slowly after about 300 yards of travel. I have had this trouble since the motor was new. Here is what I have done so far, spark plugs,stator,CDM,spark output [spark jumped over 1/4" when motor cranking] ajusted carbs and timing to Merc. specs,oil injection ajusted to specs, rebuilt fuel pump,rebuilt carbs,replaced 4 check valves,glass inline fuel filter,[no airbubbles in gas],no check valve in gas tank. Boat is a 20' Maritime Skiff with a 17" prop. ALL THOUGHTS ARE WELCOM, BOB

Re: 2001-90hp low speed miss.

That is a big boat for a 90 hp. The gearbox is a 2.3 ratio so the 17P helps. You're describing what to me is the plugs loading up (fouling out) from low speed operation. What may help is using a conventional plug rather than the surface gap.

You might ding Silvertip about the conventional NGK plugs he recommended to me and I have since installed. I don't have my service manual handy or I would tell you the number. I noticed that they are the same ones the Merc Jet of that year uses in that powerhead (use surface gaps in the regular prop drive engine); hence they are listed in the service manual. They work fine for me.

The other thing is that you may have a lazy CDM on one of your cylinders. I had one that was thermally sensitive and replacing it helped what you are talking about. Since changing that and the plug type my problems have vanished.

Tilting the engine up in the rear could help also when you are putting along. Apparently at low rpm's fuel can puddle at the rear of the cylinder and help to cause fouled plugs.

Mark

Re: 2001-90hp low speed miss.

I've found that the 75 (and probably the 90 also) are quite sensitive to idle mixture adjustment. So before doing anything else, make sure you have that right. If you promise to read the plugs frequently until you are sure they are ok, try NGK BP8H-N-10 plugs gapped at .030 - .035-in. These are the same plugs used in the 115 2-stroke.

Re: 2001-90hp low speed miss.

It amazes me that I have spent countless hours thinking about this trouble and it never entered my mind to get rid of the surface gap plugs. As for the CDMS I did buy a new one and swaped them out one at a time. It did not help. All HELP WELCOME BOB

Re: 2001-90hp low speed miss.

The boat is a skiff. It only weighs 1850 lbs. Max hp for boat is 115 hp. BOB