Saving Old Seagulls

(I tried submitting this last night but my computer started playing up and I doubt if it got through.)
My Forty Plus (SJP 330 K5) has a healthy compression; the plug (Champion D16 or NGK A-6) has a 20 thou gap; points set at 20 thou; 1:10 fuel mix (unleaded 93 octane, TC - W3 oil). It seldom needs more than two pulls to start and then idles normally and only stops when killed with the throttle control. With the choke open and at full throttle the revs rise slightly and then the motor starts stuttering and spluttering with the revs falling and rising slightly but never to near full revs. This is with the motor out of water. (Not more than 30 seconds at a time) In the water it is also very much down on power.
Help will be sincerely appreciated

Hi, Check the carby bowl for fuel contamination. Could have water in there...

Check the magneto base plate is in the right place & is not loose.

Pieter S wrote:.... stuttering and spluttering with the revs falling and rising slightly but never to near full revs..........

That really sounds like fuel starvation with revs picking up slightly before cut out; just like when running out of fuel. Agree with Hugo but would also check cleanliness all the way from tank through to jet by way of tank and carb banjo filters. Is the fuel level in carb correct ? Will it flood via carb tickler? (clean out float valve seat) If all else looks clean strip and blow out the carb jet.

Peter

yup, I'm with 40TPI

meticulously clean fuel tramsmission. With new acquisitions I have the tank off, the tap out, scour out the inside of the tank & thorughly rinse it, then work through the tap filter, fuel line, all filters, strip and clean the carb ..all blown out with the airline ...then you know that that's all OK

I have experienced the situation that Charles P describes and how wonderful it is once that screw is tightened ...S'all pretty logical really

I'm with the guys above, I too have suffered from the Loose backplate blues, boy what a difference when its right!

Would just add that all the above is by far the most lightly but if it still fails suggest you look for an air leak into the crankcase.