Gareth Jones

Thanks to all those who offered me some advice on the above problem.  I thought the most likely cause was a fuel problem so I dismantled the needle valve and discovered a small amount of 'fluff' in the valve.  This prompted me to remove the cowling over the tank so I could remove the tank and flush it out.  This revealed that the clunk was positioned right at the back of the tank.  With the fuel pick up pipe lying straight backwards the clunk inlet was almost flush with the rear wall of the tank.  Having rinsed the tank and repositioned the clunk about 5 mm from the back wall all is now well again and I flew the aircraft a couple of times at the weekend with no problems in achieving full power.  Thanks again everyone.

I have a Multiplex Skycat fitted with an OS25 FP engine.  My son and a friend bought it at the Nationals  swapmeet 2 years ago so the engine's history is unknown.  We have flown it a number of times and the engine has always started easily and initially ran well.  Lately however it has been difficult to get it to run at full power.   It runs well up to about half throttle but beyond that, although it initially seems to reach full power, after a few seconds or a minute or so, it will gradually reduce rpm and eventually die.   I have tried adjusting the needle valve,  and air bleed valve  but to no avail.  At the moment, the best setting seems to be with the needle valve about two turns open and the air bleed  set with the screw end just covering about half the hole in the vent.   I cannot see any bubbles in the fuel lines and the tank is pressurised by a connection on to the exhaust silencer.  Does anyone have any ideas what the problem is and what the solution might be?