The S4's 447 engine got a bit uppity i.e. full throttle was O.K (luckily) but mid range from 5,000 t0 6,000 rpm indicating a big weakflat spot (or was it ?) on the first flight of the day, & the second....
The occasion of performing in front of fly-in guests was clearly too much for her. It eventually took the persistence of two knowledgeable visitors in a Kolb to discover the very lightest of a 'varnish like' coating on the carb. slide needle, which I'd never disturbed and was found at least to be correctly assembled with the plastic spacer underneath the slide spring. Cleaning off the needle made my third test flight of the day a dream with the '447' operating with Rolls Royce smoothness through all throttle settings.
However that was NOT the real cure & Rotax 447 hesitation persisted. At the strip fly-in recently, a visiting BMAA/PFA inspector suggested the small end Needle rollers cage could be breaking up !! These were fitted to earlier engines instead of the later but interchangeable crowded needle. [Luckily the answer was simpler, read on].
Hesitating engine at 5,000 t0 6,000 rpm returned this week, so I stayed close to the field. 'Cleaning' needle again in carb had no benefit. I bought & fitted new 4 circlip groove needle of same & correct # to the Manual part no. using the same second groove from te top as previous - though the old one had 3 grooves (?)- also a new needle jet and an unsquashed filter thimble to fit over the main jet holder in the float chamber.
The little "O" ring intended to prevent the needle chattering in the circlip was acquired & put on too.
The new needle & needle jet was the true answer & the engine has performed perfectly ever since. Inspection of the old needle shows they are made from soft metal, not steel, & easily worn in the middle 'cruise power' section.
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