G'day all!
I have an S.C. (same as A.S.P. or Magnum) FS400AR 5 cylinder radial engine that I bought fitted to a rather nice 1/3 scale Balsa U.S.A. "Sopwith Pup". It has sat unused for a number of years and was kinda gummed up, had valves stuck open etc so I stripped it down as far as I needed to and replaced all of the bearings, freed up the stuck valves and gave it a general thorough clean up.
Now the thing is, while this engine starts and idles just fine it's top rpm is a mere 4500rpm! This is in stark contrast to the published 7000+/- rpm that it should do. Because it does start and idle so well I assume I have re-located the timing gears correctly? Why else would this engine be down on rpm so much? What have I done wrong maybe? The (much trusted) previous owner assures me that this engine used to pull this plane around just fine but now it can hardly get off the ground.
I am running 5x new O.S. type "F" plugs in it, a brand new 42% products on board glow, a Keleo collector ring exhaust (Previous owner did not have this), 20% coolpower with 10% nitro and a wooden scimitar Master Airscrew 22x12 prop. This combination is exactly what the previous owner ran (except for the glow driver, his was home-made and the exhaust, he instead had 5x long single pipes routed to the bottom of the cowling).
According to the manual the biggest prop this engine should turn is a 22x8. I tried this with a wooden Turnigy 22x8 and the best rpm I got was 4500 but it had so little thrust that it would not take off. I went back to the 22x12 and it had sufficient thrust to take off at 4100 rpm. The engine then got warmer, sagged and stopped. I thought it was maybe the warm day and the cowling now letting enough cooling through but I have since had this happen at home with the cowling removed and on a much cooler day. Also the engine idles great and when the throttle is advanced to full throttle the revs do get as high as 5200 but again as the engine warms as this throttle setting is held it sags back to 4500 at best. I thought that maybe the valve clearances were wrong so I re-checked them and opened them out to .13mm - it has made now difference. Also the main needle is not at all responsive. It can be screwed it or out one full turn while the engine is running and it makes very little difference to the engine. I replaced the main needle "o"-ring with a new one. I have also cleaned out the carby again, tried the engine without using exaust pressure to the tank (minor rpm increase when retuned this way) and done everything else I can think of to get this engine running properly.
Any ideas? It's driving me insane!
James Frater.