Dave ;

I've got a 63 (less than a year old, probably more like 6 mo., but have only 2 gall. through it) that has a rat-tat-tat sound to it when full (or close to full) throttle. I posted a ? a while back on the fun-fly forum, and heard that it might be lifter noise. A guy at the field said over the weekend, it sounds like pre-ignition! I tried a couple tanks of 15% instead of the 30% wildcat heli fuel I normally use. This seemed to quiet it a bit, but did not eliminate it. I'm nervous to run it anymore so as not damage it (if damage hasn't already been done). I know 30% fuel has been recomended on this site, and have not run it too lean. With this particular engine, though, I tried following proper break-in procedures, but to be totally honest, on the first few attempts, I bumped the starter to the prop, and BAM!!!!!, it seemed to backfire, bigtime. On closer examination, The plastic spinner, which had about 1/6" clearance (or slightly less, but was not touching sitting idle; don't read idling) from the fuse (funfly/ profile), during windup, made contact with the fuse, causing (or what I believed caused) resistance to the starter motor. (Now people reading this may say "why did you just admit that?" ; here's why > I TRY to be honest, and LOVE this motor and want to get to the bottom of this unnerving sound so I feel comfortable flying it again SOON!) I know it resisted, because the spinner backplate spun, popping the cone off, and fused to the propeller.

So............do I fool around ordering shims for the head (which I've never read to be neccesary), or do I send the motor in to get looked at, to see if the "backfire" caused some slight issues with conn. rod/timming/lifter/I don't know, problems. It seems to run great otherwise! And if so, would the warranty cover it, how long would it take, and where would I send it. I bought it from central hobbies (hope I can find the reciept!)

Please help, I need to get back to practicing prop hangin', Something this engine is real good at!

Check the valves gap. Should be .003" on both. Then go fly it and don't worry. Also don't listen to everyone's opinion, as most are wrong.

I adjusted the gap after about 1 gall., I'll check it again.


I don't mean to second guess you, and I will go fly it, it just seems to ME that that rat-a-tat sound, kinda like an A.K. 47 (above the cool 4-stroke sound) seems abnormal, I don't hear this with other 4's at the field (comparing to saito, I haven't heard another YS run). But if your telling me this is a normal sound for a ys engine to make, and isn't harming it, I'll try getting used to it and keep flying. My long story above was motivated by wondering (an idea I formulated, after hearing the spark knock scenario) if a slight bend in the conn. rod (or lifters) could have messed with timming, causing pre-igniton.


Like I said, I'll quit worrying about it, unles you tell me otherwise......again.