View Full Version : OS 50 SX-H Hyper throttle binds-sticks-jams
After about 390 flights, the throttle in this engine binds, most noticeably while opening it beyond 3/4 throttle. Wiggle the throttle a bit and it will then operate smoothly. I've torn the carburetor down and cleaned it thoroughly, but no luck. I really don't think I'm over-torquing the Carburetor Retainer Assembly which is said to distort the carburetor body in other engines. The engine is in a TT Raptor 50V2. Magnum 20% fuel, flown about 3-4 days per week throughout the year. Perhaps this is not actually a problem in flight because vibration keeps everything shaken loose, but I've NEVER ignored a problem in a heli without it immediately spinning around and biting me on the ***. I'm in the process of flipping the throttle linkage around so that the throttle arm is pulled open rather than pushed open. The Owner's Instruction Manual recommends a push-pull dual linkage installation - perhaps that is worth a try as well? If I had to guess, I'd say that some part has worn, either Code No. 29083200 Carburetor Rotor or (gulp) 29083110 Carburetor Body, whichever is made of the softer metal. Have I missed something? Some wear pattern that needs to be knocked down with some wet-or-dry sand paper?
DavidH
06-27-2008, 10:00 AM
My guess is the retaining screw has worn the slot in the carb barrel. Seen this happen many times. Try replacing the screw first, then if that does not correct it. Then the barrel.
David
Ah-ha! Beautiful. That could explain it. Thanks for the reply. Parts are on order.
DavidH, as always, is correct. I could just replace the parts in question, but, shoot, that's no fun. So, as far as I can tell, here's "the rest of the story":
The 29083200 Carburetor Rotor is in the form of a tapered cylinder or a cone with the tip cut off. It fits into a complimentary recess in 29083110 Carburetor Body. As the Rotor is inserted into the Body, obviously the clearance between the two gets progressively smaller. Eventually metal meets metal and the Rotor jams in the Body, prohibiting the rotation that opens and closes the throttle. The extent of insertion needs to be "just right": not inserted too far or jamming occurs, but not inserted too loose or an air-tight seal (with the help of the oil in the fuel) does not form. The design feature that establishes this degree of insertion is the manner in which 5581820 Rotor Guide Screw rides in its groove in the Rotor Body - the side walls of this groove (which bear upon the Guide Screw) establish how far in and out of the Body the Rotor is maintained.
Nearly all of my flying is done at or near a hover (tiny flying field). Consequently, a remarkably distinct notch was worn into the outer wall of the groove right at half throttle. The Guide Screw caught in this notch which jammed the throttle. I'm not exactly sure why all of the wear was in the outer wall with no wear in the inner wall. Perhaps the Venturi-induced suction gets applied to the Rotor in such a way that the Rotor gets drawn into the Body. In any event, I tried filing down the outer wall to get rid of the notch, but metal-on-metal binding resulted. To re-establish proper clearance between the Rotor and the Body I fashioned a shim that was placed around the component of the 29083600 Mixture Control Valve Assembly that inerts into the Rotor. Specifically, I bought a stainless steel #10 washer at True Value. Two diameters were available; I used the smaller with dimensions 0.389" outer diameter and 0.037" thickness. Using a file and sand paper I thinned the washer to 0.021" thickness, the thickness that seemed "just right" as described above. So far, this has worked very well. Any comments?