Hey guys,

Been running my 120-SC for about 4 years now. Its the most powerful 120 i've seen. 2 weeks ago the egine retainer pin broke which holds on the disc vavle for the supercharger. Called Richard up at YS and he sent me a whole new backplate Ass. for the 120-SC/NC. I also ordered a new ring for the piston aswell, (sleeve and piston are perfect.) Now after reassembling the engine it runs perfect on high end, and half throattle but when I go from idel to 1/4 thrott. after about 4 seconds of 1/4 thrott it just dies. Vavle are good, all gaskets are good, no air leaks and I have lots of pressure, when it dies, fuel just poors out the carb, seems like its rich to me, also I know its not the regulator because I changed it aswell and it did not make a difference, pulled it off another 120. Also whats settings are you running on your sc?

Thanks,
Justin Tidwell
Skymac R/C

The plunger is not closing completely. It may have a small piece of debris.

Sounds like the regulator housing is mounted backwards. Take it off and turn it 180 degrees.
Fuel is going on the wrong side of the plunger. will run at high end and pour fuel out at idle. In a inverted install at least.
Let me know
Brad

well to tell you the truth, im clueless to "what" it "was" keep foolin around and like the snap of the finger it sarted running right, im stumped, lol. But like always before the rebuild and after the ol' SC turns a little over 10 thousand R's on the ground with a 17x6 APC prop, just where I like it. Kills my heart to see these engine run at anything under 9,300 or so.

Glad to see you worked it out.

Never Under 9300 rpm????????? I have never heard or seen anyone run a YS big block over 9000RPM before. Does it last like that? The SC was notorious for failures. Maybe you found the answer in using very little pitch. They were of course designed for big pitch numbers?
Maybe SC troubles could have been related to load on the engine?

Great engine for power, I flew one myself 6 years ago. Had the power of a 140 but had to be run very rich or you had Melt down. Many did meltdown, I heard about several supercharger failures. One was a buddy of mines. Never heard a reason for the failures? Any one know why?

Brad

Well, Heres the story:

The motor was bought used from a fellow club member 4 1/2 years ago, and it had under 20 runs in his hand but he never could get it to run right, so it was sold to me for $250. I put it on a biplane similair to a dave patrick (www.barnestormermodels.com) but better when it comes to scaleness and flying IMHO. I ran a 16x6 Master Airscrew prop on this engine for about 30 flights taching around 10.8 on the ground, after around the 30th flight a vavle tappet broke, nothing horrible, a .90 cent part. fixed that and continued to run the prop for around 10 or so more flights, then I switched to a 18x6 carbon fiber and turned around 9,300rpms, after flying that for around 50 flights I finally nicked it so I then switched to a 17x6 APC and flew it for about 30 flights on the same plane. I then sold the ultimate back to one the the creators of it in trade for a nib Blue/White cermark pitts, flew the pitts twice and noticed a drop in Rpms about 1K. What had happened was the left handed threaded screw that holds on the pump valve had broke and was ever so slighty grinding into the con rod. So I called richard up and he sent me a whole backplate ass. for a fz for half price and i also got a piston ring, replaceing the factory one. Everything else in the engine is the OEM parts, even the con-rod, with the "lighting done by the screw." This engine still produces almost 1K rpms more than a brand new fz with the same prop and fuel, and like you said, this things feels and sounds just like a 140.

wow....that is a keeper. that power output is huge!!
Just a little observation. When you switched to smaller props it broke both times according to your explanation?
Maybe it likes the slower speed?
brad

well I dont really think it broke becasue of the 16x6 other than just plain use, then with the 18x6 it broke but, were not talking piston of sleeve or rod, were talking a dollar part each time, and the second breakage just happened 3 weeks ago whic was almost 4 years after the tappit.

You may be right but if you look at your sequence of events again you will see a pattern. Remove load and something breaks? Resonance vibration can do amazing things. All engines are happiest at different RPMS. the cost of the part i don't think is important. Parts never fail without a reason....never.
I may be wrong but it look odd to me?
still an awesome power plant though!
Brad

well, the second part only broke due to just a weak screw, the wrec has no stress on it what so ever and the head just snapped off of it, im think after it idealed real low for around thirty seconds it switched and ran backwards about 5 seconds before it quick, and I think that is what stressed the head on the left handed screw to snap off. you can see on the little illistration on how the con-rod looks now after the screw ate into it a little, took a polished it out but you can still see where the mark was it you look real hard.