Hello Troy I decided to use the SC 120 ,but since it had sat around for some time I thought maybe I would change the gaskets ,O-rings and rebuild the regulator and adjust the valves ,fairly simple stuff to do. All was going quite well untill I got to the reg ,Since I have a ton of extra new plungers and springs and a diaphrams this should have been a no brainer. After I got it back together I always check to see if theres spring tension and there was none ,so I turned it in a little more and ,then back it off some still nothing .So I took it back apart and set each piece next to each of the old ones ,just by looking they all seemed to be ok. So I put it back together and still nothing. I thought maybe the hole in the stopper might have been drilled to deep,cast however they build them and that wasn't the problem at all. It turned out the plunger itself was shorter by about the thickness of the head and was bottoming out rather than letting it have a spring load .So I just used the old one and things was fine. According to central all the 120's take the same part no. Could I have just got one that was made wrong or is there somthing else going on here I'm not aware of.

Another item ,none of my other 120's have what they call the regulator ballast and the SC is the only one that shows it being in there. Is this to stop some of the pressure because of the higher compression of this particular engine or is there another reason .

OK one last thing and I will leave you alone you made mention of using a smaller prop on the SC when itis capable of putting out the power of a 140, All of my 120's run a 16X8 APC and are happy doing so. Why the smaller prop for the SC ,wouldn't this be taking a chance of over redlining the engine and floating a valve ???
Sorry to bug you but I appreciate your knowledge on these matters.

Not sure on the plunger. Perhaps that is the plunger froma 53 or a 91 or something. I don't really know the differences on those parts as I don't deal with them ona daily basis. I have found it really rare to ever have a problem with the plunger. Usually its the diaphram or the gasket.

The regulator Ballast was a interesting little item. I have about 4 of them sitting here in my shop. The purpose was to keep the engine from loading up on really long term idle. YS engines were built for Pattern planes. When a pattern plane enters a spin sometimes it may take a long time at idle maybe 30secs or more that the engine will be sitting at idle, and once the spin is over you want the same instant throttle pick you always had. On some of the engines especially those without the low speed airbleed on the carb...the regulator was the only adjustment. So to have it rich enough for a good transition sometimes a really long idle during the entry into and the spin during the pattern sequence the engine could get a little rich and cool off...this could result in a dead stick as the extra fuel would put the fire out.

The ballast just slows the action of the regulator almost like putting it on a time delay. The result was during the pin, especially inverted spins the ballast kept the excess fuel from building up. Once the regualtor would open this big gulp of fuel could put the glow plug out, or cause it to sag, surge or numerous other things. This didn't work ina pattern environment and the ballast came in the 140FZ's but I didn't know they were in the SC's

Anycase...the Ballast doesn't do anything. You can remove it and not know the difference or you can re-install it and not know the difference. Only if you are in that exact pattern spin situation would you ever see the problem the ballast was intended to solve. And even then, you would probably just question...why is there a little hickup when it comes out of the spin. It does it every time...and I think the mixture is set correctly but it has a hickup. Why? The answer is the Ballast will stop that hickup. 99.99999% of the guys flying the engine didn't even realize what was happening, and only a few of the top pilots in the world got Yamada to make the change. It worked for that instance....the best solution was the air bleed screw on the carb.


As far as redline RPMS....at 8800 you are not even close. I said to keep the RPMS up to the 8600-8800 range. You will not have problems floating valve until well above that. The reason I say the higher rpms on the 120SC is because it has the higher compression than the others. So when loaded down it will be less forgiving. Letting it spin means it won't work as hard, and if you by chance run it out of fuel by accident it will likely not blow up.

I saw a 120SC that was loaded way down like a 15.5-12.5W prop on 20% nitro. The guy was practicing for a contest on Friday night. He ran it bone dry. When the engine backfired the rod broke, the crank then came around and pushed the rod through the side of the case. Literally it did the Dragster 300mph explosion. This motor was only turning about 7700 rpm at full power. It was loaded down and when it when lean it exploded. Who knows if this was a motor that was abused or if it was the first time the motor ever was run lean, I doubt it. But the guy operating it was just puzzled....It blew up. Must have beena manufacturer defect. Reality was there was not one little drop of fuel left in the tank, and it was humping full power under load, when it backfired.

So my advice is intended to keep the engine at the good RPM range. All the 120's should be run 8400-8800 and they will be happy. You can press your luck and go down to the 8200 but I would not press my luck on the 120SC. I would keep the SC at the upper end of this RPM range. I have seen 120SC's running up above 9500rpm and they take it....but this is pushing your luck in the other direction.
It is basically a 140 inside the smaller box....so pushing the limits the results will be the engine will not act as docile as say the 120NC might in the same instance. Its a supped up hopped up version of the 120NC...so its going to be harder on parts when its abused. Don't put it in a position to be abused. Too much load not enough nitro is abusing them.

Troy

Thanks again troy I understand the the load factor better now ,I try to get all of my 120's to the 8900-9,000 mark with a APC 16x8 W/O running them lean. So from what your saying I should be in good shape running the SC in that area. I seldom run flat out with the exception of going straight up, but even that is a very short burst. Since these SC engines are hard to come across period, let alone one thats in good shape, the last thing I want to do is to tear it up, by doing something I shouldn't.

Again thank you for the fast response and information.
Happt Landings Richard.

I fired the 120 SC up and all I can say that thing is a ANIMAL!!!!!!!!!!!!!!!!!! Talk about some awesome power,and all I had was YS 20/20 fuel I can't imagine what it would do with 30 % in it . As it is now it will hold a 1,650 idle all day long, at 4 K its a tad rich, regualtor I suspect and wide open was just a short burst just long enough to get a reading 9480 and it's still a little bit fat. So I guess my 16x8 is going south for that engine. I think a 16x10 will be just about perfect for it.

Thanks again for all your help
Richard

Hi Is a 4 stroke the one you are talking about?, I have a 71 inch egde 540,it was fitted with a 120 TT pro RP, it was climbing vertical like rocket,I had the TT second hand and it was ratteling a bit so I thought the bushes were worn out, so I got miself a brand new 2 stroke SC 120, I tried it and I am very desappointed with the , power I ran 4 tanks allready trough, and still wanting to overheat a full power (revs drops) with a 16X6 apc (the same I used on the TT pro)so I still running it rich, flying does the same, flyes ok for the first 3 minutes, then the power drops, I dont know if is because still stiff, or because the engine is not good, in the ground gives around 8500 revs with the 16X6 which are ok, I dont know!!, andddd the rattle still there (is not the exhasut or the frame), I guess bigger engines do have more play on the bushing thant smaller ones!!!??. any sugesstions??

I don't know what engine you are talking about, the engine I have and was talking about is a 4 stroke YS 120 SC.

Some more info would be a big help here to maybe help you deal with whats going on with your engine. This much I can tell for sure one of these items or more is your problem. Its to lean, wrong plug, fuel or over propped or even a air leak for it to be running hot.
For a engine to drop RPM's when running to hot is normal we call it sag and if you allow this to happen very often you will ruin the P&L in the engine.

Engine type and brand name
fuel and plug
Prop size and type

Is a new SC (super custom) 120 Ringed, by the manual the recomended prop is a 16X8 I am using a 16X6 APC as I was with the TT120 PRO, plug is an OS no 8, running it in my garden it drops 8500/9000 with the 16X6 revs but if you hold the throttle for a bit it gets hot. I could try as supose with a 16X5.5 or a 15X7, but seems to small for it, thanks for your time!

http://www.galaxymodels.co.uk/detail.asp?id=20402#lift
This is the engine, one more thing out of the box idle speed was perfect, 1900 revs, I had it running for 45 mins without stalling and the cylinder head was at 60 degrees celcius, there is plenty of oil and excess of fuel if I open the screw all the way, plenty of smoke

Smoke really doesn't mean a thing , its a vapor not smoke you see or had better be. A sure way to tell if your rich enough is to use a tach and back the engine off a few 100 RPM's after its peaked out. Syn oil won't give you as much vapor as say a castor based fuel will so you see less in the air.
As far as your engine goes, sorry but I am not at all familar with that brand of engine.
The SC I was talking about here was a YS 120 SC (Super Charged) 4 stroke thats no longer made.

The amount of Nitro in your fuel can cause a engine to overheat also, what fuel does the manual say to use? as a rule you can go by what the manufacture has to say, they made it and should know what it needs to keep it happy.
This covers glow plugs ,fuel, prop size and so forth. Good luck with it.