YS 120 4S - RCCanada - Canada Radio Controlled Hobby Forum
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Old 10-03-2015, 06:24 PM   #1
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YS 120 4S

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I acquired one in pieces here on RCC and when I went to put it together found the crank was bent in a spiral pattern. Would likely have taken a lot of work to straighten, so posted a request for ident on this one as there are a few versions and was not sure which would fit.

To be honest I was surprised by both the short span before replies, and the number of suggestions and offers. Gotta love this group.

Anyway, I was able to locate what I needed to put the original 4S together. In fact, I got a box full of goodies, cases, cranks, bits and pieces, to allow for a second engine to bolt together and still have lots of spares. Hoo Doggy! I think I will stay with the original dished pistons for now just to see how they run.

I had to cobble together some M8 X 1 prop lock nuts and washers. Those seemed to be MIA. Also threaded one of my spare alloy Higleys nuts to be able to start it with the cone.

NOTE: There was a rather rough side mount Pitts-style muffler in the first box and nothing at all in the box of spares so I went hunting. Seems the stock exhaust from my Saito 125a's is a direct fit.

I have my test stand nearly complete, fabbing a holder for the fuel tank the only remaining item. Was able to fire up both 120's on 15% nitro/ 19% synth and the first went nearly perfectly. Started no problem and only required 1/4 turn in on the regulator to have it idling smoothly. Slight HS adjustment a bit rich of peak and it ran through the tank no problem.

That's the fun bit. It went dry at high throttle and away flew the prop, lock washer and the Higleys nut I had on for starting. Seems similar to the Saitos. Found everything but the Higleys and will try again with more light. Unfortunately I had a look today and it nicked the back of the nice new Graupner 16/8. Hope I can dress that out.

Had quite a lot of oil coming from the front that eventually stopped and I can only assume it was what was in the case when I prelubed during assembly. Also saw bubbles in the line to the carb at low throttle, but that did not seem to accumulate much, nor affect transition. Considering the tank is wire tied to the stand there may be some agitation. The tank holder will be padded and slightly higher to be in line with carbs on standard engines, so may make a difference.

Second engine was more problematic. Still fired up but idle was rich and no adjustment of the regulator had any effect. Still peaked out pretty well and would continue to idle despite this. Another little note was when I pulled the line to defuel the tank held no pressure. After hunting for the Higleys it was getting a bit dark so did not check for bubbles on the carb line.

So, all told they both perform strongly with no surging at any throttle setting, but have to state they are not at all miserly on fuel. Estimate the fuel burn at twice what the Saito 125 sucks up. And, considering what happens at full if the tank empties in flight I can expect to go looking for new bits.

Talked to my friend that used to run these on pattern and he recommends changing the glow plug on that second engine and checking the diaphragm again. Unfortunately I could not find my tach to check the numbers.

Have quite a few OS R plugs and swapped one out. Will have a boo at the valve cover as I noticed some very slight bubbling from the left rear on that second engine. I know the AC version 91 I sold quite a while ago had pressurized this area and had holes drilled around the intake valve. Neither of these had that.
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Old 10-04-2015, 12:36 PM   #2
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Re: YS 120 4S

I do not have a lot of experience with these engines, but the ones I have run it seemed to take nearly a full tank of fuel before everything started to operate properly. On one engine it was the second tank on the next day before it operated properly. I really liked the three needle heli engine. I had one on my pattern ship. I hear that fuel with all synthetic oil makes them a lot more friendly.

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Old 10-04-2015, 02:26 PM   #3
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Re: YS 120 4S

I had the chance to deal with the regulator today; there was no contamination I could see and the diaphragm appears fine. However, found the brass piston would not provide any pressure lift, even with the screw turned in a lot, (pulled it all apart and could find no signs of anything amiss). For some reason it worked fine with one from a spare reg body, even though the lengths seemed identical. I swapped the entire unit and will try again when I'm not so tired.

Also pulled the valve cover and worked it on a flat surface. Took quite a bit to get the flange surface consistent. Hopefully it will seal better now.
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