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08-06-2018 06:27 PM  9 days agoPost 1
zonker

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South Jordan, UT USA

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I picked up a used YS 120SR engine that I'm doing a complete rebuild on. All my experience has been with OS engines. I've read on the forum that the YS 120sr works best with 15% & 20% fuel. Is that still the case (not sure why it would change but I thought I'd draw on everyone's experience....)? I also read an earlier post and it sounded like I should start with 3 turns out and go toward lean from there. Did I get that right? Thanks in advance.

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08-06-2018 09:37 PM  9 days agoPost 2
ticedoff8

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Morgan Hill, CA. USA

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zonker
I've read on the forum that the YS 120sr works best with 15% & 20% fuel. Is that still the case (not sure why it would change but I thought I'd draw on everyone's experience....)? I also read an earlier post and it sounded like I should start with 3 clicks out and go toward lean from there.
The 120SR is a vibration monster.
Make sure you never exceed 16,000 RPM; otherwise, bad things happen

The manual is here:
http://www.ysengines.us/manuals/man...ld_us/120sr.pdf

I run my YS 120SR-X on 22% nitro.
The manual says the 120SR is spec'ed to run on 15% to 30%.
I have not heard / read anything about it "works best" at 15% to 20%.
It would be cheaper to run 15% (or 22%) than 30%.
But, I ran my YS 120SR-X on 30% for 2 gal and added a shim (just in case).
I could see heat being a problem at 30%.
But I still run a colder plug, too. Just in case.

There are 3 needles on the 120SR engine
The manual says mid & HS should be 2.5 turns out (say as the 120SR-X and many of the 91).

Set the idle first.
And try to get the idle as lean as possible (1/16 of a turn and then close the throttle a little. Repeat until the engine starts to do the ring-ding-ding and surges. Then enrich the needle 1/16 of a turn and open the throttle a little).
The pinch test is a good indicator that the idle is too rich.

Set the highspeed next.
Sure, 3 turns is OK (NOT 3 CLICKS).
I believe the manual says 2 turns.
But at 3 turns, you wont really be able to buildup any heat.
But, lean the HS needle no more than 2 clicks at a time.
And only adjust the HS needle after you run the engine under a heavy load for at least a 5-count. Then land and adjust.

Then adjust the mid needle.
The mid needle on the YS actually has an affect; whereas, the OS's mid needle didn't do too much.
The OS was more like a 2-needle carb and easier to get it "close enough".
After you get the idle and the HS set, then do a full power climb and then instantly transition to a hover.
Big puff of smoke = too rich. No smoke = too lean

Believe 1/2 of what you see and none of what you hear.
Fake News will be the downfall of our Republic!

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08-06-2018 10:26 PM  9 days agoPost 3
zonker

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South Jordan, UT USA

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Thank you for the detailed explanation. I meant 3 turns, sorry I wrote 3 clicks. LOL
ticedoff8
Set the idle first.
And try to get the idle as lean as possible (1/16 of a turn and then close the throttle a little. Repeat until the engine starts to do the ring-ding-ding and surges. Then enrich the needle 1/16 of a turn and open the throttle a little).
The pinch test is a good indicator that the idle is too rich.
So like on an OS the idle needle is the one you adjust with a flat head screw driver? Right? Is that needle you are referring too?

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08-07-2018 03:15 AM  8 days agoPost 4
co_rotorhead

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Centennial, CO, USA

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ticedoff8
Big puff of smoke = too rich. No smoke = too lean
Be careful with this with the YS engines. I found with my 90SR that the engine can be running lean and still put out would you normally think is a "healthy" amount of smoke.

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08-07-2018 06:30 AM  8 days agoPost 5
ticedoff8

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Morgan Hill, CA. USA

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like on an OS the idle needle is the one you adjust with a flat head screw driver?
Yes.
But unlike the OS (+/- 45* max), the YS idle needle can make multiple turns in or out.
Personally, I start with the factory recommended and then make 1/16 or a turn (about the width of the slot in the needle) and then adjust the carb to match. Keep doing that until you have a nice, lean, idle.
It is almost impossible to hurt a motor with a lean idle.

You need to tune by ear and by the smoke trail.
That's why you start rich and then "find lean".
DO NOT try to tune the HS needle without flying at WOT for a 5-count.
If, during the 1st tuning flight, you are not rich, land right away and open the HS needle by 1/2 a turn.
It has to be rich, so you can find the rich-side of lean.
After you land, lean it by 2 clicks and fly it again. You can hear the engine go from "not happy" to "happy".
You are getting close to the right setting at WOT (wide open throttle).

When you hear "happy", you will also see the heli flying strong and steady with a good smoke trail.

Believe 1/2 of what you see and none of what you hear.
Fake News will be the downfall of our Republic!

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08-07-2018 01:09 PM  8 days agoPost 6
Richardmid1

rrProfessor

Leeds, England

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Heard it's a vibration monster also and that due to it being so large the cylinder wall thickness is thinner than the 91 so can overheat.

By sure to run an Enya #5 plug NOT an os #8.

Adding an extra shim to the head could make it run smoother if you are having vibe issues.

60% of the time, it works every time!

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08-07-2018 05:12 PM  8 days agoPost 7
ticedoff8

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Morgan Hill, CA. USA

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On the 120SR, the vibration issue is related to the (largely) unbalanced crankshaft, not so-much the combustion.
The counterweight on the big-end is simply machined as it has been done since the beginning of time.
The assumption is that the metal has a certain density and when it is machined to a certain size, it will be the correct counterweight with no further balancing from the factory.
That didn't work out so well for the 1.20cu size.

The 120SR-X (and the 96SR-X) have a crankshaft counterweight that is tested and trimmed on a rotating balancing machine as the 2nd stage of manufacturing.
They add slugs into drilled holes to perfect the crankshaft's balance before final assembly.

Believe 1/2 of what you see and none of what you hear.
Fake News will be the downfall of our Republic!

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08-08-2018 02:14 AM  7 days agoPost 8
zonker

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South Jordan, UT USA

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Does anyone know if the crankshaft for the 120srx will fit in the sr?

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08-08-2018 03:14 AM  7 days agoPost 9
dbcaster

rrApprentice

Petaluma, Ca

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Yes,it will fit.

Some arguments are sound and nothing more

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