I bought a used YS 120 engine. The guy I bought it from said it was running well, just hadn't used it for awhile. When I received it, I immediately disassembled it and ordered new gaskets, o-rings, bearings and piston ring. None of the old parts looked bad but I did not want to try my luck on them. I got my parts from Central Hobby (nice folks to deal with). They got me the right parts as well as identified my engine as an YS 120-NC. When I finished assembling the engine it turned over real smoth. I put it on the test stand with a 15-10 wood prop prop. As per instructions, I turned the prop over ten times with the throttle wide-open. Closed the throttle to an idle, put a few drops of fuel in the carb and proceeded to crank the engine by hand. It would only pop a few times so I backed the low-speed adjustment out a quarter turn (regulator) and the engine started promptly. After running for about 10-15 seconds, I heard a noise and the engine quit. Upon inspection, the prop had come loose. Retightened the prop and started it up again with no problem. Ran for about 10-15 seconds, sounded really nice, then again the noise and the prop is once again loose. I know it did not run in reverse as it blew my rag off of my bench, etc. Did this same scenario about 4 times and then decided to check on a few things. It still turns over real smoth. I noticed the ridges on the aluminum part that the prop goes up against were very smooth. Although this is my first YS, I have had many glow-engines, two strokes and four strokes, and the ridges were much more prominent on my other engines. Also when I received the engine, I got two of the same kind of prop nuts instead of the one with the indention and one with the protrusion to lock up against it, I got two of the prop nuts with the indention and was just locking these together after tightening the one against the prop.

Any suggestions would be appreciated.
My guess is that the ridges on the aluminum part are not digging into the prop like they should as the back side of the prop didn't have the normal pattern that my other props on other engines have had after tightening them down . I am also guessing that I need to get the other kind of lock nut that will go into the one that I have in order for it to truly lock intself down.
Thanks in advance for your help.

Roy Seals

Get a standard nut to back up the other one. Some guys even use a lock nut for the second nut. It is possible that the drive washer is worn but... there is not much there to start with (as far as the ribs goes).

Due to the normal crankshaft rotation, the prop nut is constantly being tightened with engine running---that's if everything is working right!---prop nuts loosen when the engine decelerates rapidly while the prop inertia breaks the nut loose---four stroke engines decelerate rapidly just from quick throttle shut-down---they also decelerate rapidly due to lean high speed fuel mixtures which cause detonation, a form of extremely advanced ignition timing that can cause an engine to stop abruptly---yours stopped, & the prop nuts were loose right?---if it looks like a gorilla & its eating bananas, its probably a gorilla---get the proper nuts, install them properly & set carb. high speed mixture correctly---hope this helps.......

Tande,
I set the high speed needle full closed then backed it out 2 full turns + a couple of clicks to be sure not to run it lean. This is what the instructions said to do.
Could I have the low-speed adjustment (regulator) to lean since I an starting it with the throttle just cracked open?

Lean fuel mixtures have lots of causes including, but not limited to adjustment settings---if your initial settings are as spec. by the mfg., then look elsewhere for the cause---restricted plumbing---air leaks both before & after the carb (fuel supply circuit & mixture supply circuit (intake manifold)--- fuel draw distance, both vertical & horiz. (not so critical with YS) . The YS uses a pressurized fuel system to deliver fuel from the tank to the regulator---the fuel tank is pressurized with engine crankcase pressure through a one way check/valve---be sure your lines are connected correctly & your tank is holding pressure---when you disconnect the check/valve after an engine run, you should hear a distinct pressurized air escape from the tank side---again, hope this helps.......

76.ta;
The NC is a nice beast of a motor and is quite easy mechanically compared to the other YS's. First off is the timing correct? With the piston at TDC, the cam lobe has a dot on it and this should be straight up! Secondly as tande mentioned, is your one way valve working properly and is you fuel and pressure lines correct? Sounds like the motor is starving for fuel. The YS line is notorious for throwing props if all is not well. More so than any other motor.
Start with the regulator flush and your hi speed setting was good at 2 turns out.

Something to check is to see if you put the backplate on correctly also. The crank has a key which fits into the fan on the backplate and this may not have been lined up(although yours is running 10-15 sec before she comes apart!). Air leak is also a cause. The ring on the forward portion of the crank was ok?

deputydog, tande
I rechecked the timing and all is well and the regulator is flush .The fuel tank is getting pressurized and I tried the check valve with my mouth and all is well.Also I rechecked to see if the backplate was on correctly and I am ok there. As far as the ring on the forward portion of the crank are you talking about the seal behind the front bearing?(Idid replace it)

Hi,
Wooden propellors compress very easily and this could cause loosening. An APC 15x10 is a good prop for the 120NC, or an APC 16x8 if running 30% nitro.

Mark

I don't know if you have an adjustable glow-driver? But you shoult try giving it a little less current. The glowdriver can also cause pre-detonation, which tends to let the engine run backwards and stop with that nasty sound. That usually throws the prop off.

Sounds like all is good without actually seeing the motor! So we are back to where you started concerning the prop, locknuts and backplate. Without purchasing those parts you can do what i've discussed in another thread. Take a piece of 220 grit sandpaper, fold and glue back to back using Super 77. Once this is dry, make a washer out of it and install between the backplate and the prop.

During this 10-15 sec run, did you advance the throttle? It may be too rich so close the hi end slightly. I've found that almost all of my YS's run around between 1 and 1 1/2 turns out.

To all the people who have been giving me suggestions.
I finally got a break in the weather here in Wisconsin and decided to give the 120 a try. I did the following as you all suggested.
I checked the camshaft timing and it was ok.
I checked the back plate to make sure that it was installed correctly and all was ok.
I reset the needle valve to full two turns from seated.
I installed sandpaper on the prop washers to help grip the wooden prop better.
I made sure the regulator was set flush to the housing, rechecked my valve adjustment, all was ok.
I put clamps on all the fuel lines at every connection to make sure the engine was not sucking air.
I checked the check valve and eveything was ok.
I rechecked all the lines per instructions to make sure I didnt hook up a fuel line wrong.

Turned the engine over by hand ten to fifteen times with the throttle wide open until I saw fuel going toward the carberator throught the fuel line.
Released the vent on the fuel tank to make sure it recieved some pressure and it did.
Cranked the engine over again by hand a few times, put a few drops of fuel in the carborator, hooked up my glow driver and proceded to try and start the engine by hand, after about 10 to 15 tries I grabbed my electric starter and the engine started firing.
With the glow plug attatched the engine was running but it was surging, picking up in rpm and slowing down in rpm over and over again.
Shut the engine off and backed the regulator skrew out one half turn.
Restarted the engine and I had no more surging and the engine ran smooth and sounded real nice.
After I let it warm up I went to advance the throttle and noticed a distinct sound, it was not a pinging or a clunk but somewhere in-between.
The noise started getting more frequent, then one loud one and the engine stopped abruptly and the prop spun loose. I know now that the reason the prop is coming loose is becasue as tandy suggested the motor is stopping abruptly and the prop is spinning itself loose.
Understand from the time the noise first started to the last noise was a matter of maybe three seconds.
Inspected the engine and all is ok. Started it again and the same thing happened.
Once again your suggestions are appreciated.

Please refrain from putting fuel in the carb. Let the engine do it! Turn your hi speed needle in 1/4 turn and start the motor. How long will it idle without touching anything? Advance the throttle just a couple clicks and let it sit for another minute. Advance another click or two. If and when you start to hear that unmistakeable noise, try to at first lean the motor hi end a click or two. If it gets worse then back it off 4 clicks. Continue with the adjustments just a couple clicks at a time.

Are you positive about your fuel lines? Looking at your motor from the front, the right nipple is pressure and the left nipple is fuel. If this does'nt get resolved, think about mailing the motor to me and I'll have a go.
Speaking of NC's! I traded mine to a buddy a few years ago and he just put it in a OMP 90 Sukhoi profile(or was it an Edge) and I flew it this past weekend. She was running perfect (I want it back....) This plane is a hoot!

deputydog,
I will give it another go and let you know.
Thanks for your efforts.
Roy

deputydog,
I put a couple of drops of fuel in the carb to start it because the instructions said to but I will not do it any more as you suggest. I looked at the fuel lines once again and all is well.
I will try your idea with the adjustments by thursday I hope.
thanks,
Roy

What %nitro are you running? You should be running a minimum of 15% preferrably 20% with 20% oil. Do you have an OS type F plug in it? If not, get one of those and install it. I have ran quite a few NCs and one of them I had to install an extra head gasket. I don't care what I did, it would kick the prop off. I lowered the compression and it stopped. Remember these are supercharged engines. High compression plus the boost can be bad! NC's had alot of carb problems. It could be that the hole in the butterfly is too small, but this mainly affects idle. There is not really and idle adjustment on these engines, and I have found that in hot weather, they tend to load up at idle and nothing will fix it. The large brass weight that is in the regulator housing is not needed either. That can be taken out and left out.

JeffH,
I have been using 10% nitro 18% oil. (keep in mind I have had alot of gas and glow engines but this is my frist YS and I am learning)
I understand why installing an extra head gasket will lower the compression and this would be helpful especially in a supercharged engine.
Could you please explain why higher nitro is prefered on the YS engines? I'm not doubting your word, in fact I believe I read this a while back before I got the YS, I just would like to know for my own curiosity.
Also when I rebuilt this engine I questioned what the little brass button was for in the regulator. I have no problem in taking it out like you suggest. I was just wondering again for my own curiosity what ist is for.
Thanks,
Roy

Based on your details, my first guess would be an air leak---maybe not correct, but the "Surging" you described is indicative of a lean mixture & it did respond to more fuel---for what its worth, try this---with engine running steady RPM's somewhere above idle, spray "Automotive starting fluid", on intake manifold connections,high speed needle & back plate to engine connection (not in carb air intake), if there is an air leak, the starting fluid will be drawn in & the RPM's will change---higher nitro % will only make a bad situation worse as it advances ignition timing even more---YS's do like high nitro, even up to 30% when all else is right!---keep us posted

Actually the higher nitro will retard the ignition! The YS's like higher nitro because it actually smooths them out. Whereas higher nitro on 2 strokes makes the needles touchy, it has the opposite effect on the YS. The needles become less touchy.

Jeff may have stumbled onto something. See if any friends have 30% heli fuel and borrow a tank or find a gallon of YS20/20 blend. Don't add anything to the fuel. Also if the age of the OS F plug is unknown then get a new one.
Let us know, Roy.

I put a new OS F plug in the motor and filled the tank with 20% nitro fuel.
I did not put any fuel in the carberator just hand spun the prop about 10 times, then hooked up my glow starter and fired the engine up with my electric starter.
After the engine ran for a short period of time (enough to get warm) I tried to play with the high speed needle setting to smooth the engine out some but did not notice a difference in the sound of the engine. I went from 2 turns out to 1 1/2 turns out.
Advanced the throttles slightly and "pop" there went the prop again.
Tightened up the prop, restarted the engine, advanced the throttle and the engine would pick up speed to a point but as I opened the throttle past that point it would start losing RPM rapidly.
Finally got the engine to run about mid throttle and started turning the needle valve in from 2 full turns out.
Up to 1 1/2 turns out the engine picked up RPM as I turned the needle in.
Tried to put a second copper washer on the glow plug as some one suggested this to lower compresion some but the motor would not start.
Took off one copper washer from the glow plug and the motor started up but again popped and threw the prop after a short period of time.
Decided to pull the motor apart to see how things looked but everything looked brand new and I inverted the head and filled it with fuel and the valves held with no leaks at all.

I have not tried doubling the head gasket to lower the compresion. (the one I have is .008 in. thick.
I have not tried spraying the motor with Automotive starting fluid while it is running to see if I have any air leaks. This is where things stand at this time.
Any more suggestions?
Thanks Roy

Go back to 1 3/4 turns out on the hi needle and turn your regulator in 1/4 turn.

Just wondering if you have yet tried a non-wooden propellor?

Did you replace the silicon regulator when you rebuilt the motor? When you put it together, did you use any petroleum based lubes? If you did, they might have harmed the reg. I have seen those regulators go bad, even though they are just a flat piece od silicone, they do go bad. I assume they get stiff. This may sound dumb, but are you letting the motor warm up? Every YS I have ever run will detonate when cold, some to the point of kicking the prop off. Let it idle for about a minute, esp on the first start of the day. get plenty of heat in the motor before trying to advance the throttle. As far as setting the needle, You need to set it rich enough so you can get the motor up to wide open throttle, then needle it to it best RPM. YS are not exactly quick to react to needle valve changes. Turn it a click or two, then wait 10 seconds. With the long path the air/fuel charge travels, it takes a bit for the needle changes to take effect.

Not once have I read the word BACKFIRE in this thread. That's what the POP sound you hear is.

When an engine sags or backfires as you advance the throttle, it's not getting enough fuel.

Put the engine back together with two head gaskets (from .008 to .016 thick) and started it up and the same thing, pop there goes the prop.
Rechecked the needel setting and it was two turns out and decided to back the regulator screw out (yes I put a new silicone regulator in when I rebuilt it.)


After backing the regulator screw out one and a half turns before being flush with the engine block the motor started and sounded good, it advanced to full throttle and responded to the needle settings. Needless to say I got all excited and put my tack on it to see what it was turning without being broke in and without leaning it out too much. With a zinger 16-6-10 it would turn 7400 RPM but know it will do much better when I have it broken in and leaned it out.
Shut the motor off using the throttle went inside to get my other tack (I never liked my tnc tack)and when I came back the carberator was full of fuel and it was overflowing and running down my test stand. Is it normal for YS's tank presure to force the carb full? I wouldnt think this is normal so now here we go for round two. What is causing this?
Thanks
Roy Seals[img][/img]

Thats the sign of a regulator problem! The reg housing could be 180 out. I remember you said you checked the spring so we can assume thats not the problem. You could have a pinhole in the diaphram. That would cause your problems also and seeing you richened the low end up a bunch....the loose diaphram would allow fuel to go into the carb!
You need to go back to flush on the regulator and whether you turn it in or out, you should only do it 1/16 to 1/8th at a time. But you are much closer to a diagnosis.