Maintaining and Running YS Engines

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Engine Break-in    YS 170  - Troy Newman NEW!
Adjusting YS 4 Stroke Engines  -Rick Mattie   
YS91/120 Standard Muffler Installation  
Maintaining and Running YS Engines - Mike McCormick   


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Some of the discussions on this page came from the NSRCA List Discussions

Engine Break-in  -  From the Web

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checkmrk.jpg (769 bytes)  This deals with the YS91AC but applies to all the YS engines.  Run the motor rich to start for the first 2 tanks on the bench or on the ground.    PowerMaster 20/20 imperfect fuel, and use a OS Type F plug. The 14x10 APC is fine, your rpm range should be in the 8700-9200 range, adjust your prop to your flying style.  Don't load down the 91AC,  let it run in the low 9000's.  Don't touch the regulator until you get a good 1.5 gallons through it,  just enough to get close to getting it broken in. When you do run it, start the high speed needle at about 2 1/2 turns out and let it run rich, it should produce a lot of smoke.  Run it rich for 2 or 3 tanks, allowing the motor to cool down between runs. Do not put a spinner on the motor until you get it broken in. The 91AC's have high compression and it will mangle an aluminum spinner very easily when and if it kicks back.   After each run, check the bolts for looseness, especially the muffler.  Good luck,  Evan Chapkis

checkmrk.jpg (769 bytes)  The YS engines require that the instructions be followed in the manual.  All my engines have been broken in in the following manner

  1. Fill the tank with your contest fuel and put on your contest prop.
  2. Set needle to whatever it says in the instructions
  3. Start the engine and set the needle a little to the rich side, and let it run a minute or so to see if everything sounds OK.
  4. Fly several tank-fulls this way (set a bit rich) and you're done.
  5. Then, set the needle wherever you want it for good. 

Under no circumstances should you bench-run a YS for extended periods because the cooling just isn't the same. In 25 years, the only engines that didn't run right were those that I "broke in" on the bench!   John Ford  NSRCA 1673

checkmrk.jpg (769 bytes)  Amen to that, John !!!  My experience EXACTLY.....YS's seem to actually break in better on the plane, with a lot of throttling, and only a slightly rich setting....never has been a problem.   Bob Pastorello

Understanding the Fuel System 
All YS 4 Cycle Engines, FZ53 to FZ140
by Rick Mattie

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As you know the YS 4 cycle engine work with a pressurized fuel system. This system is what delivers the fuel from the tank to the engine. I will start with the pressure coming from the engine to the tank first. How the engine produces pressure will be another topic.

As the engine turns, it will develop pressure. Part of this pressure is fed into the tank via the return line, or pressure line, or tank overflow line. This Is where the check valve is located. The job of the check valve is not to let pressure return back into the engine and keep pressure in the tank. The amount of pressure an engine develops is between 6~9 Ibs. at full throttle.

Now that we have a tank full of fuel and pressure, we need to control it before it floods out the engine. This is done with the fuel regulator. The regulator is just an open or closed valve that is like a gardening hose sprayer, on or off. That's it!  The regulator does not control tank pressure or any air pressure at all.

Controlling when the regulator opens and closes is the next thing. If you take off the regulator assembly, you will see a cavity in the engine case with two holes "Except for the FZ140". The center hole is where the pressure comes in from the crankshaft. The hole that is drilled at an angle is for the pressure going to the tank. As you turn the crankshaft, you will notice the center hole will open up. This is when the crankcase pressure will push on the diaphragm. At that moment the plunger will open and fuel will pass throw the regulator and into the carburetor. As the crankshaft's hole passes the opening, this will drop the pressure in the cavity and the regulator plunger will close stopping fuel from passing to the carburetor.

Most common fuel system problems:

Using in-line fuel filters is a good way to prevent junk from getting into the carb, but remember, anything that can go into the carburetor can make it's way to the fuel regulator. I have seen the following stuck in regulators and inside engines:


I haws heard, listened, talked about and even shock my head from side to side hearing some of the methods on how to set carburetors on model airplane engines. All I can say is WOW.

For the most part, I think some people really don't know what Pre-detonation is or what it sounds like. Let me tell you that IT IS BAD.

Detonation is when the piston is coming up on the compression stroke, the fuel will explode and the expanding gases will push down on the piston. This explosion will happen around 42 degrees before top dead center. This means that the fuel is starting to burn before the piston reaches the top.

Pre-detonation is when the fuel is starting to burn before 42 degrees or way too soon. This is when the engine will start working against it's self. If the explosion is too soon, the piston has to work harder to reach the top. The sound you hear is a pinging or rattling type notes usually just before the spinner and prop start to fly off.

YS Carburetor Setup

Yes the YS engines are fuel injected but it is a different story when the engine is at idle. The fuel system changes from injected to sucking the fuel.

When you first fire up your engine you always want to run it rich.....But after break-in you start to lean out the top end to the max RPM's. Hopefully you are not exceeding 9,000 rpm's ????? That is for the 140,120 and 91. The 53's like to run at 11,000rpm's.

Next you want to bring your engine to an idle. The proper idle is 2,000rpm's. Not 22 or 25 or 3000, set it at 2,000. This is when the engine starts to suck the fuel via vaccume. Above 2,000 it will start to inject the fuel.


After you warm it up and slowly bring the engine up to full throttle, set the top end to it's max rpm's and back off or richen up the needle 3-4 clicks.


After the top end is set, bring it to the idle 2,000rpm's. From full throttle set your trim so it will hit 2,000 instantly not drop slowly like 35,30,25,22,2,000. NO it has to hit 2,000 BANG.  After about 3 seconds, the engine will start to let you know if it is to lean or rich.

If the mixture is to rich, the rpm's will start to slow down 2,000 19,18,17. Lean the low end. If the mixture is to lean, the rpm's will start to speed up 2,000 21,22,23. Richen up the low end.

Remember, after you adjust the low end, always bring it back to full throttle and back to an idle to test your adjustments. DO NOT CHASE THE MIXTURE, make the adjustment within 5 seconds when you hit idle. Do not let it idle for 10,20 seconds and try to adjust again. You must bring it to full first and back down to check the adjustment,

After adjusting the low end, it should idle at 2,000 consistently for about 20 seconds.

After the 20 seconds, bring it back up to full throttle by rolling the throttle stick up. DO NOT NAIL IT TO FULL THROTTLE... Roll it. It should take you just over 1 second to go from idle to full throttle.

Another trick to test if the mixture is correct is to pull off the fuel line at the carburetor at idle.  There should not be any fuel coming out of the fuel line. If you do, you are still too rich. This is best done on the bench and not on the plane,

With this last test, many other factors can cause fuel to come out of the fuel line at idle,

I hope this will be of some help,
Rick Mattie

YS 91/120 Standard Muffler Installation

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Are you having trouble keeping your standard muffler on either the YS91 or the YS120NC? Try these techniques.

checkmrk.jpg (769 bytes)  The only connections I have had NOT come loose are those where I used teflon tape - specifically headers and mufflers (it also seals the threads and eliminates the varnish buildup). I have not had experience with gas engined planes, but zero loose nuts or lost mufflers with 26 to 120 glo 4-strokes, OS and YS. If you want to try "red" loctite, be sure to check the product number. Some reds (number 222, for instance) are "service removable" - low heat, not very aggressive.   Charlie Anchorage, AK

checkmrk.jpg (769 bytes)  I use red loctite on both ends of the steel nipple which screws into the muffler and into the engine head. The loctite deteoriates with heat, which is what allows you to be able to get it off, but it leaves enough gook in there, that I have never lost a muffler. On the other end I drill and tap 4-40 through the side of the muffler where the aft end screws on. This plus a little red loctite keeps the end on. Loctite sponsored a pattern contest in Atlanta this year and gave away a bunch of Loctite goodies to all who entered. Bob Roe, a Loctite rep, was asked what to use for this application and he agreed that the red is the appropriate thing to use.    Ken Blackwell

Maintaining and Running YS Engines
by Mike McCormick

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I have seen and read many things about running and maintaining these engines over the last couple of years; most points I have agreed with, some I don't.  However, after rebuilding and/or modifying over 200 YS four-strokes (of all varieties) I feel I can post some pointers on how to keep them running happy.  I hope this information will be helpful to both those who have experience with these engines and those new to them.

New Engines:

1)     Replace the YS paper regulator gasket with a Leak Free before you even run it.  Though the newer YS paper gaskets seem to be better than the older ones, I can't begin to count the number of these I have replaced at contests because they "grew" and occluded the fuel inlet and/or the fuel outlet holes, thus causing the engine to go lean.  I have seen this happen with engines that had less than a gallon of fuel thru them.  With the YS gasket it will be just a matter of time before it will need to be replaced.  Go ahead and replace it now.

2)     Low speed Needle Valve (NV) setting - I have seen engines come from the factory with this needle as far as 4 full turns out.  The instructions call for 1.5 turns out.  Too lean and the engine will go lean in transition, and often get the piston so hot it will expand and begin to seize on the sleeve.  This leads to premature piston failure.  1.4 pistons are a subject all to themselves, which I won't get into here, but to assure the life of your engine, please check the setting of the Low Speed NV.  From my experience, 1.4 and 1.2 FZ's usually run 1.0 to 1.25 turns out, L's at .75 to 1.  SC's are usually 1.5 to 2 out (with the "latest" upgraded throttle barrel).  See Rick Matties' article above for an excellent description of how to properly set the LS NV.

3)     True the bottom of the valve cover (VC).  If this is not done, the YS paper VC gasket will suck in and leak: engine runs lean.  Use 220 to 240 grit sandpaper to do this, the VC needs to "bite" into the gasket. On all engines: Please follow the YS instructions on how often to check the valve tappet clearance. They state the clearance should be initially checked after the first hour of use then after every two hours of use.  For those of us using 16 oz. tanks, this works out to every two gallons of fuel used.  Too much clearance WILL damage both rocker arms and valves, and neither of these components are cheap!  (Rocker set ~ $31, Valves between $22 and $26).  On engines I own and work on I set the clearance at a minimum of .0015". 

 Mike McCormick

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