This Fact Sheet is primarily aimed at the newcomer to YS Four stroke Engines

Understanding the fuel system on YS 4 Cycle engines, FZ53 ~ FZ140.

As you know the YS 4 cycle engines work with a pressurized fuel system. This system is what delivers the fuel from the tank to the engine. We 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 lbs Psi. 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 centre 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 centre hole will open up. This is when the crankcase pressure will push on the diaphram. At that moment the plunger will open and fuel will pass through the regulator and into the carburettor. 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 carburettor.

(Source Data Rick Matt YS Doctor)

Fuel system for connecting a fuel tank to a YS engine. The red fuel tubing represents the pressure side of the system and the vent tube. The blue fuel tubing is the fuel feed to the engine and the fill tube. Fuel dot's & tee's are available from Probuild.

Starting Procedure

1. Fill the tank with fuel using the fuel dots to fill and vent the tank as shown in the sketch above.
2. Open the needle valve 2 _ turns from the fully closed position.
3. Open the throttle to the idle position and turn the propeller over 10 times by hand to pressurize the tank.
4. Now open the throttle fully and turn the propeller over a further10 times by hand this sends fuel to carburettor and primes the engine.
5. Now CLOSE the throttle to the idle position and connect the glow plug cord. The engine is now ready for starting.

DO NOT ATTEMPT TO START THE ENGINE AT FULL THROTTLE AS THIS IS VERY DANGEROUS.

6. Use an electric starter to turn the engine over, it should start immediately. (Always make sure the aircraft is tethered down or have a helper hold the aircraft to stop it leaping forward
7. Let the engine warm up fully before opening the throttle.
8. When you are satisfied the engine has warmed up, gently open up the throttle to about half to allow the fuel tank to fully pressurize. (If you immediately open it up to full throttle the engine will go lean and throw the propeller, ouch...!!!)
9. If the engine sounds ok and is not labouring gently open the throttle fully and adjust as detailed below.
10. Once the engine has been fully run in and adjusted it will very rarely need to touched.

ALWAYS STAND BEHIND THE PROPELLER WHEN MAKING ADJUSTMENTS

YS Carburettor Set-up

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. The needle valve should be set rich for at least 12 flights and avoid running at full throttle for long periods. But after break-in you can start to lean out the top end to the max RPM's. Hopefully you are not exceeding 9,000rpm for the 140,120 and 91. The 53's and 63's like to run at 11,000rpm.

Next you want to bring your engine to an idle. The proper idle is 2,000rpm's. Not 2,200 or 2,500 or 3,000, set it at 2,000. This is when the engine starts to suck the fuel via vacuum. Above 2,000 it will start to inject the fuel.

TOP END: 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.

LOW END: After top end is set, bring it to an idle 2,000rpm's. From full throttle set your trim so it will hit 2,000 instantly not drop slowly like 3,500, 3,000, 2,500, 2,200, 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 1,900,1,800,1,700. Lean the low end.

If the mixture is to lean, the rpm's will start to speed up 2,000, 2,100, 2,200, 2,300. 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.

Another trick to test if the mixture is correct is to pull off the fuel line at the carburettor 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.

• Bad Check Valve
• Piston ring worn out causing to much blow-by
• Leaking intake valve

(Source Data Rick Matt YS Doctor)

Filters

Using in-line fuel filters is a good way to prevent junk from getting into the regulator from the fuel tank. But remember, anything that can get into the carburettor can make its way to the cylinder, so always use an air filter as well. It could save you 100's £££ in repair bills!!!

Propeller Selection

The choice of propeller is of great importance. Here I will try to explain in a simple way to choose prop for aerobatic use

Over the past 10 years, APC has almost been dominating the aerobatic scene. APC is a very efficient propeller design, especially the tips, which make them very quiet and efficient.

We want good static force, used during long vertical climbs, which keeps the model climbing without loosing too much speed. The static force should be greater than the weight of the model. This force is very much dependant on the diameter of the propeller, the largest volume of air as possible should be set into movement by the propeller. One more advantage of using large diameter is that the "rotating disk" acts as an air brake during vertical dives.

Speed is needed to make the plane fly, but we are not looking for as high speed as possible. We want a speed fast enough to make the plane fly steady, but not too fast which will make the turn-around program feel and look very stressful.

The speed is dependant on the propeller pitch.

The maximum engine RPM used during flight is about 8000 - 9000 rpm.

With this criteria in mind we can select a good propeller size.

At our disposal is the engine power (EP), which will be loaded by the propeller. An increase of propeller diameter (D) will increase the engine load. The greater the propeller pitch (P) will also increase the load of the engine. A simple formula would be like EP = D x P. So we must make a compromise between static force (diameter) and speed (pitch).

(Source Data Swedish F3A Web site)

Suggested propeller sizes for YS four strokes are as follows: -

YS63FZ
Keep the RPM in the 10,500 to 11,500. APC 12 x 7

YS 120 AC
Recommended RPM: 9000 max APC 14,5x13 / 15x12 / 15x11

YS 120SC
Recommended RPM 7800 to 8000 APC 14,5x13 / 15x12 / 15,5x13N / 16,5x12N / 16x12

YS91FZ
The correct range is 8,800 to 9,300 RPM. APC 14 x 10 / 15 x 8

YS 140FZ
The normal working-range is about 7500 - 9000 RPM.APC 16x12 / 15 x 12

YS140L
Works well from 8,300 to 9,000 on props up to 18 x 6. APC 15 x 12 to 16 x 12

PROPELLERS ON ALL MODEL AIRCRAFT ARE DANGEROUS! ALWAYS WEAR EYE AND HAND PROTECTION WHEN STARTING AND RUNNING ENGINES, AND ALWAYS HAVE A HELPER HOLD YOUR AIRCRAFT DURING STARTING AND RUN-UP. ALWAYS STAND BEHIND THE PROPELLER WHEN MAKING ANY ENGINE ADJUSTMENTS.

What is the recommended fuel for YS Engines?

The recommended fuel for all YS engines is 20% OIL and 20% Nitro Methane. For optimum performance, 20% Nitro (or more) is a must.

Nitro Methane
Nitro is a power enhancer as it increases the amount of oxygen available in the fuel for combustion. The down side is that it generates more heat. Nitro methane will not increase rpm, but as it's combustion produces more energy, the engine will develop more torque, allowing the use of bigger propellers, or better operation under high loads. A classic example of this is it will give a big performance increase in vertical climbs.

Oil
The YS factory recommends no less than 20% oil in the fuel mixture. Many commercial fuels contain less than this. Use of these fuels may cause damage to the engine and/or poor running characteristics. Not to mention invalidate the guarantee.

Fuels containing large percentages of castor oil can cause fuel pump/regulator trouble. We recommend you use 100% synthetic oil in the fuel, but 18% synthetic and 2% castor is suitable also.

At Probuild we can supply top quality fuel from Model Technics, blended to your requirements at competitive prices. Or on request we can obtain Cool Power pro-Pattern fuels.

Fuel consumption
YS 63FZ: 1/2 oz per minute

YS 91FZ: 1.1 oz per minute

YS 140L: 1.5 oz per minute

YS 140DZ: with venturi restrictor- 1.75 oz per minute. Without venturi restrictor-2 oz per minute.

All tests were run with Powermaster YS 20/20 fuel and standard prop sizes for each displacement. Use of other fuels and prop sizes will affect fuel consumption slightly.

(Source Data YS Performance USA)

Glow plugs

Fortunately there is very little to be said about glow plug selection for use in YS Four Strokes, unlike 2 Strokes, there is only one choice.

We highly recommend the OS Type 'F' glow plug specifically designed for four strokes. This year YS have introduced their own Glow Plug, which has basically the same construction and characteristics as the OS type 'F' and is equally as good.

The plug should last at least 20/30 flights, but a good sign as to when the plug needs changing, is when you feel a slight hesitation when throttling up in the air.

When preparing for a weekend competition, a few days before when practicing, run the engine with a new glow plug, so when you arrive at the competition you can guarantee the plug will not let you down and the engine will be running at it's best.

Maintenance

Generally YS Engines require very little maintenance if looked after properly. Here are a few tips to get the best out of your engine.

Most common fuel system problems:

• Cracked fuel tank: Split at a seam
• Clunk line hitting back of tank: As fuel line gets older if will stretch and go soft.
• Fuel line and Fuel line "T"s: Not holding pressure.
• Silicone Rubber: I have found many engines plugged up with bits of silicone.
• Diaphragm: Check for cracks or torn around the edges.
• Regulator Gasket: Plugging up holes for fuel and or pressure.
• Dirt in Carb: Yes even if you have fuel filters...

(Source Data Rick Matt YS Doctor)

Tappet adjustment
After the first 12 flights check the valve clearances and re-adjust as necessary. Also check for leaks from gaskets and fuel tubing. It is a good idea to re-check valve clearances every 25 flights. (Always adjust valves cold)

Every 2/3 gallons of fuel run through the engine remove the cylinder head and clean the valves. Inspect for excessive wear on valves and rocker arms.

Diaphragm
If you loose fuel tank pressure or the tank will not pressurize check the regulator diaphragm and the non-return valve to the fuel tank for leaks.

When re-assembling the regulator the tab on the side goes forward.

Always replace all gaskets upon re-assembly. One possible exception to this rule is the valve cover.

Fuel Tank
Use a good quality fuel tank, Tettra or MK are recommended. Remember, they are operating at 6~9 lbs Psi. at full throttle.

Watch for leaks, especially ones that only appear under full pressure.

Cleaning
Very simple really, use soap and water or methanol. DO NOT USE PETROL.

Storage
During regular daily/weekly running YS engines don't need to be run dry after each flying session. However, the engine should be stored with the engine inverted to allow fuel to drain from the regulator housing in to the crankcase. This is to avoid the oil blocking up the tiny holes in the regulator housing making it difficult to prime and start at the next session.

At the end of the season or when the engine will not be run for long periods it can be run dry and again stored inverted.

Servicing
Why not treat your YS engine to a full service at the end of the season.

Here at Probuild we offer a full servicing & rebuilding service. We also have a full line up of YS engines in stock, along with possibly the largest stock of spares in Europe, as well.

Please call for full details and service costs.