I'm having a tremendous problem with my YS 91. I only have about4 flights with this engine and is killing me!
here are my problems or symptoms:
1) At iddle the engine have a change in rpm (about 100 to 200).
in about a minute after it accelerates and desacelrates, the engine suddenly quits. (i changed the spar several times)im usng an OS plug!!!

2) At full throttle the engine is only giving me about7,000 to 7,500 rpm with a APC 13 x 11. (i think this is too little rpms)

3) In one of the flights I inverted the plane and the engine lows in rpms and then quits!!

other considerations: im using Cool Power 15% nitro!
I dont know if this is good, but my high speed neddle valve is about 1 1/16 open... (is that too much closed?, but thats when the rpms are at it highest)

thanks for your answers....
Alex

Alex,

Is your engine used in an in-cowl application? If so, it sounds like it may be overheating or it is running a little lean.

I have no experience with the 13x11. I have used 14x8 and 15x6 APC. I run 20/20 fuel and get about the mid to upper 9K rpms. The 15% fuel may cost yoiu some power. Try a 14x8 and see what happens.

Azhar

Alex,

this is a common problem of all YS engine.

I have two YS91AC with the same problem what you had today.
fortunately, all of them are recovered to normal.

First of all, check all screws, the screws are loose when exit factory or after first flight. it cause the pressure inside the engine very unstable. escpecially the valva cover and air chamber. becareful in tighten all screws.

Second, check the codition of the gaskets (valva cover and air chamber), replace it if broken where will cause pressure leak.

Third, screw out the regulator and carefully clean inside. sometime the lubricant oil will stick around there.

Nelson

What type of Os plug are you using, just out of interesT?

thanks for your responses! my engine is in cowl! i will cut the cowl a little bit more, because right now the muffler and cylynder head is tight. I will check also al the scews!!!
I will try today again, to see how it works!
By the way how closed you have your high speen needle valva????
Keep flying,
Alex

Just in case you don't know, to increase cooling, increase the size of the hole that lets the air OUT, or decrease the size of the air that lets the air IN.

set the high speed needle 1 1/8 turn.

an OS type F or YS 4 stroke plug must be used.

ok i am in the process of making the cowl holes a little bit bigger, that is the hole in the muffler and in the cylynder head!
My high speed neddle is now about 1 1/8 open! i will try tomorrow how it works!!!
Thanks again,
Alex

Alex,

just a note. The needle valve adjustment may vary slightly from engine to engine. The % of nitro will also determine your needle setting. The best way to set it is to use a tach and set it about 100-200 rpm below peak.

Azhar

yepa thats right! Im using Cool Power fuel. This fuel have 15% nitro, not 20% YS recommends, but i think it shoukd work.
Hope my engine gets better today!
Keep flying,
Alex

Alex,

I have a YS .91FZ on my pattern plane. I've tried the 15% route before, but $adly, you won't get much over 8500 RPM with it (14 x 10 APC). Try a tank of 30% heli fuel and you will have a new engine on your hands. You also have to run an OS-F or the YS 4-stroke plug in this engine.
Remember, this engine has high compression and performance. Also, cooling is a must. Good luck!!

I agree..
I just came from my starting the engine and I think i discover the problem..
I found a lot of bubbles from the small line from the diafraghan to the carburetor.... I think this is a major problem. thats why i have this changes in the rpm in iddle... what do you think guys?? i replace the line to make sure the line wasn't broken...
I think i have to send the engine to YS!!!
Any suggestions
Alex

I have bubbles in the line of my YS .91 also. . . . try some 30% heli fuel and an OS-F glow plug. . . .

If that doesn't fix it, send it in. . . .

Alex,

The bubbles is normal between the regulator and carb. You seem to be having a problem with engine overheat. Open up the cowl some. The YS's do not like to be hot.

Azhar

wow bubble are normal in this engine! I though they weren't in any motor!
umm i'm confused now!!I think the bubbles have make my iddle go bad!!! i'm using OS plugs! wow i dont know what to do?

Are the bubbles large? Do they show up entire throttle range?

Is the engine inverted, upright, or side mounted?

What kind of plane is it installed? just curious.

tiny bubbles at low throttles are fine, but should not have much bubbles above half throttle.

As Keith said above, you won't get much out of the 91 with 13x11 if cowled and low nitro. Getting good power with 14x10 is with minimum of 20% nitro.

On a test stand upright mounted, even a 10% fuel can run the engine fine.

One of my first YS 4-stroke engine, cowled, inverted in a brand new Pattern plane quit low altitude, broke my bank account once.

Reason? Low nitro 15%, Not enough cooling.
25% nitro and a little more cooling to the cylinder solved all.

Rule of thumb for cowl opening that I heard is that, Cooling air outlet has to be at least twice the inlet opening. Mostly for Pattern applications with enclosed pipe.

Inverted application need slightly leaner idle setting in my applications.

One way to find it out bubble is to replace the blue YS tubing (hard to see through) with normal medium size fuel tubing. I use Prather medium only.

Make sure there is no air leak in the entire fuel circuit.
Try minimum of 20% nitro.

I've been running all my YS 4-strokes with minimum of 23% without any problem of cooling. Now I am running 30% in all my engines including OS 91 2-stroke.

ok the bubbles in my engine are from low, half and even full throttle!!!the bubbles are a lot, not one, two or three!
the engine is inverted and is mounted on a AW EDGE 540 60/90!

I run the engine with the cowl!!! still with the problem!
I email YS and they told that the fuel im using could work with this engine, but the performance will not be the best!
I checked the tank is good and also replaced the fuel lines and still is bad!


One think that I noticed is that from the tank to the inlet there are no bubbles, but from the Diafraghm to the carburetor the bubbles came, and a lot!
Any other advice,
Alex MBA

Alex,

You might want to replace the gasket and see whether you are still experiencing the same problem. It sounds like you are having a leaky gasket.

Azhar

Yeap.. air leak! But thats it for me! I take the engine out and i am gonna send it tomorrow to YS. They told me to send it. So tomorrow is gonna be FedEx!!! well hope i receive it soon so i can fly after i move to the United States!!
Keep flying,
Alex, MBA

my engine is packed and ready to go tomorrow!! This Su*%$!!!
Keep flying,
Alex.............

Welcome to life with a YS .91. . . . .

my engine is gone to the YS Service center!!! I hope they fix the problem!!
Alex

waiting, waiting, waiting waiting!!

I just bought my 91 and was having the same problems you are talking about. Because i also was using omega 15% ,a friend at the field said run 30% heli fuel ,so i tried some it is different motor on 30% . The idle is strong and solid , and the power is unreal ,and it wont die. OSF glow plug. I Love This motor. :thumbup: TomBob

wow. i hope minw woeks good after serice repair from YS. i wanna take this engine to the limits!!
Alex