T-ster

I've had this YS .91AC for over 10 years in a Goldberg Extra (several of those years just sitting). It had been running ok, but down on power slightly, I thought. Anyway I flew it the other day and it ran fine (except I did have to speed up the idle slightly from normal), then for the 2nd flight of the day would barely start and would keep dying. Now it won't start at idle and it may run at 1/2 or full throttle for a while, but will run slightly erratic. Once I throttle down it dies.

One odd thing, there is fuel dripping from around the throttle arm.

I went back and changed all the settings to stock (high speed, low speed and pump) with no luck (running new Powermaster 20/20 fuel, YS glow plug and APC 15x8 prop).

I checked the valves and they are ok, changed the glow plug, changed the check valve. I took it out of the plane and checked all bolts, they were tight. It has pressure when I unhook the check valve, so don't see the tank as being an issue. I also changed the fuel lines on the engine. All of this with no help.

Any suggestions?

w8ye

My Feedback: (16)

The main reason is your regulator is sticking. It is gummed up?

But also sounds like the seals on your carburetors are bad

Flyboy Dave

My Feedback: (21)

....I just went through the same thing with a .91FZ....

....replace the diaphram no matter what it looks like.

FBD.

krayzc-RCU

My Feedback: (31)

yeah that always works for me 2

Flyboy Dave

My Feedback: (21)

....yep, if the diaphram isn't perfect....

....'yer lost in the tuning department. [:@]

FBD.

w8ye

My Feedback: (16)

The diaphragm is in the regulator

T-ster

Thanks for the advice. I ordered the diaphragm from Central Hobbies and will anxiously await it's arrival. (Ordered 2 so I have a spare....)

w8ye

My Feedback: (16)

Get ack on here when you are putting it in for more comments.

The cover goes on in a definite diresct that you should observe before you take it off. The screw is normally set flush with the surface.

abacro

I have your EXACT same set up! YS 91 ac in Goldberg 300,it's set in my basement for 3 years as i was flying my WildHare 300lx. took it out to the field back in may,had same problem your now having.!our not going to like the solution to the problem,but you'll need to replace all 4 bearings in the motor to fix it. Changing the diaphram,all the seals and gaskets woent help as the bearings have partially rusted and are causing to much friction on the crank shaft. This fixed mine,and now it's running as good as ever.By the way,i'm running Byron YS 20/20 fuel,a gruptner 14/7.5,and fox rc long idlebar plug. I get 9800 rpm's on the ground and over 10 grand in the air.My 11 pound 300 flys just great with the 91AC on it.


Good Flying,

Art

T-ster

Well, replaced several gaskets and replaced the diaphragm, but no luck. It will run better than before, so 1/4 throttle to full throttle runs fairly well. Once you get down around 2,000-2,500 rpm it will idle for anywhere from 0-15 seconds, then die. About ready to toss it and get a 1.10 (plus the .91 just doesn't have the umph to pull a Goldberg Extra).

Guess the bearing are the next thing to checck, but just not sure it is worth all this aggrevation.

plbrand

Looking for a little help with my newly aquired ys 91ac mounted on a Ryan ST-A. The engine/plane had been sitting for some time - don't know precisely how long - before I bought it. On it's initial couple of flights, it ran fine. Now, the engine starts perfectly, idles fine and runs well...for about the first minute or so of flight. Then it seems to loose power, fade to about idle and stay there. If, after a handful of seconds, I sort of 'work' the throttle up and down, the engine will come back to full song. After landing and keeping the engine running, it will do this surging on its own while sitting on the bench. Adjustments to idle and hi jets don't seem to make much difference. The fuel system hold pressure well. I see fuel leaking a bit around the air chamber o-ring. I tried a new F plug,I replaced the fuel line in the tank itself, I can hear the clunk move properly, there do not appear to be any leaks in the diaphragm, the check valve appears to work properly and I'm pretty well stumped at this point.

Any help, suggestions or resources would be very much appreciated.

Thanks to all,

Paul Brand

abacro

Yours is a straight forward simple fix T-star. Remove the valve cover and turn the engine over by hand, You'll see that the valves are sticking in it. Useing ur thumb to apply pressure to the valve Spring , work the valves up and down a few times with a bit of penitrating oil on them. They should snap back up quickly after you release the valve spring. Next is to put a few drops of the oil down the push rod tubes to loosen up the Valve Tappits! And yes,they get gummed up and stick also. After this is done,reinstall the valve cover and fire it up! Should now be running smoothly again.

Hope this bhelps you out!

Art