Hey Dave,

I have a YS91 that I sent in last Summer for a tune up. All gaskets were replaced etc. It's had about 2 gallons of fuel run in it and it's been working great until recently and I can not figure out what the problem is.

It holds pressure fine. I've changed the glow plug (twice) just to be sure (OS F type). I use 20/20 Power Master fuel and am swinging a 15 x 6 prop. The engine is mounted inverted on a UCD3D.

It idles fine somewhere around 2100 to 2400. The transition from low to high is quick and powerful. At full throttle I have a fairly thick stream of smoke coming from the engine. A tach shows the engine turning around 9500. In the air it runs great at low to mid-throttle... but if I do an extended up line at full throttle it loses significant power. Unable to keep the plane even in a hover... when I back off the throttle and let it coast back down I can hear the engine surging. Sometimes I can work the throttle stick and get it to come back to life, but most of the time it will die.

I've tried richening the top-end a bit, but then it gurgles when I'm testing it on the ground. I've also tried leaning the top end, and the engine died during level flight.

As odd as it sounds, the bad behavior of the engine seems linked to the up-line more than the throttle. I've done some tests flying it level at full throttle and it seemed ok(?). I think I could fly the tank empty with the plane level... but that would defeat the purpose of flying a UCD3D! lol!

My needle settings are: top end - slightly less than 1 1/2 out; low end - 1 1/2 out; regulator - flush with top.

As a last observation, I did notice that if the engine is idling and I raise the nose of the plane, the engine will die. Again, the nose up attitude caused probs.

Thanks for your help Dave,

Bobby

Is the clunk hitting the back of the tank?

Hmm... the tank configuration hasn't changed since the problems started... but I'll pull it and check... I guess it's possible the internal fuel lines have moved.

I'll get back to you either way.

Thanks Dave.

Bobby

Well... I checked the plumbing inside the tank and it looked fine. So I took it back out to the field to give it another whirl... and it ran better. Not perfect, but better. I was able to fly several flights without it quitting on me. It surged and sputtered a few times, but I was able to wiggle the throttle just enough to keep it going. Unfortunately, I don't have enough confidence in the motor to fly the plane like I'd like to.

I'm baffled by its behavior.

Have you done anything with the regulator? If not, time to start playing with it. My first move would be out about 1/2 turn.

OK. I'll give that a try.

I've read what the regulator 'does' I'm just not sure how it affects the engines behavior. The explanation(s) that I hear a lot is that it affects the mid-range and/or the fuel flow.

Turning it out will richen, or increase, the fuel flow, correct? For just the mid-range? After I turn it out will I have to re-set the low and high end needles?

When I turn it out, what should I listen for? I've thoroughly read the YS manual and printed out several articles from the net (including the YSDoctors white paper) but nothing gives a good description of what will happen to the motor when you mess with the regulator. Will it gurgle at mid throttle if I turn it out too much?

I feel like I'm close... I just need to understand this motor better so I can make educated adjustments.

Thanks for your help Dave, I do appreciate it.

Bobby

The reg controls fuel flow in the mid range. It does not control pressure, but the amount of fuel admitted to the engine versus throttle position. It will sometimes have a slight effect on the top and bottom end, but not much.
Don't be afraid to play with it, just remember where you started so you can return it to that point if you need to.