YS FZ110S Violent acceleration and sudden stop
 

After the engine is started it idles well but cannot be accelerated to full throttle. When it is slowly accelerated to the lower mid range it hammers violently and loudly and then stops suddenly. This action causes the prop to slip and has worn the splines off the drive washer and the spinner back plate.

This is a new engine that is still in the break-in phase and has had four trouble-free flights lasting from 3 minutes to 5 minutes at less than 9000 RPM. The first sign of trouble occurred before the final flight when the engine quit on the runway before take off. The engine was restarted and the flight was uneventful.

No amount of adjusting the low or high-speed needle valves has any effect on this condition. Adjusting the regulator clockwise made it worse. Adjusting the regulator 2 turns counter clockwise enabled the engine to accelerate to the upper mid range before it quit.

With the regulator at that new setting, all of the fuel in the tank will run out (not drip out) through the carbureter and onto the ground while the engine is not running. The engine is inverted. The only way to stop it is to relieve the pressure in the tank. By the way, this engine has always leaked fuel from the carbureter while flipping the prop to start.

Thanks for any help.

RE: YS FZ110S Violent acceleration and sudden stop
 

SOunds like a blockage in the fuel delivery system causing a lean condition which is why opening the regulator made it better but that also, as you found out, will cause fuel to flow freely due to insufficient spring pressure.

Check your fuel lines for kinks and the filter for blockage. Also, try taking the needle valve out and flushing it with fuel in the event there is a bit of dirt in the orifice.

RE: YS FZ110S Violent acceleration and sudden stop
 

I agree with petec, there is something blocking your fuel delivery. Check the regulator. Very likely there is a piece of debris of some sort holding the regulator open. You do use a fuel filter between the tank and the engine, don't you?
Paul

RE: YS FZ110S Violent acceleration and sudden stop
 

+1

Sounds like classic lean YS high needle. As petec sugested check for leaks in the fuel system. You will want to screw the regulator back to the original setting or you will be overly rich on the low end.

RE: YS FZ110S Violent acceleration and sudden stop
 

As a side note, the loud and violent hammering you mentioned is caused by the engine being run too lean. It can and will cause significant ( read expensive ) damage to your engine.
Paul

RE: YS FZ110S Violent acceleration and sudden stop
 

I agree with the other guys that this engine now is running lean. The hammering noise is called pre-detonation, and as has already been said, it will cause you real problems if it continues. But I think your problem has shifted around 180 degrees on you. I think at first your regulator was set a little rich. That's about the only thing that can cause fuel to drip from the carb. If the engine were mounted upright, this fuel would be ingested into the engine and it would idle rough, but with it inverted it just drips out. This problem isn't uncommon with an engine that you're still trying to break in and fine tune. Once you get it sorted out, you may find the original setting was just a tad bit rich.

But now, as the other guys have said, it sounds like it has leaned out, and my bet is trash in the regulator. Mainly because you said the fuel all runs out of the tank after you shut it off. The only thing I can think of that would explain that is the valve in the regulator is stuck open, probably blocked by a gob of someting.

I'd recommend that you start out by getting, if you don't already have one, an exploded diagram of your motor. Look it over carefully, so you know what to expect before you take anything apart. Then take the regulator apart, make sure you know exacty how everthing was put together originally, and flush it out with your fueler. Flush it in both directions. Check the diaphragm for a pin-hole or tear, that might explain the symptoms you're having, but I'll bet it's just dirt. Put it back together, turn the screw back to approximately the place it was when the engine was new, (flush with the housing, or slightly below), and try it again.

Hopefully, problem solved?

David

RE: YS FZ110S Violent acceleration and sudden stop
 

Gentlemen,

Thanks to all of you for your troubleshooting suggestions.
Today I had a chance to take the Yak and it's misbehaving YS 110S out to the back yard for a short test run and it appears that the problem is cleared up and the engine runs satisfactorily.

Attention was given to the fuel supply system as suggested. I blew out the needle valve and carb with compressed air. The regulator was disassembled and inspected with a magnifying glass before and after flushing it with fuel but no debris or imperfections were found. Those are the only actions that were performed. There were no problems with fuel tank, fuel lines, filter or check valve.

Since there was no obvious way to enter the lower half of the YS 110S regulator to view the diaphragm, I blew my breath into the pressure input nipple while covering the output nipple with my finger and it didn't appear to leak. This was done while the regulator was disassembled.

Maybe I should not have assumed that this problem was uniquely YS. After all, I've been flushing out needle valves and carbs since my Ohlson & Rice .29 and Fox .35 control line days. Thanks for steering me back to the basics.

RE: YS FZ110S Violent acceleration and sudden stop
 

Glad you're up and running again.

RE: YS FZ110S Violent acceleration and sudden stop
 

Sometimes they're just like a woman....

You just gotta talk sweet to 'em! :D

David

RE: YS FZ110S Violent acceleration and sudden stop
 

I have the same problem.

RE: YS FZ110S Violent acceleration and sudden stop
 

with a woman or an engine?