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YS 120 FZ Going Lean

Old 11-14-2014, 05:53 AM
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Default YS 120 FZ Going Lean

I have a YS 120 FZ that goes lean after several perfect engine run flights.
This happens on start up. Once started I let it idle for about 30 seconds and then advance the throttle. It starts to go lean and would then throw the spinner. This has happened 4 times with this engine over the years I have owned it. This engine has had about 10 gallons of fuel through it.
I disassemble the engine looking for a tare in the diaphragm, or bits of something clogging the fuel system. I haven't found anything yet, and after reassembling the engine it would work perfectly.
The previous 3 flights had a perfect engine run , good idle, and good top end with no detonation and plenty of smoke.
I use Morgan 20 20 fuel with OS F type glow plug, and a Bowman ring. I own 5 YS engines, and am familiar with maintenance, repair, and operation of these engines.
Has anyone had this type of problem?
Old 11-14-2014, 02:13 PM
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speedracerntrixie
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Sounds like something is leading to a pressure loss in the fuel system. It could be anything from the tank stopper developing a leak to the diaphragm not sealing. Maybe even a bad check valve.
Old 11-15-2014, 07:07 AM
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That sounds like a possibility. After about 2000 rpm the engine starts to run on the pressure system. Under 2000 rpm it is just like any other engine as far as fuel supply is concerned (not pressurized/supercharged).
I guess it is time to cheek that also. I wouldn't usually consider pressurization an intermittent problem.

Greg
Old 11-17-2014, 06:27 AM
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Greg, I'm not sure I can agree with the engine not running on tank pressure below 2,000 although that would be a fairly low idle. All YS engines I have owned will hold pressure in the tank for hours after a flight. If you have one that does no then I would think that confirms that you have a leak down issue. As far as being intermittent, if the engine sits for a week or so I can see where the methanol dissipating and leaving the oil behind in the check valve thus giving a seal with a worn unit. It could take a couple flights to wash this oil out and then the valve stops sealing. Speculation of course.
Old 11-17-2014, 09:48 AM
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It certainly does switch from pressure to no pressure at 2000 rpm. Here is an excerpt from a YS Fact Sheet.
Located at:
http://www.julton.net/Fact%20Sheet%20YS%20Engines.html

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.

My engines will also hold pressure for extended periods. If I go out the next day and didn't disconnect the lines the previous day, there will still be pressure in the tank.

As far as the intermittent going lean problem, it has only happened 4 times in the 10 gallons run through the engine. About 4 years.

Greg
Old 11-17-2014, 09:59 AM
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Interesting, there must be some sort of valve that reduces the fuel pressure below 2,000 rpm as we know the tank stays pressurized. One of the first things I learned about YS engines is to keep it at a medium idle and allow the tank to fully pressurize before transitioning to full throttle and always adjust the high speed needle with the engine at full throttle. I have always done both and have not ever had a prop spit off.
Old 11-20-2014, 10:13 AM
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I have seen where the check valves will unsrew them selves and leak air. also are you running a filter? take the regulator apart and flush every port in it. it doesnt take much to clog the tiny ports in it. open your needle 4 turns or more and build ou some pressure by slowly openening the throttle. you can hear it going lean by going real real slow. open up the regulator to so that it is super rich. sometimes the fuel left in them will gel up and need to be cleaned out by new fuel runnning thru it. once up and running well go back and re-set everything so that it idles well.
Old 11-20-2014, 11:03 AM
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On all of my check valves, I take a piece of X-Large silicone fuel tubing and slip it over the check valve. This will stop the check valve from unscrewing, and also keeps it from leaking any air.
I guess the point of my original question was why it would only go lean once a year, even though I fly this particular plane several times a week. There isn't time for fuel to evaporate and gel up. Seeing as the engine runs perfect the rest of the flying season, I don't really see a need to be adjusting the regulator.
At this point I am going to take the check valve apart, and see if there is anything stuck in it that would allow the tank to depressurize, and thus go lean.
I have a felt filter in my fuel pump line from the fuel jug, and also have another felt fuel filter in the fuel tank. I use a three line system, so there is no fuel back flushing anything out of the tank filter.
Thanks to all for your input.

Greg

Last edited by OldRookie; 11-20-2014 at 11:21 AM.
Old 11-20-2014, 01:36 PM
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ok I missed that you fly it regularly. I remember some talking about the FZ's having a fuel puddling issue if mounted inverted and the rush of fuel caused it to spit props. i will check with the guys this weekend.
Old 11-20-2014, 09:55 PM
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ok talked to guys that had run these in the past. They said the plunger could get stuck sometimes. they would grind down the little hump under it but don't grind it flat. this would give it a bit more travel and no more issues. something to think about.
Old 11-21-2014, 04:05 AM
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I guess I don't understand "the little hump under it" comment.
The shank of the plunger is straight with no hump, and the end is flat not rounded. A silicone end is then slipped over the end of the plunger shank upon assembly.

Greg
Old 11-21-2014, 04:12 AM
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This is off topic... Sorry. OldRookie - do you actually fly in the winter time up in this frozen place we call 'home'?
Old 11-21-2014, 05:18 AM
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I lived in St. Paul in '92-'93. I flew exclusively helicopters back then but did fly the one winter I was there. Kept myself and the heli warm in a Suburban. Popped the hatch, got the heli fired up, flew until fingers were numb then piled the heli and myself back into the Suburban until I warmed up and then put in another flight. Great fun.
Old 11-21-2014, 05:41 AM
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Originally Posted by speedracerntrixie View Post
I lived in St. Paul in '92-'93. I flew exclusively helicopters back then but did fly the one winter I was there. Kept myself and the heli warm in a Suburban. Popped the hatch, got the heli fired up, flew until fingers were numb then piled the heli and myself back into the Suburban until I warmed up and then put in another flight. Great fun.
I will run my cars from time to time, but even that is usually short lived. I'd rather ice fish. LoL.
Old 11-21-2014, 06:34 AM
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I'm not a winter flyer.
I have made several winter flights, and can't say that I enjoyed any of them.
About the only time I get out in the winter, is if there is a New Year day get together.

Greg

Last edited by OldRookie; 11-21-2014 at 07:34 AM.
Old 11-21-2014, 08:22 AM
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I have to say living in California has it's advantages........and we pay for them LOL
Old 11-22-2014, 12:44 PM
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I went through the whole check valve system, and all lines and fittings.
I didn't find any dirt or debris in the check valve, the high speed needle seat, or any of the fuel lines or fittings.
I also pumped the fuel tank up to 8-1/2 lbs of pressure, and after 6 hours the pressure didn't drop at all. I would say my system is pretty much air tight.
I included a picture of my check valve with the fuel line pulled over it, and a picture of the check valve flapper. Everything seems to be OK.

Greg
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Old 11-22-2014, 01:00 PM
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how is the plunger? that is what sticks causing it to go lean sometimes
Old 11-25-2014, 04:14 AM
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I finished up going through everything.
The plunger is in good shape. The diaphragm is in good shape, lies flat, and no tares. I blew out all lines and found no debris.
I checked the valves for proper clearance, and put everything back together again. I'll try it out, but it is looking like the times this has happened before, didn't find anything and it will run as usual.

Greg

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