pjakew

My Feedback: (4)

Having a problem with a YS .91 AC.

When I fire up the motor with the glow stick it is fine. It will idle o.k. with the glow stick on, but once I take it off it will die. I changed out the plug so the plug is not the issue. I think it might be rich or lean but I am not sure. Now if I run it at medium or full throttle no issue. When I pull back the throttle to idle it runs for a few seconds and then stops. If any of you guys can give me some pointers that would be great because this motor is driving me F*&%ing Nuts.

Prop MA 14X8
Fuel Cool Power 20%

speedracerntrixie

My Feedback: (29)

Jake, what plug is being used and how old is your fuel? When I had one of these I used an OS F plug. If there is enough moisture in the fuel it will affect idle is well but that also will show up as elevated temps at full throttle. Do you find that you have to keep it fairly rich to keep temps down?

pjakew

My Feedback: (4)

I do have an F plug in there. Now the motor is inverted, but I have another in my Hurricane that is mounted the same way with no problem. It does seem to be rich, because a good bit of smoke comes out. The fuel is about a year old, but again, in the Hurricane has no issue with the same fuel. Im perplexed.

MFLOOD3800

My Feedback: (51)

It may just be the low end needle. It could be lean and starving for fuel, or rich and not keeping the plug lit. Either way just try 1/8 turn in or out.
Sometimes the regulator on the bottom needs to be adjusted. If this is a used engine (Got from some body else), they may have messed with it. Normally set it flush with its case heigth.

If the regulator is old and the diaphram is leaking it may not be keeping enough pressure at idle.

There are many variables and you just need to try one thing and begin the process of weeding out potential problems from the list.

Now if this is for a warbird racer, who needs low idle anyway.
Wide open baby, wide open!!!

pjakew

My Feedback: (4)

Well, I will pull the motor out the plane and give it a once over. I need to get an engine stand to work on the thing while it is running.

still4given

If it runs OK with the glow driver on it is rich. The low end needle is an air bleed so turning it in richens the mixture. So try turning it out a 1/4 turn at a time until you get the desired result.

speedracerntrixie

My Feedback: (29)

Has the fuel been sitting in an airtight container for a year? Is a year old and recently opened? was opened a year ago and has fittings that will allow air to get to it? If the fuel has been exposed to air at all for that time it will have a fair amount of water in it now. Maybe dosent affect the other engine noticably because it is broken in and well tuned.

When I got my .91AC the original owner stated he could never get it to idle. As I do with any used engine I did a tear down and found the cam gear was out by one tooth. Called Shadel and was told that the engine would run reasonably well that way except for idle. I re assembled it correctly and it ran and idled great.

You have one that runs correctly so I'm not going to suggest bad tuning as you obviously know what you are doing there.

pjakew

My Feedback: (4)

Don't let that fool ya, just because I have one motor running right doesn't mean a thing. Maybe I just haven't got around to screwing it up yet

still4given

Hi Jake,

The glow driver won't help an engine that is too lean to keep idling. It will however, keep the plug hot on an engine that is too rich and allow the engine to keep running. Have you tried backing out the low end needle a 1/4 turn at a time? I bet that helps.

Blessings, Terry

pjakew

My Feedback: (4)

Im going to fiddle with it this weekend if the rains hold off. I just need to sit down and tinker with all the suggestions, but I am as busy as a one legged man in an &ss Kicking contest right now with work. Thanks for the suggestions. Once I get to sit down with the motor I will give her a go.

pjakew

My Feedback: (4)

Well I fiiddled and farted with the thing with no measurable success. So I bought another Y.S. 91FZ off of here and that thing runs much better and am much happier with that motor. The one issue that I do notice though is that even this motor will stop if it idles for too long, but I took a tach to it and it was at about 8k rpm at full stick. Isn't the optimum rpm for a Y.S. .91 closer to 10k? Could my prop be bogging down the motor? I am running a MA14X8 so maybe I am running a too large of diameter prop? I guess I could go with an APC 13X9 or 10 or something, but I was just paranoid that the radial cowl of the 190 would affect the thrust since it is like a big salad bowl behind the prop. Should I just prop this sucker down and get the rpm up? This is where those with the experience and "know how" chime in.....

BTW: What is the best rpm for idle? I have it currently at about 3.6-3.9K

speedracerntrixie

My Feedback: (29)

I was running an APC 15X7 on mine. Never hit it with a tach but level full throttle would rip the prop. Fuel was Powermaster 20/20. Properly adjusted any engine should idle as long as you want. I stand by my first suggestion, fuel it up with fresh fuel. If you do and don't see an improvement after adjustments I will buy lunch next race. My guess is you have the needles set more open then what would be considered normal. Tell tale sign that the fuel has moisture.

Iron Dog

Jake,

I run anAPC 12x 12N on my YS .91FZ. That is what has been powering my P-47 all year, and it has a radial cowl, too. Back in the day when the .91AC was the main motor used for racing, Sweeney, Sullivan, and others, found that it gave the best performance forthe purpose of racing. Jim has always used the APC12 x 12N on his YS.91-powered Fw 190Ds. I recommend giving this prop a shot.

pjakew

My Feedback: (4)

Well, I cracked open a new bottle of CP 20% ProPattern, and so the fuel variable is not an issue at this point. The needles are open and there is good smoke, but could the moisture issue, if it was the issue, be due to higher moisture in the atmosphere. It does seem wetter here in Mendocino County with fog about everyday now that Fall is here. I got some props to play with so I will make some changes and report back.

Jimmy Skids

My Feedback: (2)

Jake,

If it's not fuel, needles, or valves, it could be as simple as the plug. Check it again.... I know you said you replaced, but sometimes they go bad right away. Also, a small trick.... pull the element down a tad with your xacto blade. I would also state that this motor is happier at mid 9's for RPM vs. mid 8's..... however, with that said I ran a 14/10 APC on my motor at 8.9 to 9.0k at the last race and my motor was very happy on 20%.

If you want help tuning I'd be more than happy to work with you on set up at the next race down at SAMS or when your back in Sacramento.

I just went back and looked over your original post and I see two things in it that I feel need addressing. First, I've found that Master Airscrew props are a lot harder to tune based upon how the "flatten out" when you run them up and restrain the plane. This is my personal opinion, but from what I've found it is easier to tune a motor using an APC prop. Secondly if your running rich like your 1st post leads me to believe you may have damaged the plug element right from the first run after replacement.... take it out and compare to a new plug.

Lastly.... I would add to Iron Dog's prop suggestion..... The two props I think are good on this motor.... APC 12x13N (I won a lot of races with this combo), and the out of production APC 13x12N if you can find it (this is the prop that APC MADE FOR THE .91).... I have few of these left and for sport flying love them on my Mustangs and Spitfires..... but I must admit, I'm pretty happy with the 14x10 and the "gas mileage" has gone up.

Take care,
Jimmy Skids