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YS FS-91AC mounted inverted in WM Mustang 60 problem

Old 02-01-2009, 02:42 AM
  #1  
jamesc43
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Default YS FS-91AC mounted inverted in WM Mustang 60 problem


I have a YS 91AC 4 stroke engine mounted inverted in my World Models P-61 Mustang 60 with a 15X8 prop.
The engine runs flawlessly. The plane flies fantasticly around 1/2 throttle. The plane had great speed at full throttle
and has never let me down. I run Magnum 30% heli mix in it. Today I noticed that when I did a half loop the engine
(now sitting upright) started smoking more than it regularly smoked. I rolled it back over pretty quickly because
I didn't want any major issues while flying. I've never known a mustang to fly dead sticks very well and I do not
want to try it if I don't need to.

I want to know why it smokes more while the plane is inverted (and the motor is right side up).

I also had a YS 91FZ installed right side up in a GP Big Stick. I have flown it a lot and I never had this issue. The only issues I had with it was when I attempted to fly real high (guessing about 3000ft above our flying field ground level) is the engine stopped once which ended up with a dead stick landing and the fuel tank stopper popped out. Both of these issues happened while the plane was decending down from the high altitude. Since then I have stopped doing those thing and I have had no more issues.

P.S.
The field we fly at is located at 6200 ft altitude. I would say the planes fly anywhere from 6200 ft to 7500 to as high as 8000 ft.

James in CO
Old 02-01-2009, 09:26 AM
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petec
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Default RE: YS FS-91AC mounted inverted in WM Mustang 60 problem

Considering how the fuel system works on the 91AC there only two things I can think of that would give you more smoke when the engine is straight up as opposed to inverted and bothof them involve unburned fuel or oil residue.

First would be the bit of fuel that is in the valve cover is now being dumped into the combustion chamber when you roll from the engine being inverted to being upright. If you were to pull the valve cover off your engine you would see some holes around the intake valve that lead from the valve cover area into the intake tract.

Second would be if you were using a 91FZ muffler as opposed tot the straight 91AC muffler and the outlet now became turned up instead of down which might keep a bit more oil residue in the exhaust stack where it would cook off and produce more smoke.
Old 02-01-2009, 11:09 AM
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Jimmy Skids
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Default RE: YS FS-91AC mounted inverted in WM Mustang 60 problem

James,
This is very normal. All my YS (running headers only) do the same thing when I roll the planes so that the head is up allowing accumulated oil in the valve cover to flow into the motor and burn off leaving a little heavier smoke trail. In fact when I have trouble seeing a smoke trail I will sometimes invert the plane to see if I can get a "puff" just to see if it is the motor running to lean or if it's my vision for a particular day/sky. If yours has a huge amount of smoke then there could be something wrong, but if it is only for a short period you should be good.... Next time you fly the plane go inverted and hold inverted to see if the trail lessens after a full field pass. If so then you probably are seeing the same thing.
Jimmy Skids
Old 02-01-2009, 11:44 PM
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jamesc43
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Default RE: YS FS-91AC mounted inverted in WM Mustang 60 problem

I'll give that a try when I have it out next time.

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