Lean runs

In addition to having short runs due to the back plate problem, I have had a series of lean runs about the time I am in the vertical eight. Level laps before it, in it, or just after it. Then I have a very fast plane that gets ahead of me in maneuvers. From 5.3 to 4.5. This is with an OS .35 FP, blocked boost port and hemi head, OS needle assy. Or an OS .46 LA-S, stock. The only thing I have found is the muffler had come loose, about 1/16" at the front, just barely loose at the rear. My question is: will the loss of muffler back pressure allow the engine to pick up that much rpm? I haven't flown the wrecked ARF Nobler since discovering this loose muffler. It wasn't from the very low pass and sudden stop. Never let the top loop of the vertical eight get too big. Ha.

I've had the muffler line come off a K&B 4011 a couple of times and there was noticable speedup. I think your loose mufler would cause the change you mention.

Those symptoms sound familiar to me. It's the classic "runaway", which can be caused by multiple factors. One thing is that engine is getting warm from its load. Another is that fuel tank is partially empty, and with some uniflo tanks can develop a "bubble" near the fuel pickup vent. This bubble starts to introduce air into the fuel, which leans the mixture. If you are using muffler pressure, the extra pressure can make the problem worse.

There can also be a resonance in the fuel system, which manifests itself at a certain point, and it can introduce air into the fuel system.

Loss of muffler pressure during the flight will often tend to lean the mixture (decrease fuel pressure). And when the engine speeds, the added muffler pressure is no longer there to enrich the mixture.

If you are using muffler pressure on your uniflo, try running it with none. (You'll see a larger variation in setting between ground and flight mixture, which you have to compensate for in the ground needle setting). If this affects it, you have a clue.

You might end up taking the tank back off and moving the terminus of the uniflo vent a bit further from the pickup.

Also, try a shorter (or longer) fuel line or removing or adding a filter to see if it affects a resonant vibration.

A smaller venturi can help, decompression can help, a flatter wider (or different) prop can help.

Good luck. It's almost sure something to do with the tank.

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"This wallpaper is dreadful. One of us has simply got to go." -Oscar Wilde (his last words)

LAST EDITED ON Oct-26-04 AT 01:46 PM (CDT)
 
Thanks guys. I think it may be the fuel and vibration. It does not have more than 25% oil (11 & 11), 10% nitro. It is in a modified ARF Nobler. I have added balsa vibration dampners between the motor mounts. It did the same with the Brodak .40 which caused me to cuss the engine. Sorry B.40, it ain't ya'll. Hmmm. I think it may be the needle valve #####'y or the tank. More likely the tank. I was running a B-Y&O 11-5, set at 10,000 on the ground, which gave me 5.3 in the air, upright and inverted. No muffler pressure, but the uniflow vent is very close to the outlet tube, so I will change tanks. I have also read on here where at times it is a good idea to make the uniflow inlet opening about 1/2 by putting a bit of solder in it and redrilling the opening. I remember doing this on pressure tanks in combat, before we stole our babies passifiers, LOL>
amazing how this never seems to happen at the locl flying site, but only at a cold, windy contest. Ah, the gods of chance strike.
I guess I cannot abreviate assembly!! Too many (s'es) after the (a). LOL

LAST EDITED ON Oct-26-04 AT 10:51 AM (CDT)
 
I think, yes it could do it (muffler loose). Also, how much castor are you using in the fuel.

My TT36 from Leonard will do exactly what you describe if I dont use about 25% oil. I take 20% oil Sig Champ and add about 2-3 oz of castor. Keeps it under control because it runs cooler.

Also, are you running muffler pressure to the uniflow? This isnt always necesary, but can help in extreme cases.

Curt

PS: I have worked around this problem with the FP 35 and 40, Tower 40 etc, but have not seen it as a problem with the LA 46's

Ty, the first thing to check, provided you are using good fuel(22% oil mininum)is for air leaks in the fuel system. Make sure you can pressurise the tank by pinching the fuel lines and there are no leaks. I usually suck on the fuel line and put the tip of my tongue in the end.Sounds kinky, but it works if your tongue will stay sucked to the end of the fuel line. If the fuel system is ok, then try a filter in the fuel delivery line to help act as a mini sump to help eliminate bubbles in the fuel system. Hopefully your not using the stock os remote needle as they leak, even with those rubber o-rings....We use tigre needles....

Ty-

I have had similar problems with my box stock FP .35 (tongue muffler, now about 1:25 on the engine) on my P-Force. I believe that Larry's analysis is correct, but would add that in my case, despite the plane being very accurately and well built, there seems to be a fuselage nose resonance very close to where I want to run the engine at the 2-4 break. The fuel then is prone to foaming. I want a rich 2-cycle in the air, but for probable reasons similar to what Larry mentions, have to set it too rich on the ground. It finally bit me a few weeks ago, when an experienced friend set it too lean on the ground, and it cut out in a negative-g maneuver and fell into the circle.

My tanks were Smith uniflow metal with pentagonal cross section - rectangle, but with pickup in angled out right side. These tanks worked just fine with my Fox .35's (SkyRay) on uniflow. In my case these were the symptoms on the FP .35/P-Force combo:

1) The engine leaned out permanently within 1.5 laps in the air, or as soon as I did a wingover or loop. Only once did it do so when left to run a while on the ground. So I had to set it extremely rich at takeoff, which left barely enough acceleration to get off grass at low a.o.a. I got overly long flights at 4.4 sec/lap on less than 2 oz. of fuel. When I hit it just right, I got 4.8-second laps and reasonable times.

2) The engine ran more dependably when I plugged the uniflow vent and vented only through the overflow. Rather than richening the mixture in flight, the uniflow leaned it more than normal venting. So I figured it was short circuting the air, due to vibration.

3) Muffler pressure caused the engine to operate on a knife edge - one or two clicks richer or leaner would stall it on the ground. Unuseable.

4) The engine ran slightly leaner inverted and quit sometimes on outside (greater than -1.0 g) maneuvers. Wingovers were not critical. That told me that the tank was mounted too high.

5) When set to a certain localized lean 4-cycle, the needle valve knob vibrated at high amplitude. Sometimes the mixture would then change, but usually it needed only to be changed by 3-6 clicks to calm down.

6) Once it started to run away without this outward sign, and I squeezed the tank against the profile nose. The engine then richened to the point of stalling. I was unable to repeat this phenomenon on later occasions. The funny thing is that that vibration was not visible on this plane. My SkyRay with a Fox .35 vibrator and much more flexible nose generally has no such problem on the SAME tank.

Bill Werwage helped me with it one day, but we were unable to get consistent results, although we verified that it would not lean out on the ground of its own accord when set at its normal best.

What I had tried to solve the problem:

1) Replaced the venturi with a smaller one from an LA .25 - slight improvement only.

2) Added head shims - not improved, so I removed them.

3) Lowered the tank 3/32" to where pickup was even with glow plug/venturi C/L. There was a slight improvement. Next step would have been to mount the tank lower yet.

4) Put a couple drops of ArmourAll in a new gallon of Powermaster 5/22 fuel. No significant improvement, but might have helped.

5) Experimented with different foam rubber pads between tank and doublers - no change.

6) Put fuel filter back in line - no change.

7) Plugged uniflow. This made the plane stuntable with perfect setting, but rich setting on ground was still required, and engine still leaned too much/hesitated sometimes in negative-g maneuvers.

8) Replaced tank with two others of similar configuration, one a 3-oz Brodak uniflow. Results were the same, except that the smaller Brodak tank resulted in surging. The other tank had already worked well on the Fox .35 SkyRay, a much more vibration prone plane.

I believe that in my case, the uniflow in conjunction with vibration, was putting air into the pickup (maybe on the Fox, the problem required just a needle adjustment solution). I was going to try a clunk and perhaps cut the uniflow tube shorter. Unfortunately I fell victim to the "One Last Flight" syndrome.

Well, FWIW.

Serge

Hey Serge,

Stock 35 & 40 OS FPs are big run away engines. Do your self a favor & send it off to an engine tuner.

You hit the nail on the head. I have had this happen several times. When there is no muffler pressure you are running in a vacuum. The best case I had was with a 4 stroke on muffler pressure. Kept getting inconsistant runs until I went to straight uniflow. Had to turn needle in a full turn. DOC