Birdman3

Hope some of you engine junkies will have some quick answers for me.

I bought a GMS .47 (remote needle) and broke it in last week. Ran pretty good for a day, though quite rich at low/idle, and hesitant to "blow out" to high rev when throttled on. Ran good once it wound out to WOT, but dead-sticked several times.
I tried to fine tune the idle the third day (low speed needle), and couldn't get it to stay running well. Sounded real rich and blubbery at idle, slow choking transition, then it would wind up and sound fuel-starved til I opened the hispeed NV about two full turns, then it would peak, and I'd back it out another half turn or so to drop a few hundred RPM's. As soon as I throttle back, she floods and stalls.

Then I noticed that when it wound out at the top end, it was drawing air from the tank (right from the first line at the stopper). Re-plumbed=no fix. Swapped out the tank with known good and different fuel lines= still doing it! What could cause that?

I'm running this on a Tower Voyager. The Voyager is designed for an inverted engine mount. I ran my .40 LA upright on it without any trouble (only affects the thrust line by 1/8").
Looking at it, I'm thinking the tank's too low, and the .47 is affected by it, not the single-needle LA. The TOP of the fuel tank is about 1/2" or more below the NV and carb if I mount upright. So I flip over the mount and put the .47 on inverted, and now I'm thinking the tank's TOO HIGH, as the centerline of the tank's gotta be almost half an inch ABOVE the NV and carb.
I give this a try. If I leave the PLANE upside down in the cradle (engine upright) it will start and run pretty well.(still draws air at WOT) As soon as I invert it, fuel begins dripping out of the carb. I can still get it started and wind it up a bit, but I still have the SAME PROBLEMS! It is rich and floody at idle/low, takes a bit to get it to wind up, then it sounds fuel-starved and lean (RPM's going up and down rythmically, every second or so) so I richen it A GOOD FULL TURN OR TWO, and it will wind up, then STARTS DRAWING AIR FROM THE TANK AGAIN!

Two different fuel tanks, three sets of line, upright or inverted, I can't get past this!

I'm running Powermaster 15% and a 10x7 Master Airscrew. She's blowing out a lot of oil during these runs and it appears pretty dark, real brown, almost blackish. Glow plug looks black (too cold?).
How can it be drawing air right from the tank? The tank is wedged in thru a fomer, so I suppose vibration could be a problem, but I can't pad it with foam, it rests right on the former!

Looking for all the pointers I can get!

Thanks a bunch!

RaceCity

If I were a betting man, I'd put my chips on a fuel foaming issue which is being caused by
jamming the tank in between the formers without any padding.

Quickest way to slap a mojo on your motor is to foam up the fuel. The black oil is likely
coming from the motor running lean/hot when the foam starts flowing through the lines.

Have to tried a smaller tank? One that will allow more accurate positioning AND some padding?

Not a lot of padding...just enough to keep the tank from contacting any part of the models structure.

??

'Race

Birdman3

Thanks Race. I'm willing to try anything!

RaceCity

No problem Bird....

I'm partial to GP or Hayes tanks, but Sullivan makes an assortment of tank styles to address
some of the more tricky installations.

(ARF Mfrs should take note....their tank compartment engineering often REEKS!!!)

Take a few measurements of the tank compartment of your model, then go to the TOWER website
(www.towerhobbies.com) and research the various tanks for one that would fit best.

8-10oz tank should be MORE than enough if you use some part throttle from time to time in your
flying.

Good luck, and keep us posted.

Regards,

Race

Birdman3

Thanks for the pointers, guyz.
I opened up the nose belly yesterday and cut out as much of the former as I could, and reconfigured for a Du-Bro 10 oz. tank isolated on all sides by foam. Ended up with the engine upright, figuring if I have to make any compromise, it might as well be for the most convenient operating position.
The 10 oz. tank has a wide, flat bottom area, so its height is as short or shorter than the stock 7 oz. tank.
Took her out for test flight yesterday, and she ran out a 12-minute clock before dead-sticking. After I got her in, I was pleasantly surprized to find out that she had simply run out of fuel! She drained the tank right down to about the last 1/2 ounce!

Thanks again for your help. Advice was on the money.

p.s.: Tank height appears to non-problematic. The NV & carb are still a half inch or better ABOVE the TOP of the tank!

RaceCity

Good Deal!!!

Glad everything worked out for you.

Have fun flying the model..

'Race

DarZeelon

Scott,


Don't forget the height of the tank, before closing up the plane.


The tank's center-line must be at the same level as the fuel jet in the carburettor.

Most trainers and some sport planes have the carburettor jet much higher, when the engine is in upright position.

You can add padding above, or below the fuel tank, to make this tank height directive as true as possible (it must be between + and - 3/8" from the fuel jet level).

If you don't take care of this issue, the problems will not go away, but will change instead....

Birdman3

Thanks for the pointers, Dar.

As mentioned above, she ran out the whole tank of fuel, even though the TOP of the tank is about 1/2" BELOW the jet.
The high speed valve appears to be open more than typical, and getting the low speed jet balanced is still tricky, but I'm not done fooling with it.

Any pointers on what to look for otherwise?

Bird

DarZeelon

Scott,


Many engines have very, very good suction, enabling the engine to empty the tank, even when it is mounted way too low.

BUT,,, does the engine run and sound the same, not too rich, when the plane is inverted???

I don't think so...
It is way too rich, since inverted, more fuel is getting through.

You don't want this to happen, so the tank center-line height = fuel jet height is the only way to go.


Even if you cannot get it exactly, get it within no more than 3/8".
Unless you have a Perry/Iron Bay/Cline pump/regulator; so you don't really care where the tank is.

Sport_Pilot

Unless he is using a uni flow tank configeration then this is incorrect. The tank center line should be below the carb, I have adjust them as much as 1' below when both carb and muffler is restrictive. When mounted with the tank about 1/4 inch below the engine leaned out when inverted.

DarZeelon

Hugh,


Getting the tank center-line at equal height as the carburettor jet is what all magazine engine gurus have been saying all along. Just get into the engine column in all leading R/C magazines and read; Harry Higley, Clarence Lee, Peter Chinn and others.

The tank mounting in many models is what makes side-mounting the engine (90 degrees to the right of the model, with the exhaust stack pointing down), so popular in many advanced models.

It puts the carburettor jet at crankshaft height, with the tank directly behind the engine - Presto! - the height is correct.

In an upright mounted engine, fuel jet level == fuel tank centerline level would be the case too, unless slow carburettor and crankshaft passage airflow, allows part of the fuel to fall out of the mixture. The engine then feeds partly on the pooled fuel....

When inverted, that extra fuel just leaks out of the carburettor....
There is no difference in the venturi depression, between an inverted condition and an upright condition, so the same amount of fuel is sucked through the jet, when the fuel jet level == fuel tank centerline level.

If your personal experience is different, there must have been a fuel pressure difference resulting from another issue, or fuel falling out of the mixture, as I described above, in your specific engine.


I am not going into an argument; just explaining the logic behind my position and that of engine gurus all over.

Uninflow tanks are designed so fuel pressure change will be minimal from full to nearly empty tank conditions.
I believe not even one in a hundred has a uniflow tank installed in his model...

Sport_Pilot

No I don't think they all do, in fact I recall some specific information from George Aldrich that explained this. I recall CL recommening that the center of the tank be below the carb fairly recently. When up right the pressure vent is un restricted the pressure to the carb is the muffler pressure plus or minus the fuel head from the carb. When inverted the fuel pressure is the muffler pressure minus the fuel head which is now restricting the muffler pressure plus or minus the head from the carb. See the differance? When upright the muffler pressure is unresticted, when inverted the muffler exhaust gas must pass through the fuel and the pressure is reduced by that amount. If you look at most ARFS the tank will be such that the spray bar will be at the 3/4 point of the tank.

Razor-RCU

My Feedback: (50)

Nevermind-

DarZeelon

Hugh,


I would want to read that information from (the late) George Aldrich.

The fuel tank and the muffler are actually two connected chambers, in which the pressure is approximately uniform.
The very low flow speed through the muffler pressure line (equal to the flow speed through the fuel line to the carburettor), dictates no actual pressure drop, even when the inflow from the muffler encounters a head of fuel, through the submerged pressure tube.

If you are still unsure of this, do not use an up-bent tube for muffler pressure; instead use a short stub, which is around the same height as the tank center...

You will have to 'fill her up', holding the plane's nose straight up, or you will give it only half a tank...

This way there is no difference whatsoever in the 'head' encountered by muffler pressure.

Sport_Pilot

Not sure of what you are talking about. I think you are saying that because the flow is low that there is no pressure differance. That does explain the fact that short inverted flight will not cause the mixture to significantly change. But if you go a lap or so around the field inverted you will see that the mixture of a center mounted tank leans out on most models.

Actually you may not notice, I doubt the differance is much more than 100 RPM and that would be an increase if you richened it up 300 or so rich. I had a Fokker Tri Plane that I had to center mount the tank, actually slightly above, and it ran slightly better upside down, but not by much. I guess it depends on how much muffler pressure you have, many of our planes have a lot which decreases this effect.

DarZeelon

Hugh,

I am saying the average pressure in the muffler (which varies and pulsates continuously), is about equal to the pressure in the tank. It is so whether the muffler pressure tube is submerged in fuel (as when inverted), or in the air (gas actually) above the fuel, like when upright.

If the fuel flow is very high, the muffler pressure will be higher than the tank pressure and more gas will flow from the muffler to the tank.

I have not yet seen what you just described.

Ed Smith

All of these tank problems, lean, rich, frothing, bubbles etc. etc. could be eliminated with the use of the Tettra style bubbleless tank. I do not know why they are not used more often.

But then there would be nothing to talk about on threads like this.

Ed S

Birdman3

Thanks guys, for info and insight.

I'm not sure I can get the tank height ideally suited, although I can lower the engine mount, or try the 90 degree mounting position.

I did not fly the plane in any attitude other than upright (except for briefly knife-edgeing the turns) , so I don't know how it performs during rolls or loops.

I'll continue to update the thread with results of further test and experimentation for the reference of all.

Thanks again!

Bird

Birdman3

Updat:

After spending FOUR HOURS one day fine-tuning the lo vs. hi jets, I got it to idle smoothly, transition smoothly, and run well at WOT.
Flew it about 8 flights since then, including several loops and quite a few slow rolls, generally the engine performed very well.
Last Wednesday sheinexplicably dead-sticked during a routine throttle-up after a pass. She made a hard wheels-down ditch landing in an adjacent field.
After some shop time examining for internal damage and touching-up, I took her out tonite for a sunset cruise. Initially, it appeared my foaming issue may have returned(perhaps I repositioned the tank when I was inside the fuse, maybe increasing the vibe problem again), but after a little tuning I got her to run for a brief sortie. Running a little rough. As you advised, Dar, perhaps the troubles are not behind me as I had thought.


Bird

Birdman3

New update 10-5-04:

Discovered a troublesome Needle Valve allowing air at WOT.
Swapped it out and she runs sweetly.
Two flights today running out 13-minute clocks.
Seemed to lean out a little in vertical, but more test flights will detemine if she's flyable!

Bird