This is a VERY interesting conversation. And it really does make me think..... that if an engine runs fine while going straight up in the air with the tank several inched below the carb,why would it make a difference while in level flight if you flip upside down? I will say,I too can hear a lean/rich difference many times though while doing this as well as diving and stuff. Not sure what to think,but I do have one question.....

Is it easier for an engine to RICHEN and flood itself like when flying level upsidedown with the tank then higher then the carb then it is to.......LEAN itself out when flying straight up in the air? Perhaps engine are simply more tolorant at "taking" fuel,then "accepting" it?

...maybe this could explain all this,because it sure does seem like both sides are right.

I don't know about the physics but I fly a spitfire with an inverted ST 90 and no pump. The tank centreline is about 2" above the carb when level. The engine must be started with the plane inverted to avoid syphoning but once started it runs fine, upright or inverted. No special tank plumbing or anything, it is just a simple two line system.

1/4" to 1/2" is being pretty obsessive IMO, the engines still run fine when the tank is 8" below the carb when the plane is pointed vertical.

All the comments made , have made sense , and the discussion has been very informative .

I would use the process of elimanation and try all the info you have been give to solve your problem . We can only give our best guess, becuase we are not hands on with your problem .

Short answer, Yes, many people tend to fly their planes in a lean condition for long periods, until the engine finally fails, then when they open the engine find that the liner is distorted and discolored (A bad sign that the engine operated in lean condition for a long time).

Not so many people tend to run their engines in rich conditions because they quit so fast (very common situation at most fields) or are way underpowered and is not what a pilot wants. (less tolerant)

I have got my plane back together and did a test run today.

I tuned the engine until it ran fine “in my opinion”.
Then I held the plane inverted for quite a while and nothing seemed to happen.
I flipped it over again and played around with the throttle and then flipped over again inverted and held it in my hand again for a while and this time the rpm dropped quite a bit. I did a few more tests whit all kinds of throttle settings and needle settings. The engine never stopped, but did not sound very reliable.

The plane is stock built, so the tank is there where it is supposed to be as designed. I really wonder if the height of the tank is really that important, the designers are so reluctant about the position of the tank. Many ARF planes have their tank outlet lower than the carb inlet.

About my engine tuning. I could run full throttle for about 14 minutes, with 5% nitro, with humid weather on a ASP .46 engine. Is that about normal? Or am I tuned way to lean? I always fly not longer than 7 a 8 minutes, so I had no idea how long my thank would last.

Also how many bubbles are you supposed to see in the fuel line when running full throttle? I can see quite a few.

Thanks,

Filip

None. You should see no bubbles. This is causing you to run lean. I would check the fuel line for a pin hole. Pay close attention to where the fuel line slides onto the tubing at you tank. That is normally where the line is cut. Also be sure that you tank is mounted wrapped in foam. It should not be touching wood. It should have foam isolating it from the planes structure. At higher RPM's the tank will sometime vibrate so much that the fuel foams if not wrapped properly.

Like he said, bubbles are bad.

I've used the "sintered" brass dubro clunks to great effect to cut down bubbling on the fuel lines.

Remember though that in a nose down attitude with less that 1/2 of a tank, it becomes almost impossible to avoid bubbles as the fuel depletes.

Since the clunk is not permitted to "travel with the fuel" most planes have a tendancy of deadsticking on nose down aerobatics as the tank runs below 1/2.

What I see I can hardly call bubbles, it is more foam that collects on the top curve of the fuel line that comes and goes.
It could be indeed that the fuel in my tank foams too much. My tank is touching wood on many places.

After reading through this thread, I have to agree with opjose. I would first look at other solutions such as replacing the glow plug, checking your plumbing / clunk,not enough foam around your tank, and carb fuel mixture. The height of the tank is pretty insignificant.