Nitro engine dies when pointed in downward angle?

by Dane
(Minnesota)

I have an OS.46AX that sputters / dies out when airplane points at the ground (both straight down and even at a 45 degree angle down) The low and high needles are set and the engine idles great, throttles up through mid range - full range excellent and back down to idle no problem.

I have a 3 line setup in the tank (1.clunk to carb 2.fuel/defuel with clunk 3.vent to exhaust) The clunks are not sticking in the tank...and the situation happens with full / nearly full tank also. Running 10% nitro

When I ground test it I can hold the aircraft in any orientation: straight up, upside down (on it's back) sideways (either side) but when I point the nose down (around 30 degrees and lower) the plane wants to die. On idle it will last about 30 seconds...at full throttle I'm lucky if I get 4-5 seconds. 4-5 seconds straight down is a LONG time....but 4-5 seconds at a "dive" angle isn't too long really.

Thanks

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Nitro engine dies when pointed in downward angle?

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Jul 26, 2009
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Heavier clunk
by: Haidar

What I found out is that your clunk is not that much heavy to immerse un the fuel when you plane pointed downward.
You can use either a heavier clunk or use a silicon tube with a thiner wall thickness than the one you are using now (Inside your fuel tank).

Using an extra clunk for refueling line doesn't do any thing for you except the risk of preventing the main clunk to play freely inside the tank during harsh maneuvers.

May 07, 2009
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Results
by: Dane

Thanks for the suggestions everyone.

Tonight I installed all new lines and hardware in the tank. (Kept the 3 line setup) The clunks are about 3/8" from the back of the tank.

The results:

~Full tank: Engine will stay running when at downward angle / straight down.

~About half full - empty: Engine will "starve" for fuel, level out quickly and engine will remain running.

I feel this is as good as it's going to get. I trust the plane / engine enough now to take the plane up again for some more testing in the air.

Thanks

May 07, 2009
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Switch the Lines...?
by: Matt

Hi Dane,

Is your fuel tank set up like the image below?

I agree with Bill that it sounds like the clunk for the carb my not be just right. It may not be completely stuck, but the length of the tubing may be causing it to not move around as freely as it should.

If your set up is like the one above, there is a very easy way to check this. Switch your fill line with the carb line and see if the problem still happens. You can easily do this without removing the tank.

Well, kinda easy, if you have a set of alligator clips to get a hold of the fuel lines. Easier than pulling the entire tank out.

As the first guy commented, the line is definitely sucking air from somewhere. If switching the lines don't help, I'd suggest replacing all of the tubing on all three lines making sure the clunks are free, and the problem should go away I would think.

Keep us posted,

Good Luck!

Matt

May 07, 2009
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Engine dies pointed down
by: Bill

Without seeing your setup it sounds like the pickup clunk isn't able to move down with the fuel. If it runs fine in all other angles it would surely seem like fuel starvation because the internal tubing is too long and not allowing the clunk to follow the fuel.

May 07, 2009
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fuel sputter
by: Anonymous

You may want to remove the fill line to the tank and install a "T" in the carb feed line. Automotive vacuum fittings work great! You do this and you will need to pinch the line between the "T" and the carb to prevent filling the crank case. Are the fuel lines in the tank tight on the, I assume, brass tubing? Gotta be an air leak somewhere!