When I hold the plane vertical with engine idling, RPM would slowly decreases and eventually the engine quits. I see a few bubbles in the fuel line before the engine quits.

The low-speed needle setting seems right. When I pinch the fuel line at idle, the RPM would increase after 4~5 seconds. I tried richen and lean the needle, but the engine still quits whenever I hold it vertical.

I set the high-speed needle slightly rich so that when I hold the plane vertical with full throttle, RPM would increases a little.

Are there any other tips for engine tuning? Right now I'm using idle/full throttle pinch tests to determine my needle setting.
Thanks a lot!

When you say "eventually"... how long is that?
Keep in mind that if it's longer than four or five seconds... you're probably fine.
(Can you think of a flight condition where you'd be dead vertical, nose up, at idle, for longer than that?)

That said... you might make sure the muffler to crank seal is good, and that there are no
leaks in the pressure line. Still... I have to believe that most engines at idle will eventually
sag lean and die, if the plane is held completely vertical for any length of time.
There's just not enough pressure to keep forcing the fuel up to the carb.

I could be wrong, of course... never tried it myself

<edit- typo>

First thing I would do is check the fuel lines. Check were they attach. See if there any holes. Check the line to the clunk see it it's got a leak near the stopper. When the plane is pointing up it's out of the fuel! My $.02 Good Luck

C_Watkins,

I would say the engine runs for around 3 to 4 seconds before it quits. You might be right regarding the plane pointing straight up at idle the engine just won't run. It's hard to think of any situations, both in full scale and in R/C, where the plane is pointing straight up with engine idling, even when doing 3D/aerobatic.

The fuel lines are good. As I said before there was no fuel starvation; I only saw a few air bubbles in the fuel line when plane is straight up at idle. However I'll make sure the muffler is tight.
Thanks all!

I'll admit I "assumed" something too... based on your original post.
I'm "assuming" that it runs fine at full throttle, nose up and vertical. Correct?
If not, then I agree with the previous person who said definitely check out fuel lines and such.

If it's fine at throttle positions other than idle, and it flies ok... no problem.
You might add a one-way checkvalve inline on the pressure side... to hold a bit more pressure,
but I can't think of much else, off the top of my head, that might be of any assistance.

Well... I would consider the engine dieing in the manner described.... normal.

Engine at dle... minimum muffler pressure to the fuel tank. Nose straight up... maximum gravity to overcome getting fuel to the engine.

The bubbles he's seeing would be air entering the fuel line via the fuel jet in the carb. Nose straight up at idle is not a good test for seeing if the engine is set correctly.