Anyone have a suggestion? Just finished a Brodak Thunderbird. Power is a new O.S. 40 LA-S (the remote needle valve style). Fuel tank is Brodak recommended 4 oz. #BH-541.  Well, engine runs perfectly, on the ground that is! Tethered to the stooge she runs great. If I release and pick up the plane and mostly point down, air bubbles appear in the line between tank and needle valve. Engine stops almost immediately. Inverted and up seem OK.  Down gets it. I have pressure tested the tank (all OK), change positions of tank, even changed engines. What the heck is going on? At this point I'll try anything. Help!
Jim in Tennessee

Maybe I should just take her out and fly. Seems strange that even slight motions down would shut it down. I have been out of C/L for quite a few years. Just getting back to it.  What "static" (non-flying), position changes should the engine/tank set up tolerate?
Jim

Thank you very much. Now I understand. Will fly (carefully) tomorrow.
Jim

HI Jim,

If oyu crank the engine and rest the outboard wing tip on your foot, you can terst teh needle setting when you rotate the nose up or down.  This does take a full tank to do, but it works pretty good to set an initial tank height.

(not my idea, I stole it from Lou Crane!)

Bill <><

I'm kind of lost here. Is the plane now vertical along the wing axis? In other words it's now in the same position  it would be if hung by the leadouts. Then moving the nose up and down means "yawing" the nose with rotation about the OB wing tip??  OR are you rolling the plane with rotation about the OB tip??

Hi Dick,

Well my run for the Presidency was kinda shortlived, wasn't it?? 

Yes, the plane is held as if it was hung by the leadouts.   You do "yaw" the plane if you will.  This does roughly approximate a nose up/nose down flight path to a degree.  The head in the tank changes, but the fuel stays in the "wedge" so it will run for almost a full tank if you want it to.

Bill <><

Thanks for 'confessin' to swipin' my idea. I know it's in here somewhere, another thread, another day.

The idea is a way to imitate the angles of the forces on the fuel in flight. We have gravity and centrifugal force. CF is usually 3 or more times as much as gravity. Both forces act at right angles (within a skeeter's whisker) to the long axis of the fuselage - say through the engine shaft?

Start the engine. Hold the fuselage centerline horizontal through all of the following.

Turn the wing leadouts straight up and set a comfortable RPM so you can hear if it leans or riches out in the next moves. Bill mentioned resting the outboard wingtip on your foot. Two reasons: Keeps the ground/tarmac from scuffing your wingtip, and gives another point 'hoding' the model. The wings vertical position is basically a resting place while you think about what you're doing.

We don't need to duplicate the exact angles of the CF plus Gravity 'resolved' force. Any angle that we can pretty much repeat, large enough to cause an RPM change if the tank isn't right, is good.

The angle is in ROLL attitude, not nose up or down!

Once you got it running, comfortable, and you've  settled down, ROLL the model (fuse centerline horizontal) to about 45° 'cockpit UP' and tach or listen close to the engine.

Then, ROLL the model (fuselage centerline still horizontal) to about 45° 'cockpit DOWN', and tach or listen close, again.

If there's much difference between the two rolled positions, do it again until you are sure which atttitude makes it go richer (or leaner, just be confident you've got it nailed.)

Shut the engine off: Nose down, other wingtip up, and a finger over the uniflow vent tube is about quickest and safest.

The engine went richer to the side that the tank was "too high" in the rolled position. Lower it some. (If you judged by leaner, then the tank was too LOW in the rolled position where it leaned out. So, you'd raise it some.)

Adjust the tank height, and try it again. Going through the wig-wag and shut off a few times should get you to where there's no audible RPM difference between the rolled 45° positions, and very little RPM difference on a tach.

At least, before a first flight, this should put you close enough that the engine won't flame out on you because of tank height problems. For me, most often, little to no further tank height adjustment will be needed for even run both ways.

Thanks guys... will give it a try.