what would make a cox .049 black widow (metal tank and backplate) run poorly while upside down?  It seems to run fine if you hold the plane and flip it over but when you launch it the engine slows down and dies only while inverted. I am running sig 25% that I just got last week.  It is getting annoying as I can only do loops and wingovers... no figure eights or inverted.

Anytime a reedy gets to running poorly You might also check the tiny o-ring that seals the tank to the backplate. A real source of trouble. Turning it over may only mask this problem so that you may think that only inverted is the problem. The way these engines are designed it does not matter to the engine itself what attitude it is in. Just so long as the fuel is not foaming, The engine gets the fuel needed (pick up tube) and there are no air leaks (gaskets and o-rings and in some cases the needle valve threads).

Robert

Jeff,

How are the tank vents positioned?

Upright vents will breath normally when upright, but are covered with fuel when the engine is inverted. That could account for the engines slowing, and eventually stopping. The engine draws only air through the vents upright, but when inverted it must draw air through the vents, and through the fuel covering them, which can lean the engine too much.

There was an illustration on one of the C/L forums showing how to turn the vents to the inside of the circle, and plumb the vents with fuel tubing, so they don't drain half the fuel while handling the airplane.

As I recall, it was a loop from one vent, all the way around the tank to the other vent, with the only opening in the fuel line towards the outside of the circle.

Anyone remember how that's done?

Bill

Many Black widow tanks have the "stunt tank" like the golden bee and do not suffer this problem. Typically most will rotate the cylinder to an inverted position as to simply bolting the engine upside down.

If it has the plastic backplate there will be only one nipple and the overflow will be flush. Can't do the "loop to the other vent" in this case.

Robert

        Jeffrey, its obvious that the plug is working due to the engine running upright. Is it possible the plug is on the tired side and its drowning out? I've been quite surprised how they will run with a new plug. I've found that using a smaller prop 5x3-5x3 1/2 can really get these engines going. I've found that when you start upping the nitro, the coil in the plug really gets hammered. The 25% is a very effective fuel for the black widow. I'm actually even surprised that the engine was running with the missing o-ring. It should've been sucking air with extremely poor fuel draw. While refueling the fuel should've been coming out of the venturi screen hole. If your firewall is painted, the raw 25% could've softened and removed the paint. I've had paint chips get clogged in the intake just enough to create problems but the engine still ran. The debris usually finds its way under the reed eventually. I would try to dissassemble the tank and use a straight pin through the vent pipes to make sure they're not blocked as well. I hope some of this info might help, good luck. Ken

Is the engine assembled with the cylinder vertical or horizontzal with the head to the outside of the circle?

When you pull it apart to check the reed for debris and ensure the venturi and vent tubes are not restricted take a close look at the fuel feed line. These tend to get brittle over time and it may have a chack in it that is exposed when inverted letting it suck air.

Re-assemble with the cylinder horizontal, this help eliminate the differences between upright and inverted.