I have a Cox Killer Bee mounted on a control line plane that I built. The engine is quite new, but well broke-in. I'm using a 1/4oz Brodak Perfect Wedge fuel tank. When I try running the engine it starts right up but then the vibrations from the engine start to shake the plane and tank and air bobbles fill the fuel line killing the engine. I balanced the prop, but this still dose not help.

Is the hole in the prop square to the face? Does one tip follow the other as it rotates? Also, make certain your engine is solidly mounted.

Possible causes not in any order:

prop unbalanced or badly warped
prop hub not square, causing prop bolt to bend
bent prop screw, move up to grade 8 hex bolt [5-40]
motor mount too thin or too soft
internal parts of engine getting sloppy

Generally speaking, I don't bother balancing 1/2A props. Even badly warped rubber props run smooth at 20,000+.

You can do something that will contribute to all around smoothness if the engine is running in ultra high range [above 24,000 for a Cox], shave material off the inner skirt of the piston.

Check to see if the piston-conrod connection is reasonably tight. If not, you to get a reset tool. If you push the rod in and out while holding the piston, you should feel almost no movement.

What prop are you using?

The great Walter Musciano once published this advice in a book published in 1956:

Make sure the fuel tank is securely wedged into place or else vibration will cause the fuel to foam in the tank causing erratic engine runs.

Obviously he was able to stop foaming not by fiddling with the engine or the prop but by making darn sure the tank could not possibly move. I suppose it wouldn't hurt to make sure all 4 of the engine bolts are absolutely secure as well.

Robert

I usually put a 1/4" piece of semi-soft foam between the tank and the fuse for my CL.