Saw your post on RCUNIVERSE just now.

Note that the suggestion to check the alignment of the engine won't do much.

The Venture allows the motor to move in only one direction, that being up and down, then only slightly.

You're stuck with the alignment of the clutch pinion gear, the large drive gear, and the tail rotor drive gear due to the way the three axles - clutch start shaft, main rotor shaft, and tail rotor drive pulley -- are held in the chopper. The gear mesh is determined by the fact that the six different bearings are held in the frames by molded in recesses that fit the bearings. None of the bearings (therefore none of the three shafts) can move horizontally in the frames.

If the engine is pushed up too high into the clutch bell, you will end up with the clutch shoes binding with the clutch bell. The drive train will still turn, albeit with some difficulty.

A potential problem that COULD bind the drive train in flight is if the large drive gear has enough play in its hub that would allow the large gear to tilt slightly. This would cause the system to bind enough to periodically bind while the rotors are turning. My first Venture suffered a bad case of wah wahs due to this problem (periodic binding of the drive train while flying, causing the tail of the machine to wag back and forth).

The problem you have occurs only when you have the tail rotor drive belt tensioned...that tends to tell me you've got a problem in either the tail rotor shaft bearings, possibly the shear pin that pins the rear pulley to the shaft has slid out of place, or the bearings at the drive end of the tail drive belt have a problem.

It's possible that a bearing is shot or full of loctite.