I am NOOO carb tuning expert at all, but from what you've described, and what I've read, I can't imagine how the intake would be causing your problem.
Certainly would be easy enough to test though: take it off and mount the filter directly to the carb and see what happens.
All the other intakes I have seen like yours are either straight or made from a single 90 degree piece. The extra length is supposed to help smooth out the flow of the air before it goes through the carb.
I suppose it's possible that your set up, with the joint in the middle, might be creating turbulence in the air stream? Still, I would think it should be better than just attaching the filter directly...

If you have a filtered opening pointed into or somehow catching the wind and it's fed to an airbox, the airbox will do a lot to handle the increased air pressures.  But fed directly into a carb the pressure would increase with speed.  It wouldn't draw much fuel through the opening.  I know you know this already but allow me to explain it for other member's benefits.  With that setup, air is being pushed through the carb instead of being sucked through.  It creates air pressure in the carb that is greater than the air pressure outside of the carb.  The carb needs pressure that is lower than outside air.  It is that low pressure that creates a vacuum effect and pulls the fuel up through the jets.  Pressurized air may in fact push into the jets and make it harder for the gas to come up from the float bowl.  That type of setup works best when combined with an airbox and FI.