HI

I wonder why your LHS suggested to change the fuel....it doesn't have anything to do with your problem, it should run with 10% or less nitro without problem (if the fuel is ok..)

You have a problem with your tank, with your lines.....maybe you have connected the lines wrong by accident... maybe the clunk is now pointing forward pinching the tubing (common after a crash or hard nose landing) shake vigorously the plan and the clunk will loose.

How are you tunning the engine? if you screwed the tunning up....put the air bleed screw half way closing the hole, have the needle valve in a rich setting...go full power peak the engine, open the needle valve 3-4 clicks from top and then check the transition....you adjust with the air bleed for a good idle and transition, but not touching the needle valve!

Saludos,

Jorge