I have a yamaha xv250s 1996.unreliable tick over ,hesitates on opening throttle .thought it was a blocked jet.?

have taken carb to pieces and no jets were blocked,also diafram is ok.what i have now found is if i dont connect the 3 wires that go to the carb heater and the thermo switch (these are 2 units screwed in to the carb) the engine ticks over and the hesitant throttle are ok the bike runs fine,but when i connect them back the problems come back.any help or advice would be most kind .thanks dave