Trouble with a new Thermoquad Installation

Hmm... sounds like you might have a vacuum leak. Try spraying WD-40 or similar around the carb base and notice if the engine speed changes.

You also might have to do a little tuning to get it to run just right on your 340. ThermoQuads are very sensitive to incorrect tuning and have to be totally calibrated to your exact engine setup. BTW what's that 340 got in it?