What fuel should I run in my 340?

The gas octane should not affect the idle quality. The octane level controls or limits the detonation at acceleration or high load. As stated in formulaS340 reply look for plug wires, vacuum leaks, and timing. Also check check vacuum at idle to see if it varies a lot; could be valve leak or misfire. Also if the idle setting of carb is too rich, it will idle bad but drive good.