I don't know what plug the Fuji uses but if it is not a resistor plug but has the product line that would let it be one, get a resistor plug.

You note that the hrottle rod is "2-26". I'll presume you meant 2-56. A 2-56 throttle rod is pretty flimsy and could easily be vibrating bad enough with the engine running to be making contact with some other metal component. Change to a minimum of 4-40. Even then if it's a long run support it in something non-metallic that will prevent it from banging into things.

The problem is only with the engine running, so vibration and not just ignition noise could be the, or part of, the problem. Are all the hardware pieces tight everywhere? Landing gear, linkage connections, motor mounts? What about servo, battery, and switch connections? You have a lot of things to look over and before you try to fly is an absolutely great time to do so.