Change the metal throttle rod to a "nyrod" type rod and use plastic clevises at both ends.

Even an electronic ignition and metal throttle rod will give you possible interference.

A short threaded rod at each end is fine but not over about 1" long for adjustments.

Then try it again.

You may also want to check your kill switch wires making sure they are not too long.

Don't try to run the kill switch wires back where the receiver or servos are located.

Keep the switch as close to the engine as possible with short wires.