With the throttle closed and the engine idling, there is a very strong low pressure between the intake valve, the carb's slides (or butterfly, as the case may be) and that low pressure is exposed to anything in between the two... like an air leak. Standard atmospheric pressure will try and fill that vacuum.
So, if there is any kind of an air leak between the two.... the idle mixture goes lean. Since the idle mixtures can be a bit rich to keep it ticking over at low speeds, an air leak can momentarily speed the engine up and advance the mechanical ignition timing which can speed the engine a bit too.
And when the mass of air flow and mixture rushes in, that fuel mixture gets used up faster and the engine slows down again, retarding the timing a bit... etc etc ...starting the whole process over again.
It should not be crazy jump in RPM just a few hundred RPM pulse up and down.
However, as I think about it, it sounds a bit more like a combination of carbs out of sync, idle speed is incorrect and the idle mixture screws not set the proper idle condition mixture.
Bruce
1977 KZ1000A1
2016 Triumph T120 Bonneville
Far North East Metro Denver Colorado
