G surges when we try to idle

Just got her yesterday.

Our new G runs smooth and strong at full throttle and  also runs well down to a little below half throttle.  But the governor starts surging  at about 1/3 throttle and slower.

If I sneak the throttle down very gradually I can sometimes get it down a little slower before the surging starts.

So far I have cleaned the carb.  By that I mean that I blew compressed air through all the orifices.  I also replaced the spark plug wires which were in very bad shape.

Both the carb cleaning and the better spark plug wires greatly unproved power at the top end but didn't help much with the low speed surging.

What should I try next?  Thanks.

Surging at low speeds can be a hint of carburetor running lean on the idle circuit whether by adjustment or by crud I don't know about the G carburetor.

Surging like that is caused by the engine slowing more than the governor expects when the throttle is closed, so the governor opens the throttle which gets the high speed circuit in the carburetor working that's not lean and so the engine speeds up more than the governor is calibrated for so the control system oscillates.

Lean at idle can come from dirty idle passages or low gas level in the float bowl or a small intake manifold leak.

Gerald J.