[Dick Boneske]

The bucking problem can be a power valve spring that is too strong, causing the power valve to open intermittently when there is not enough vacuum to keep the rod up off the valve.

Another cause of the bucking off idle can be ignition timing. Ideally, with vacuum advance operative, you need between 6 and 10 degrees initial timing and about 34 degrees total timing (with the vacuum disconnected). After confirming this, connect the vacuum advance and check total timing at about 2500-3000 rpm in neutral.needs to be less than 50 degrees. Some vacuum advance diaphragms allow much more than that, causing problems with surging and detonation under light load.