Your timing can be 5 degrees late and it won't cause a sputter. Even 10*, and maybe more. I know because I have a dash-mounted, dial-back timing device with a working range of 15 degrees.If the timing is at least close,think about it; as long as the coil can light the fire at the sparkplug tip,
and you have already proven that it can by driving thru the problem;forget about ignition.
If it's related to throttle input, it's a mixture issue.IMO you have exactly 2 possibilities, the transition circuit, or the pumpshot.
The transition, in the case of a metering rod carb, is controlled by the step-up rods and more specifically the springs under them........which have to be synchronized to the manifold vacuum.
...And that leads to suspecting manifold vacuum, which leads straight back to the basics, testing the compression,the idle-vacuum,and maybe the valve springs, or lash.Then the fuel, the fuel level,the fuelpump output,finally the carb.You can keep on working on the carb, or jump in anywhere you like, but I doubt you'll get to the bottom of it in a reasonable period of time. I'm old, and I learned the value of basics many many years ago, about spring of 1968.
There's only one way IMO, that it could be timing related, and that would be if it was severely over advanced or severely late, both of which lead to vacuum issues, and thus step-up issues, and thus transition issues.
In any case
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