Engine sputtering at about 65

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, and I test these things, cuz I'm a curious guy.
>I'd be more concerned that it had too much timing.And I'd be most concerned that the Vcan had died. But,if the Vcan diaphragm works, and 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; at least for now.
>If it's related to throttle input,as you describe, it's most likely to be a mixture issue.IMO you have exactly 3 possibilities, a cold-running engine, the pumpshot,or the transition circuit.
>as to a cold engine
Cold engines require extra fuel on account of not all the fuel gets burned, instead sticking to cold surfaces, then tearing off in sheets, and passing through the engine,some of it unburned. The hotter you can run them the less compensation they require. Of course there's a limit to how hot you can go. I run mine at a minimum 205*F.
>As to the pumpshot;
Figure out how far your throttle is open at cruising speed. This may require some ingenuity on your part. Then stop the car, and stop the engine. Now open the carb to that amount, stare down the primaries, looking with a flashlite, and ever so gently open the throttles just like you do with the footfeeder; There better be a small amount of fuel delivered from the pump-nozzles, and it needs to not be a dribble.
>As to the transitions
Your carb has multiple transition circuits.
There are;the Idle to idle plus transfers; that plus the mains; and that plus the M-rods.And they are all air-flow driven,and/or vacuum controlled.
In your case,per your description, the primary suspect after the temp and accelerator pump,is the transfer to main transition, at part-throttle tip-in.
At 65mph, the M-rods should be stuck down hard,by vacuum. Your butterfly valves may or may not be at the top of the transfer slots, depending on the load being presented to the engine, and the throttle-opening being used to achieve the power to achieve the speed.The vcan should be at max timing, and the total all-source timing should be more than 40*@2200. Firing on all cylinders this can easily done on the transfers, by even a lazy SBM.
Ok so, you're cruising at 65, and start rolling into the throttle:
The very immediate thing that happens is that the manifold vacuum drops. Immediately. Simultaneously the Vcan may start dropping timing, but doesn't necessarily have to yet . Also simultaneously..... The accelerator-pump should deliver a bit of fuel, in response to the drop in signal at the T-slots.
If you then stop the throttle tip-in, the pump shot ceases and the engine vacuum stabilizes, bringing the Vcan back on line, if it relaxed. At the new higher cruise rpm, the D should be supplying a new slightly higher amount of centrifugal timing, and everything returns to normal.
But if you continue increasing throttle opening, eventually the T-slots get to max flow, and by this time your main boosters need be on line. But the boosters better be delivering pretty small droplets of fuel, cuz the big ones may not burn, and go straight out the tailpipe. Or, they may drop into the bottom of the manifold and stick there like raindrops do on your windshield, slowly creeping down the runners. Eventually they rip off and plow into the cylinder, and most probably never finish burning in the chamber.These may even be the cause of your sputter, but I don't think so.
So the trick is for the boosters to deliver those small combustible droplets. The carb does this by mixing air into the fuel, BEFORE it gets to the nozzles, inside something called the emulsion tubes. I'm suspecting a problem in these, or in the transition to these, iF the problem cannot be blamed on the temp or the pump-shot.
>As to the transition to power
The power 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,
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,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.And you would see that unusual timing on the vacuum gauge
In any case
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