Should the crossover ports be blocked off completely if the valve isn't used? Sacrifice a little cold-bloodedness for a better summertime highway use? See where I'm goin' with that?
The only practical reason for the elimination of the riser valve is to allow the carb plenum to remain cool so as to allow passage of a denser fuel charge to the cylinders. One would have to live with it while cold on the street or at competition, the charge will less likely separate @ high RPM.
Like Charlie stated, a hot IGN SYS is going to help tremendously (less plug fouling from puddled fuel and long accel pump shots). If a street engine is used mostly as a driver, the valve is important (IMO)
This all changed with heated air cleaner inlets and electronic fuel injection.
And how many times have you looked down and the choke cable was still pulled?
FORGOT -
If you have a hot or somewhat hot street engine, yes, you want to block the crossover as exhaust pulses will cross over.
FORGOT AGAIN -
Unless you actually remove the valve or disable it (wiring it open), do not block the crossover. It was part of a service back then to check and lubricate the valve.
(All of the above is IMO as your results and mileage may vary)
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