New engine nearly top to bottom, runs great in driveway sputters horrible under any load. Tried different carbs, no diff, timing set right( all new MSD including Dist.). Wraps up great parked, but not so good driving. Fuel system more than adequate, everything very gonzo on car.

I had that problem once with a new motor. Problem was fouled the plugs real bad trying to start it. Replaced plugs and it ran fine.

Maybe both carbs were lean.

Maybe you got a bad part in your new MSD ignition system.

Maybe the fuel pump or filter is bad.

Peter

Could be a vacuum leak. Check all connections and lines, and even around the carb base. Use propane gas, (don't light it), just let the propane gas out and run it along the lines and carb. When it finds a leak, the rpm's will rise.

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Joe G.
ChicagoChevelle68
Any car past 1972 is just simply transportation, really!