Lots of folks remove the OE Mikuni carb and install a Weber. It gives better performance is the consensus. I never had a Mikuni only the Weber. Try doing a fluid change oil and trans and coolant if it's old then tune up cap, rotor, wires plugs, Coil. See how it runs then. If it is a Weber carb there should be a vacuum hose from the driver side bottom of the carb to the vacuum connection on the distributor.

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