If the engine is running out of steam by the time it gets into mid range rpm, I'd first check the secondary vacuum actuator. The diaphragms in them are prone to splitting whether they are driven every day or left sitting for 5 years. Easy to test if it's working (with the engine off) - simply disconnect the small elbow hose running into the top of the vac actuator, fit a long hose onto it, open the carb to wide open throttle and suck on the other end of the hose (at this point the secondary should open right up). If it won't hold vacuum, it's toast.
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