Weber secondary cct. if coasting is OK and full throttle is terrible. Check for cleanliness and function of the carb. Compressed air is great for checking these things. Possibly blocked pump jet nozzle as well. Does it bog down or pop when at full throttle. pop /carb backfire is a lean condition , bog down is to rich. Sounds like it consumes Oil none the less. being a former turbo build i'm going with broken rings. Performed a compression test? Does compression improve if you squirt some oil into the cylinder? if not. the valve guides could be out of tolerance as well.