Yes, getting 1/2 of typical mileage is sure sign you're dumping too much petrol into engine. If you're already at less than stock adjustment on mixture-screws and it still appears to be too rich, it would indicate something beyond external adjustments.
So most likely there's something non-stock inside carbs themselves. Non-stock jet sizes, perhaps even non-stock needles and/or heights. This is most likely issue since most common issues are in opposite direction with clogged jets & circuits causing lean mixtures.
Don't assume printed number on jets is actually what's being used. You'll want to measure jet-sizes by inserting soft-copper wire of increasing size until you find one that's snug. Then measure wire with micrometer/calipers.
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