Also, crap in the air filter (clean it anyway), fuel filter (clean any inline filter anyway - if there's no inline filter get yourself one - flush the tank and clean the petcock filter), or the exhaust (if it's been sitting or a while)
If it's not that, it could be that the jets or feed are all clogged up on the second carb, if you remove the carbs to check the second carb slide it's worth doing a full clean and check anyway. Takes a while, and you need to be methodical, but worth the effort.
Check the inlet rubbers for leaks / splits at the same time.
Simon
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