He hasn't said if it runs ok off the choke.
What if there is a fuel supply problem due to clogged fuel filter or fuel line?
All of the above is sound advice but I'd be tempted to keep it simple at first.
As aslready said, drain tank and shove some new fuel in there, then check the fuel filter (it's easy to remove when tank is drained etc) and check the fuel supply line out of the tank to the filter, then the line from the filter to the pump, and then the line from the pumo going over to the carbs. You;re looking for blockages AND kinks or perished rubber.
If that all checks out (it' only take about 30 mins and is dead easy) then I'd then sigh and start dissassembling the carb body, after I had a fag and mug of tea