I'm going to put money on the accelerator enrichment pump diaphragm being shot or the jet is clogged. It's the small square assembly on the side of the carb with the lever arm pointing straight down. It gives the primary throat on the carb an extra shot of fuel when you jump on the throttle. Look down the throat of the carb with the choke butterfly held open and twist the throttle linkage a few times (engine is off) - you should see a straight jet of fuel shooting across the throat of the venturi. If it's a trickle or hardly and sign of fuel being pumped out, it's dead or had debris blocking it. You can try taking the pump cover off and unblocking it but there's a risk that the gaskets will be crap. Use a screwdriver that is is good condition and fits the screws firmly as the screws on the Mikuni are prone to stripping heads The accelerator enrichment is not a separate part and should come included in an overhaul kit (can be replaced without removing the carb but I'd be thinking about rebuilding it completely) Also check the fuel filter and eyeball what the fuel is like as the tank can accumulate water condensation and contaminate the fuel if it's been sitting for a while (plus fuel goes funky if it's old...)