Is it struggling, I mean really struggling to start? Check the vacuum advance unit on the distributor for leaks. I ran into a situation recently where I'd bought a wrecked truck for parts and the PO complained about 'the dreaded Mikuni flat-spot' even after a full carb rebuild. I checked the timing and it was right at idle but it fell flat on it's face in midrange. I dialled the distributor in and all but ironed out the flat spot, then it refused to kick over properly.