That thing has a crazy fuel system on it. Dual fuel tanks with one valve check valve type fuel caps. Vapor lines from the two tanks to a expansion tank, On the tank is a rollover valve, and then there is a anti backfire valve in the line between the tank and the carb inlet. if any of that system gets clogged it will cause either fuel tank pressure issues, or vacuum issues with the tanks. The rollover valve and the anti backfire valve may be one way valves since the fuel caps may be the anti vacuum valves for the tanks. You may not be able to blow through the valves you may have to put a vacuum on then to check operation.
I would also do the check that Fish mentioned because it could be a ignition module issue also.
I would also do the check that Fish mentioned because it could be a ignition module issue also.