1) charge battery, check battery
2) clean all battery cable ends and contact points ( be sure cable size is big enough, even 12 volt system benefit from large diameter cable )
3) get generator cleaned and/or checked ( if it has an alternator, go to next step )
4) clean all generator or alternator wire contact points
5) clean, check voltage regulator wires ( disregard if it has an alternator )
6) with it running, check voltage at battery
A) if it won't start, or turns slow, pull starter off for a cleaning and check
B) if it won't start, clean and file points and clean spark plugs ( don't mess with timing )
C) jack up each corner, check for brake drag ( you cold have faulty wheel cylinders or brake hoses from setting )
D) clean carburetor and change or clean fuel filter last if everything above is OK
You could just have bad gas, so add fresh to it or drain and refill with fresh. Water in fuel makes for really bad running engines.
When it does start and run, get it up to road speed and drive it a while, the miles will help it.
Heat flap in the exhaust manifold won't do what you are having trouble with. The heat helps fuel stay vaporized, on hot days it likely not needed, on humid days you could get a little stumble every now and then.
