Ok lets start easiest first
1 pull the gas cap off and try it (vent may be semi clogged
2 check your air filter remove it and run it
3 check the fuel flow pull off the feeder line with petcock off, put a paper cup NOT STYRAFOAM under the line turn on petcock if flow is slow check screen at the petcock ( fuel flow may be limited by rust)
If this does not work and flow is good stop and check your ign points for proper gapping and loose connections and make sure the shaft isn't worn and wobbling wiggle it see the play.
The if all is ok remove your carburetor and pull the float bowl down remove and clean all your jets and needles clean em thouroly and replace you will need a gasket kit I am sure if the bike is 40 years old sorry the cork gaskets are done.
while the carb is off inspect the boot leeding into the engine for cracks but if its idling I doubt you have a intake leak unless the idle is super high.
Hope this helps
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