Why won t my bikes start when they re hot?

i have two old honda bikes. Ones a 175cc road legal enduro from the 70s and i have a 500cc naked street bike from the 80s
they re both in pretty good shape and i got them with low mileage for their ages

the 175 is kick start and the 500 is electric

both are difficult to start if they ve been running for a while and they re hot. I usually need to wait 15-20 minutes for them to cool down if i go to the store or fill them up with gas or something

any idea why?

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