CB400f won't start after stopping when HOT!

Hi!
I have a 1975 CB400f in good condition. Clean tank and carbs. Have been using the bike around town without any problems.
Yesterday I took her out on the road and drove about 40KM with revs steady around 4000rpms. I stopped for a chat with a friend and when I tried to start it she didn't fire at all! Cranked over, but no ignition.
I let her cool down an hour and tried again and she fired right up and I rode all the way home for another 40KM without a hick up.

I run straight pipe and open filters with #38 Slow jet and #85 main jet.
Anyone with any insight to my issues?

Happened again today. Tried again after 10min and started on first try.....
Hondaman thinks it could be the Dyno S or the Coils that are overheating.

Did you leave the ignition on while you were chatting?

HM is usually on point, and that sounds like coils going bad. Are you running the OEM coils or aftermarket with the Dyna S?