70hp Johnson won't start when warm

[Valv]

I have a mid 70' 70hp Johnson 3 cylinders.<br />It starts right away when cold and runs fine, but after use it for 10/15 minutes I cannot re-start it again unless I wait many hours. I replaced coils, plugs, rebuilt carbs, starter. It has spark on all 3 cyls when cranking and plugs don't look wet.<br /><br />Any hints?<br /><br />Thanks.<br /><br />Val