Bike wont start while warm and sitting for a few minutes

[kk4dbi]

Hello, my gz250 turns over fine with a little throttle when cold (its hot outside now) but after I get off my bike and go shopping or get the mail for say 1-5 minutes the bike wont start it just keeps turning over. I don't push it if it doesnt start after a few cranks I let it sit so I don't flood the engine. This doesn't happen every time I use the bike but it has done this today and yesterday. When I sit with the bike about 5-8 minutes after the startup attempt it will usually turn over (as I curse and pout). I don't want to get stuck anywhere..

Here is some background on my bike:
7800miles
Some oil leaks out of the head when I ride it at 65mph for a few hours
Carb has never been cleaned to my knowledge (purchased the bike at 5k)
Bike has been parked outdoors its whole life.
Fuel is good (this has been going on for several tanks by now, just been getting worse recently)
New spark plug.
The bike runs excellent when it does turn over so no ignition problems.
If anyone has any input that would be awesome
Thankyou