My TL250 can be finicky when starting hot, like 90 degrees and low humidity.  Are there any tricks or methods that may help?  With near full choke and maybe 1/3 throttle it starts very easy cold.

So it's difficult to start only when the ambient temperate is high?  How does it start when the engine itself is hot?  Have you checked the valve clearances?

The TL250 was prone to over heating. Are you running the stock carb? You might go up a size or two on the pilot.

8 hours ago, lineaway said:

The TL250 was prone to over heating. Are you running the stock carb? You might go up a size or two on the pilot.

Additionally,

1. Check the timing.

2. If a carburettor heat shroud is not fitted then buy or make one.  

 

I have not been able to test with hot engine and cool air temperatures.  It is hot engine and high air temps.  There is no heat shield.  The front muffler (bulge in exhaust) is near the carb and I wonder if wrapping it in heat shield would be the same as a heat shield.  

It has a Chinese copy of an OEM type 22 mm Keihin carb.  I did have to increase the throttle slide cut away to get rid of an off idle stumble.  I wonder if I could have gone too far.  I have what I believe to be the OEM TL250 slide to copy but no other OEM carb parts.  I'll check timing and valve clearance.  

If it is a non-original carb, how did you set the float level?  If the valve clearances are set perfect, I’d be visiting the float level to possibly address the off idle stumble.  Carburetors need to be isolated from heat that can boil the fuel and from vibrations that can turn the fuel in the float bowl into froth.  Plug read should tell if carb settings are rich or lean, post up a photo of your carb setup and sparkplug after running for a while might help us to assist.

Will it restart easily immediately after a hot shutdown? That would be a clue whether heat-soaking is the issue.  Is this bike new to you, or is it a new problem?

Could be something as simple as using "winter" vapor-pressure gasoline in the summer.

I checked and valve clearances are good.  The carb is a nice looking copy of an original so float height is 25 mm per the manual.  The manual is a little confusing in that it says 25 mm to carb body but looking at the picture the gasket is in place so I set it 25 mm to the gasket.  I cleaned the carb and will put in fresh gas to see what happens.