XT225 - engine shuts down in extreme heat
 

Hi Folks, Greetings from Bamako. We've made it through Benin, Togo, Ghana, Burkina Faso, and now we're finally in Mali. It's hot! and the bikes are complaining. I'm on a Yamaha XT225 Serow from the early 1990's. In the heat of the day (110+ degrees F), the engine shuts off, like I'm running out of gas. If I wait in the shade five minutes, it starts up again without a problem. I recall reading something about this phenomenon in Chris Scott's Adventure Motorcycle Handbook, but I don't have the book with me. Something about gas vaporizing in the fuel line so it doesn't reach the carb. Any ideas/suggestions? The carb is pretty well adjusted, and the bike runs great in the mornings and evening. Unfortunately we've got lots of distance to cover, and we can't simply avoid riding in the mid-day heat. Thanks! Christoph PULAKU | the fulani photo-documentary project

I had a similar problem after rebuilding my XT600Z. For me it was the twin petcocks either side of the fuel tank were joined by a T piece before reaching the carb. I had underestimated the length of fuel hose required leading to:

1) the fuel hose was to close to the engine allowing the fuel to become fairly warm
2) there was a U bend in the pipe. Only instead of u it was n if you get my drift.

This allowed for for the heated fuel to create a vapour lock preventing further fuel flow. Whether you have this issue (vapour lock in a proud bend of the pipe) or simply your bike is overheating and the jet sizes of your carb were on the tad rich side to begin with I couldnt say.
Heated air does affect the stoichiometric ratio as the Mass Air Fuel Ratio lowers with less dense heated air.

You might also want to check obvious issues such as fuel flow to the carbs and continued good flow once the carb bowls drain bolts have been loosened. If you do have a vapour lock fuel flow problem your engine / exhaust will start making popping noises and loose power gracefully before cutting out. If it's a richenning out issue the engine will sound like it's burbling and hesitantly bogg down before cutting out.

vapour lock
 

The problem has all the symptoms of vapour lock. Check that the line from the tank to the carb runs everywhere well clear of hot parts. If you have an inline filter installed it should be somewhere where it gets free cold air flow, so not behind the cilinder or something.
If the bike runs great in the mornings and evenings there'll be nothing wrong with jets or settings - the higher temperatures in the middle of the day would maybe make it run a little rough at idle, but would certainly not make it cut out at speed as if running out of gas.

Did you install an aftermarket tank? They tend to reduce 'ventilation' around the cil head / carb area.
You could consider to wrap fuel lines and filter, if installed, in aluminium foil to keep the heat out.

Luck!