Bike won't start after sitting in the sun?

[california_rookie]

Here's an odd question. Well, at least I think so. Is there any plausible explanation for why my bike takes forever to start after it sits in the sun for a bit? Even after just half an hour of sun-bathing it can take at least 10-15 sec's of turning over before it'll cough and sputter to life. I find this odd because it starts after only one or two revolutions under normal circumstances and fires up immediately even in the frosty morning hours.

My theory is there's a build-up of hot air in the intakes and air box which I seem to remember hearing is harder to combust than cold air. Or maybe it's a chemical thing? Hell, I dunno. Maybe that's why I'm asking. Any help would be greatly appreciated.
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[Claud 14.7 to 1]

When I start my bike from cold I only need a bit of gas to get it started. (Or choke and a tiny bit of gas, on a cold UK/Winter day)

When the engine is hot the bike needs more throttle when turning over to get it started.

Do you mean starting the bike from cold, when it has been in the sun or when, for example, you have ridden it, left it in the sun for a few minutes and then gone to start it???

Try more throttle when turning over.

Engines stay hot/warm for a long time, especially sports bikes. 1/2 an hour, the engine will still be warm.

[Minky_monkey]

What about fuel evaporation?

I've experienced it with cars before, all I've done is undo the filler cap.

Not sure whether it would be a similar problem, but hey, it's worth a try!

[dodsi]

Your just gloating because you have sun, Over here in good o'l Britan we have no sun or warmth!

[stinkwheel]

Ok, here's my story to explain it. I made it up myself so I don't know if it is correct.

Bike has a vaccuum fuel tap. When you start the bike, it starts up on the fuel lurking in the float bowl from when you last ran it. In the sunshine, the fuel in the float bowl evaporates and exits via the vent. Therefore when you then come to start it, there is no fuel and the engine will not start immediatley.

The fuel can only run into the float bowl when the engine is turning over (and applying vaccuum to the tap), it takes a few revolutions for enough fuel to run in to start the engine. Hence the delay.

If your fuel tap has a 'prime' position, try setting the bike to this for a few seconds before attempting to start it, this circumvents the need for vaccuum to make fuel run in.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[Robin]

Same sort of thing as I was thinking Stinky. I've had a similar problem when the bike has been left standing for a while. A couple of seconds with the tap on 'prime' worked.

[BlackSheep]

Its called vapour lock , certain makes of car also suffer from it
i used to have a BMW 316 that would occaisionally do it if left in the hot sunshine , as someone said above just put your fuel tap onto prime for a couple of seconds before you start your bike
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GSXR 1100 StreetFighter

[Kickstart]

Hi

Is it fuel injected? If so the vapour lock is not that likely (a 40~50psi pump should overcome it).

Also worth a check is any evaporation from the tank, which probably goes into a carbon canister, and is then drawn from there when the engine is started. This might be getting the mixture very rich when first started.

All the best

Keith
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