I'm wondering if anybody has any ideas about a starting problem on my GS500F K4 when it's hot.
Symptoms / Story as follows:

The bike fires up fine from cold. I ride it about 10 miles, enough to get it properly warmed up, then stop at a shop to pick up some goods. I'm only in the shop a couple of minutes, but when I get back to the bike it won't start. If I leave the bike to cool down, just over an hour, it will start up straight away, but if I try to start the bike any earlier (30 or 45 minutes after the initial problem), it won't fire.
When it won't fire, I have a full set of lights and the engine turns over quickly (new battery). I've tried priming the carb just in case it's a fuel problem but this has no effect at all.
The bike has been in regular (almost daily) use since I purchased it 8 months and 2500 miles ago, so hasn't been laid up at all.
The day before this problem first occurred, I took the bike on a 40 mile non-stop ride, with no problems at all, so the problem doesn't seem to occur if the engine stays running.

My guess is it is flooding. You can try checking the plugs, are they wet when you have tried (and failed) to get it to start? I'd check that, then work from there.

You could also have a petcock failing.  When the bike is running, it can successfully make a vacuum.  When it's off, it may be struggling to get fuel to the carbs.  Have you tried starting the bike in the "Pri" position with the petcock?

My guess, its rich. I like to set these bikes up to be rich everywhere except at idle, just to avoid this problem. Likely your float has worn and its gone high. Even 1-2 mm will do it.
Cool.
Buddha.

Sorry, but I don't think it's a fuel problem.
I've just re-created the symptom (10 mile ride then switch off). If I try to re-start the bike immediately, it fires up, but if I wait for just over a minute and try to restart, it won't fire up at all although it turns over OK. If I take off one spark plug lead and use a spare spark plug against the cylinder head, there is no spark. The same occurs on the other plug lead as well. This would suggest some sort of problem further up the ignition circuit. Because I've got the same problem on both plugs, I'm assuming it's not a coil problem (odd if both coils have failed in the same way). This means an expensive igniter box or signal generator replacement.

I thought K4 had one common coil?

It could be the pickup coil if both cylinders are affected at the same time after warm up.  04 and later GSs have one pickup coil and 2 ignition coils and it does sound like a coil problem.

It looks like the problem is indeed the pickup coil.
I've now swapped out, in turn, first the ignition coils, then the igniter box. Each time I've got the same results. As far as I can tell, the only thing left in the ignition circuit is the signal generator, so this is now on order. Fingers crossed when it appears.

Others have also had problems with the signal generator with heat.

Sorry about not replying for a while. It took a while to get a new Signal Generator (pickup coil) assembly, which is now fitted, and seems to have solved the problem.
Thanks to everybody for their help.

 

Glad you got it back up and going.