Hot starting

Check you haven't lost the spark? is the coil Hot? Is the carbon brush in the dizzy cap stuck? Is the air hole in the petrol cap blocked?

I had a magneto that had the same symptoms/illness, not sure your bike has one though.

As Al says check the spark.
Very common problem with an old magneto.  Easy cold start and fine when engine running.  However once engine stopped there is no chance of starting again until everything has cooled down.  The capacitor inside the magneto is the problem.  Rebuild magneto [Paul Wolf] or electronic ignition [preferred].

You might get away with a new capacitor.  Check "Easicap" by Brightspark.

Does sound like possible bad condenser, but some bikes need a press on the carburettor tickler for starting under the circumstances you describe.

OH yea? How is it there is the cry of EI every time a mag misbehaves? The cost of EI can be as much as the cost of a mag rebuild-£400. With EI you need to fit an ignition switch, you are now totally reliant on a decent battery and battery charging. Conversion to 12V is preferred. Is the dynamo up to it? An alternator will have sufficient power.

I replaced my Concentrics with the correct Monoblocs.  When hot, I now need to give it a lot of throttle opening...which Amals don't usually want on starting.  (I must sort out the pilot screw.  But drawback with monoblocs is poor access)
it starts starts normally when cold

Irregular usage makes it difficult to get the tuning just right. I have  given the pilot screw a half turn (in or out) and used the bike for a few runs and checked the plug condition . Even a quarter turn out of the pilot screw can clean up the plug . and the bike will go from a reluctant warm starter to a first kick   warm starter with just a quarter turn outwards (anti clock) . Summer and winter require a slightly different setting. A new carb makes a huge difference to the reliability of the idle .

Its most likely that the mixture is too rich once warmed up.  Try half a turn out on the pilots 

hello, is the choke working ok.  just a thought i had similar symptoms with a thorspark ign and dynamo setup. the bike would charge ok when engine was warming up. until it got up to normal temp. put the lights on then the charge would slowly drop off. then the bike would come to a stand still, with a semi flat battery. i later found it to be the what i will call the drive clutch to the dynamo, was slipping when hot. mind you i assume yours to be alternator setup.  barry

1) have you tried hanging plugs?
2) they can dry out when hot and need another tickle or air slide closed

hello,

tight tappets / lack of compression,

barry

On my 600 with coil I find I have to open the throttle say 1/4 to start when the engine has been warmed after a ride but little or not throttle when cold.

Not hanging! Changing
I don't know where editing posts has gone to!

You would think that a slightly too rich pilot setting would be an aid to starting , and it is !, but only in the colder months . Not helpfull in hot weather or re starting when hot.

hello, David .Click on the highlited part of your post. then the edit function appears at the top of the list of posts . When signed in of course.
Barry

Al Osborn's advice about checking if you have a spark when it's hot will tell you whether it's electrics, if you have a spark move onto investigating the fuel supply / mixture with the carb.

When you start it from cold, do you use full choke and also tickle the carb? If yes, then I presume when you start from hot you do neither.
If when you start from cold, you do NOT need full choke and a tickle of the carb, then when starting from hot it is probably way too rich.
This is assuming the spark checks etc. have all been passed.

I told you EI is NOT the answer.

This hot starting issue was almost certainly caused by the carb soaking up heat from the engine while not running. When running the flow of air and fuel keeps them cooler.
Modern 'petrol' doesn't seem as tolerant of hot carbs as the old stuff was. On some engines 1/8" spacers aren't enough and 1/4" or more is needed.

Yes, I agree with Darren about the carb being the most likely cause of the hot starting issues.

 I recently had similar problems with an old bike not starting after parked for about 20 minutes (not a Norton) and went through some of the stuff that Geoff done only to come back to the carb being the starting problem.

I was told that E10 petrol has a greater alcohol content and evaporates more easily in the carb float bowl when hot.

Which sort of rings true because I had no starting problems when I first used the bike over 20 years ago when we had proper petrol, only now when I started riding it again after all that time the hot starting issue appeared.

Sorry if this starts another rant about E10 petrol. 
 

... I had similar problems with a 16H. I was vainly kicking away when an elderly chap stopped beside me. He said he's been a Don R in the war and they'd always had the same problem. He advised me to tickle the carb and try again - and it worked. He said it was heat soak into the carb vaporising the petrol, and being a stub fitting there was no way of fitting an insulator.

In this case I know there's an insulator but it would be interesting to put a hand - or even one of those infra-red thermometers -  on the carb and check how warm it was.