Hi my sons o9 ktm50 needs choke to start when hot ?

the chock must be bad.
on all carb cars , since 1964 i remove it and make it a manual choke
then hot starts are easy, hot, its not touched. (instant starts hot)

chokes this old never work right, they are TOO old
the spring in side (bimetal) is NO GOOD. it fails to open properly.
try setting the choke calibration more lean.
if at higher altitude, even leaner.

the book is free. see it here
http://www.fixkick.com/sammi-stuff/index.html

The choke is a device on most old carbs (varies by country)
that closes a butterfly valve, (google that) and starves the engine of air, thus the word "CHOKE" as in Choking it.
this causes higher venturi vacuum and more fuel flows from the jet nozzles.
if the chock is set too rich the engine floods
if set too lean , you have to pump the gas pedal to keep it alive.
if the bimetal spring is slow, it floods
if the choke is electic the heater needs to work or the choke sticks on.
if the choke is water heated, the water needs to flow there. again
the motor floods and stalls.

rtm. read the chapter on the Choke, the book is free.
http://www.fixkick.com/sammi-stuff/index.html

Electric choke? Try retrofitting a manual choke. I think the electric choke is closing over a hot engine, thinking that the engine's cooler than it really is. A manual choke will allow you to start it with no choke at all when you know the motor's already warmed up.

whats the compression ? NO less than 100psi !! when were the crank seals last replaced ? is fuel gettin to the spark plug ? carb clean and new fuel ??

If it's cold, you need the choke to richen the fuel mixture until the engine warms up. Use your hand to partially cover the air intake. Once it starts, use your hand to feather the amount of air you allow to the engine until it smoothes out.

Hi, Replace with correct sparkplug. You have a bad plug replace it and you're good to go.

Hi, you may try draining the fuel from the float bowl through the screw at the bottom of the carb as it may have become contaminated, try leaving the choke on a while longer, if it still stalls then I would remove the carb and clean the jets and check the fuel flow, checking the float valve action before reassembling

I would pull the plug to make sure its not fouled. If it is then replace it with a new one. Then try starting it. As far as choke operation, pull the choke out, start the bike let i idle for a minute or two, push the choke back in and continue to warm up the motor.

just wondering if anyone can help me. I bought my son a second hand for the christmas of 06 (2005 model). He has not used it since last summer as I have not had the time to take him out on it we both tried starting it the other day without success. I am not a technical man and I cant find anything on starting it. We didnt use the choke, but I am just looking for some help to have it up and running for him, as he has been deprived of it. Thanks very much lads, he and I will both appreciate it!! ,I would pull the plug to make sure its not fouled. If it is then replace it with a new one. Then try starting it. As far as choke operation, pull the choke out, start the bike let i idle for a minute or two, push the choke back in and continue to warm up the motor.,,,

The normal reason why a hot engine doesn't start is the choke. Your fancy new range rover will have a computer controlled choke that will be activiated by a temperature sensor. maybe this has failed or is disconnected
In my ancient range rover I have a manual choke but essentially the same engine. I know that if the motor is hot I can't start it with the choke on.
The way the choke works is it restricts the airflow into the engine so that there is a richer fuel / air mixture which is needed for a cold start. If the engine is hot and the choke comes on the engiine floods with petrol and there isn't enough oxygen for it too ignite in the cylinders. To start a flooded engine you have to open the choke and crank the motor without pressing the accelerator. This pumps out the petrol vapour and eventually it will fire and start. If the computer thinks it is cold because of a disconnected or faulty sensor it will put on the choke whenever you try and start and the engine will flood.
I think a vapour lock is unlikely unless you driving in extremely hot weather (above 40 degrees celsius).