Big single four-strokes are always a bit difficult to restart after a stall when the engine is HOT. So some smart engineer at Keihn came up with a brilliant idea that was called a hot start button. It's designed to leak a bit of cool air into the intake to temporarily lean out the air/fuel ratio to help it start in those hot start situations. The bigger the engine, like your 620 the more it helps.
For cold starts the hot start button needs to be closed (less air in) , and the choke should be engaged (more fuel in).
When an engine is cold it usually needs a richer air/fuel ratio to get it to start because less of the fuel will evaporate on the cold surfaces in the intake. Basically you throw in more fuel than the engine really needs in the hope that enough of it evaporates and becomes usable to start the engine. As the engine warms up and the surrounding surfaces in the intake get hotter they'll more easily evaporate the fuel, so you can back the choke off and stop adding the excess fuel. But you probably already knew all that stuff. :-)
I'm not sure which carb you have on your bike but if it's equipped with an accelerator pump type like the Keihn FCR, or a Mikuni BST you need to be careful not to roll the throttle excessively when starting it. With some of the habits we all developed riding two-strokes it's pretty easy to accidentally roll the throttle multiple times and wet foul the plug.
This was originally posted on the Yamaha USA website around the time the YZ250F first came out. While it's specific to a Yamaha with a MANUAL compression release (your bike may have an automatic compression release), the basic procedure is time tested and solid info for any modern four-stroke. It's basic the way I still teach first time four stroke MX riders.
To further enhance the enjoyment of these products, please follow the step-by-step instructions as explained by Doug Dubach, Four-Stroke Motocross Champion and Yamaha test rider. These helpful tips will ensure quick and easy starting of your Yamaha four-stroke motorcycle.
COLD ENGINE STARTING:
1.Make sure the bike has fuel, then turn on fuel petcock.
2.Pull out choke knob (black knob on left side of the carburetor).
3.Prime the engine by giving the throttle two full turns (only if the bike has not been started in a day or two).
4.Apply firm pressure to the kick-starter with your foot until you hit distinct resistance (this is the compression stroke/hard spot).
5.While keeping pressure on the kick-starter, pull in the compression release lever and push the kick-starter past the compression stroke/hard spot. The kick-starter needs to only move about 1 to 2 inches past the hard spot. That is all!
6.Release the compression release lever and return the kick-starter to the top
7.Now, kick to start. Do not touch the throttle, as the engine will start and idle on it's own
8.If the bike does not start, repeat steps 4 through 7 only until the engine starts.
HOT ENGINE STARTING
1.Pull out the hot start knob (red knob on the left side of the carburetor).
2.Follow steps 4 through 7 until engine starts.
3.You may have to apply some throttle after the engine starts until you get the hot start knob pushed back in because the carburetor is sending a lean mixture while the hot start knob is out.
The only difference between hot and cold starting is which knob you use, choke or hot start. There is no need to deviate from these simple steps. Remember, DO NOT touch the throttle during hot or cold starting. Also, there is no reason to prime the engine again on the same day of riding.
OTHER HELPFUL TIPS
It helps to become comfortable with steps 4 through 7. By becoming familiar with these steps you will become more efficient and will be able to perform them quicker. The faster you can get through steps 4 through 7 the quicker you will get going.
You need to give the engine what it wants! Understand that the choke will give the engine a rich mixture and the hot start will give the engine a lean mixture. If you have kicked the engine several times during a cold starting procedure and it still won't start, maybe it has too much fuel. Don't be afraid to push the choke back in and use the hot start, even though the engine is cold, this will give the engine a lean mixture and that might be just what it wants.
The KTM 620 is a really solid engine and should provide you with some serious fun.