Honda 125 wont restart when warm.new plug dirty black gray,is it too rich even when new it would stop ,I would blow into the tak then Ok but that is a different problem

Is the spark plug set at the right gap if the old plug is light brown its running OK if it darker or ioly its running rich or very light its running too lean

Hi! I assume your problem only applies when the engine is hot. This could be one of several things:
1 - bad seals 2 - broken engine body (heat expands it and lets air in / compression out) 3 - related to above, but happening on top end (cylinder/head)
All these are hard to fix at home, but there it could also be related to a carb problem... imagine your scooter is running too rich, it will run ok for the first kms, but after you stop and because the plug is dirty and/or the engine is flooded, it wont start anymore.
When this happened did you check the sparkplug look (is it dark? dry or oily?)
Hope this helps! Regards

the jetting should not be a problem, perhaps the air cleaner element is dirty or the element even if it looks ok sometimes needs a new one, the smaller motors often rely on the right intake vacumn through the filter for the carbs to work as designed

Black smoke means that the fuel mixture is way too rich (too much fuel), and the mis-fire is most likely due to carbon fouling of the spark plugs because of the rich condition of the air fuel mixture. Be sure that the air filter is clean, and that you are not running the engine with the choke on.

maby you have touched somthing on the carb when you changed the air filter or maby spark plug is to big or maby faulty try new one or your old one

Yeah the mixture is too rich.Didn't say but are both plugs sooty?Anyway,if jets have been ok then all they could be is dirty and you'd have other problems.Take air cleaner off and make sure choke plate is fully open with choke lever off.Adjust all the fuel/air screws for proper idle and throttle response.Check voltage of spark from coil.Then try differant heat ranges in plugs to get hotter fire.Just a thought.Are you sure it's soot and not oil residue?Oh yeah,when you have new plugs in before you fire it up wipe out the ends of the pipes,real clean.Then after it runs awhile look at the pipes.Sooty? gray?black?Gray or lite tan is best.Try heat ranges til you're happy.

The only way to know if the jetting is correct is to see how well it runs - and check the spark plug color.
You can start by warming up the engine and adjusting the idle mixture. Do this by turning the idle mixture adjustment screw (there are only two adjustment screws on the carburetor: one is the idle speed adjustment, the other is the idle mixture adjustment). Turn the idle mixture adjustment screw in or out until the fastest idle speed is attained. Then adjust the idle speed with the idle speed adjustment screw.
Now ride at full power and see how well it performs. If the engine cuts out or surges at full throttle, that means the main jet is too small. If the engine produces black smoke from the exhaust and doesn't seem to produce enough power, then the main jet is probably too big. Confirm the main jet correctness by looking at the color of the spark plug: white or light tan = too lean (main jet too small). Dark brown or black = too rich. Dark tan to brown color = perfect.

have you checked your air filter. If its dirty wash out with air filter cleaner or petrol and replace.