Hi, My KX is starting but will only run for about 10 seconds before it dies out, the sparkplug is black and wet, i have stripped and checked the carb and all was perfect. only other thing i can think of is the coil ? Thanks. Alex.

Engine model number?

Check the crankcase oil is not contaminated with fuel.

Inspect the sparkplug. If its oil fouled (black sticky wet deposits), do a cylinder leak down test.

If plug is oil fouled, you could replace the plug and it will run but it will likely oil foul in an hour or so or even minutes... it would confirm rings are shot and the leakdown test not necessary, but be sure fuel is not getting into crankcase.

It's also possible the decompression mechanism has failed. It will start with the extra fuel from priming then die. Could also be impulse air to fuel pump... model number?

Double check air fuel mix screw settings for new carb... again model number?

There needs to be fuel, air, spark and compression. It may not start with starter fluid if it's already flooded. It may not start if a valve is stuck open.
You can take the sparkplug out and check to make sure it's not got excess fuel. Plug should be dry.

it runs but sputters.
that can be 2 things.
First idd your sparkplug cap has had it (get a new 1 from the parts store)
2nd it could be that your getting to much fuel in top end of your revband.
easy to check just run your bike a few min with a new sparkplug and look at the color.
Wite is not enough fuel.
Brown is perfect mix.
black is to much oil and if black and wet its to much fuel.
try this test and maybe you'll find it

CHANGE THE SPARKPLUG IF THAT DOESN'T CURE THE PROBLEM REMOVE AND STRIP THE CARB AND CHECK ALL OF THE JETS THEY WILL LIKELY BE CHOKED.

change the plug then drain the carb and ck compression and that will tell u the story

Check if sparkplug is wet or oily when removed,clean and heatup old sparkplug in kitchen flame.put it back again an start engine. Stop engine and remove spark plug,check if colour is light brown.If not or black check if choke is not permanently on ,clean carburettor.

the catalytic converter maybe pluged .

Just a very quick check,Swap the sparkplugs over.Don't put in new ones.If the problem follows that plug,it is only the sparkplug.Hope you get it fixed

Check the filter inside the fuel tank
Check the fuel line for cracks or holes
Clean the carb

If still not working,replace the pump diaphragm on the carb

hi

check the screen filter from the carb,check the assembly needle-spring -lever,check the fuel filter ,check all the membrans and gaskets from the carb,check the sparkplug pipe if it is fit snugly on the plug.

regards savumihai71