Kdx 125 has good spark but wont fire up

pull cover on magneto and check for loose pickup coil.

So you've tested a spare spark plug sitting outside grounded to cylinder and you defintely have good spark?
If so....is it getting fuel? Is the enricher (choke) working?l

clean the carb,new spark plug and do a compression test,,being that old id say it needs some new piston rings

Piston rings may be sticking after sitting for so long. It may serve you well to remove the top end, paying special attention to the timing alignment as you do. Visually inspect the piston and rings. ensure the rings can move freely in their grooves. Look at the piston for signs of blow-by. A burned up surface of the piston that extends past the rings onto the skirts. You may simply re-ring this machine and it would solve your problem if you view these problems.

It may need rebuilt maby it does not have enough compression to turn over i would suggest getting a compression test done at a local bike shop. If thats the case it most likely needs rebuilt.

I'd start by thoroughly cleaning out your carb.,...check/replace the main jet with a leaner, [higher #], jet...change/clean pilot jet also,..spray carb cleaner in all holes and blow out w/compressed air. Re-check your float height and/or any damage to the float pin+tangs etc. Maybe try a 50:1 gas:oil mixture. Perform a leakdown test,..air will cause boggs as well. *[p.s.; does bike blow oil out exhaust?].

Even tho u have got a spark dose not mean the spark plug is ok try changing the spark plug and i would recheck u carb to see if the flots in the bottom of the carb are still inplace correctly if u have any more problems please get back to me i will be more than happy to help i have worked on at least 100+ of these bikes

im thinking if it has spark and getting gas and has compression, the bike will run. sometimes a "WEAK" coil will give a good spark, but under compresion its not enough to ignite the fuel. if thats not the case, take the air filter off and spray starting fluid into the carb and with NO CHOKE, give it full throttle and kick it over. if that dont do it try compression starting it, tow it for some distance if you have to. if that dont do it, then there must be something wrong with the carb. jets, fuel air adjustment. thats all i have. good luck.

Based on the information given, I think you may have a hole in the top of the piston. It sounds like pre-ignition, ( AKA detonation and engine knock ). Pull the cylinder head and check the piston and cylinder head. Sometimes the top outer edge of the piston disintegrates due to the heat. Replace the piston and rings and remove the carbon from the head. Be sure you run 32/1 mix on gas and oil. That's 4oz two stroke oil per gallon of gas. Only use premium gas.

Remove and clean the carb. Be sure the carb mount bolts and carb manifold bolts are tight. An air leak will cause the engine to run hot.

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sounds like the rings are fried and needs a new piston and rings.. for an 80 there about 110 bucks for the whole kit