My 125 yz has lost compression in the motor and will not kick over it has spark

If the plugs are fouling from excessive oil then it may be that the oil ring is broken. That will be scoring the bore and accounting for the low compression pressure. To do compression checks the motor has to be hot and the throttle wide open. Check for blocked filter before testing. IN the short term run a hot plug . ( where the porcelain insulator is very long in the plug) as this will help it burning off the oil .

They can do this from time to time. What can happen is the expansion chamber gets pumped with fuel vapor and then can be ignited, causing a large backfire. I hate to attempt to answer your question with a question, but how old is the piston and ring? A ring won't last very long in a 125. For racing, it's changed every race. For joy riding 15-20 hours of run time and it's time for a ring at least. Weak compression can make it impossible to start. If you have perfect compression the cause for the non-start could be something as simple as a burned up or fouled spark plug.

Start by re-ringing the motor (you can do this for about $25 inside of 2 hours time). Clean the carb out, clean the air filter, fill the tank with fresh fuel and replace the spark plug (NGK B9ES - from Autozone for $1.89). If the bottom end of the engine is good, the above steps should get it fired up - providing your ignition system is good, and based on the apparent backfire, I would say you have spark.

Scott

Check the "Basics" first. Clean Air Filter, Check the Spark Plug. When you kick it over, Is there resistance on the kick starter which means it has compression. Is it hard to start? When the motor gets tired, It will lose compression and be hard to start. I would start on the Air Filter and spark plug. Also, Does it have fresh gas in it?

Kinda sounds like your needle valve is sticking in your carb and it is giving it to much fuel

By what you have written,I take it the motor still turns over.Take the spark plug out and see how much compression you have.If there is very little,it may have been the piston seizing.When they cool down,they can sometimes be moved again.This is the most likely cause of your troubles.If you have compression,take the s/plug out,put the bike in first gear and see if you can turnover the motor AND g/box.What you are looking for is a broken part in the g/box that locked up the driveline.Do this in each gear.Sometimes you can take the pipe off at the head and see into the combustion chamber through the exhaust port.Otherwise the head needs to come off.Hope this helps you

MAKE SURE YOU HAVE SPARK AT THE PLUG
MAKE SURE YOU HAVE COMPRESSION
REMOVE PLUG PUT FINGER OVER HOLE AND KICK
IF YOU HAVE BOTH OF THOSE TRY CLEANING THE JETS IN THE CARB

try to change the magnetinc there are 4 or 5 or 6 coil or dynamo one of them is damage

• If a hole in the piston then install new piston and rings. Next, clean the carbon off the head and install a new head gasket.
• If cylinder wall is scared the piston has seized up. Pull the cylinder off and take it to the dealer. Have them bore the cylinder and return it along with a new oversize piston and rings.
• While at the parts counter, get a new stock spark plug and a supply of Yamalube Two Stroke Motor Oil. You need to add 4 ounces per gallon when filling up. This will be a 32/1 gas to oil ratio. NEVER ride the bike without at least the needed 32/1 mix.

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check your compression I had a 94 yz that done that when i checked the compression it was really low so I put new rings on the piston and that's all it took to bring her back to life

2 stroke bike are very dependent on compression. and being a bike that is 12 years old my best educated guess is that the top end needs to be rebuilt. i usually have to have the top end done on my honda about every three years.