Troubleshoot the Yamaha YZ 85 Two-Stroke Dirt Bike

Besides releasing dirt bikes for fully grown riders, Yamaha has introduced a few youth models over the years intended to introduce young riders to the sport. The YZ85 is one such bike, modeled after Yamaha’s adult-sized YZ250 and YZ125 bikes. In 2009, the engine was redesigned as a two-stroke, adding additional power and speed. As with any vehicle, the YZ85 requires regular maintenance. For owners experiencing problems with the engine overheating or failing, Yamaha recommends specific troubleshooting steps before taking the bike to a shop.



1) Check the radiator system if the engine is overheating. Wait until the engine has cooled down before proceeding.

2) Remove the radiator cap and check the coolant level in the reservoir.

3) Turn the engine back on if the coolant level is adequate. If the engine continues to overheat, take the YZ85 to a Yamaha dealer for inspection.

4) Check the cooling system for any leaks if the coolant level is low.

5) Have a Yamaha dealer repair the cooling system if a leak is present.

6) Refill coolant if a leak is not present and restart the bike. If engine overheating persists, take the bike in for a professional inspection.

Fuel System

7) Check the fuel tank to ensure that there is enough fuel in the bike if the engine stops or will not start.

8) Move on to the next section if there is an adequate amount of fuel.

9) Refill with premium unleaded gasoline if the there is not enough fuel. Restart the bike.


10) Use the kick starter to check the compression.

11) Move to the next section and check the ignition system if there is compression.

12) Bring the bike to a Yamaha dealer to inspect the compression system if there is no compression.


13) Take out the spark plug and examine the electrodes.

14) Take the YZ85 to the shop if the electrodes are dry.

15) Dry the electrodes with a cloth if they are wet and inspect the condition of the spark plug.

16) Replace with an NGK BR10EG spark plug if the original appears defective or broken.

17) Set the spark plug gap between 0.020 and 0.024 inches.

18) Open the throttle halfway and operate the kick starter again.

19) Take the bike to a dealer for inspection if the engine still won’t start.

Extra Tips:

* Certified Yamaha dealers have the tools and training necessary to repair the YZ85. For mechanical failure and serious repairs, Yamaha advises that owner’s take the bike to such a dealer to ensure repairs are successful.

* Gasoline emits explosive fumes, so do not smoke while troubleshooting the fuel system.

* Allow the engine to cool fully before removing the radiator cap, to ensure that hot gasses and fluids do not burn skin.