How To Adjust the Valves on a Honda 300EX


Adjusting the valves on a Honda 300EX ATV is relatively uncomplicated. The most time-consuming aspect of the procedure is removing parts and components to access the rocker arms and valve adjusters. Anyone with basic mechanical knowledge and a set of metric tools can adjust the valves on a 300EX. Many experienced riders and mechanics can do the job in less than an hour. If you’re adjusting the valves for the first time, have the service manual for your Honda 300EX on hand to aid with identifying the individual parts and components associated with the procedure.

Things You'll Need

  • Metric sockets and ratchet
  • Metric wrenches
  • Allen wrenches
  • Flat-tip screwdriver
  • .004-mm feeler gauge


  • Remove the rear fender-seat assembly to access the battery. Disconnect the negative battery cable and move the end of the cable away from the battery. Turn the fuel petcock lever to the closed position.

  • Pull the vent hose off the gas cap by hand and remove the cap. Depress the push pins that hold the gas tank cover in place using the butt of a ratchet handle. Pull the cover toward the back of the ATV and lift it off.

  • Locate and remove the metric bolts and Allen bolts that secure the one-piece front fender unit to the frame. Ease the fender unit forward enough to disconnect the headlight wires and ignition wire at the back by hand. Lift the fender unit off to access the fuel tank heat protector.

  • Depress the push pins where the carburetor air vent attaches above the heat protector. Remove the clutch cable from the cable guide above the protector and move the cable to the side. Loosen the metric bolts that secure the heat protector in place and lift the heat protector off to access the valve caps at the top of the cylinder head.

  • Loosen and remove the valve caps at the cylinder head with a metric wrench to access the intake and exhaust valve rocker arms and valve adjusters.

  • Remove the small timing cover at the left side of the engine crankcase with an Allen wrench to provide a view of the timing marks and the indicator-pointer. Remove the larger cover below to access the crankshaft bolt.

Valve Adjustment

  • Put the 300EX transmission in neutral. Attach a metric socket on the crankshaft bolt where the larger timing cover was removed. Rotate the bolt clockwise as you observe the intake valve rocker arm at the carburetor side of the cylinder head. Rotate the bolt until the rocker arm is fully up. If necessary, rotate the crankshaft bolt again to determine the fully up position.

  • Note the position of the timing mark “T” in relation to the timing indicator-pointer where the smaller timing cover was removed. Rotate the crankshaft bolt a small amount either way until the mark is directly in line with the indicator.

  • Loosen the lock nut at the intake valve rocker arm adjuster with a metric socket and ratchet. Slide a .004 mm feeler gauge between the lower face of the rocker arm and the top of the valve stem. Turn the rocker arm adjustment stem with a flat-tip screwdriver and adjust the clearance so the feeler gauge slides in and out with some resistance. Tighten the lock nut securely.

  • Rotate the crankshaft bolt clockwise as you observe the exhaust valve rocker arm at the opposite side of the cylinder head. Rotate the bolt until the rocker arm is fully up. Loosen the lock nut and adjust the clearance using the feeler gauge and the screwdriver. Tighten the lock nut securely.

  • Reattach the valve caps and tighten each. Reattach the heat protector and reposition the clutch cable in the cable guide. Reattach the carburetor air vent. Reattach the remaining components, wires and the fender unit in the reverse order as removed. Reconnect the negative battery cable before reattaching the rear fender-seat assembly.

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