How do I Adjust Valves on an XR400?by Tracy Underwood; Updated November 07, 2017
Items you will need
XR400 shop manual
Metric hex wrench set
Metric combination wrench set
Metric socket set
The Honda XR400 is one of the most ubiquitous dirt bikes ever sold. They have proven to be reliable and trouble-free. But as with any other mechanical device, there are certain maintenance tasks the owner or a bike shop must perform periodically. Valve adjustments are one of these tasks. If you own one of these bikes, it is to your benefit to learn to adjust the valves yourself. You will save time and money, and it is easier than it looks.
Turn off the fuel petcock. Follow the directions in the shop manual to remove the side covers, seat and gas tank.
Remove the crankshaft access cap and the timing access cap. The crankshaft cap is in the center of the engine cover on the left side of the bike. The timing cap is just above the crankshaft cap. Remove both -- turning counterclockwise with a hex wrench.
Locate the valve adjusting caps. They are on top of the cylinder head; two each on the front and back. Use a shop towel to clean any debris off the caps. Turn each cap counterclockwise with a wrench to remove.
Loosen the lock nut in each valve adjustment port, under the caps you removed in step three. This takes a 7mm wrench.
Use a 12mm or 17mm socket and a breaker bar to turn the crankshaft counterclockwise to align the "T" mark in the timing access hole.
Make sure the rocker arms are at top dead center (TDC). Use a flat blade screwdriver to back each valve adjuster out slightly.
Adjust the intake valves. These are the two valves on the rear of the head. Insert the 0.10mm blade of the feeler gauge between the adjuster in the rocker arm and the sub-rocker just under it. Turn the adjuster screw, until it lightly contacts the feeler blade. You should be able to slide the blade with only slight resistance. Tighten the lock nuts.
Adjust the two exhaust valves on the front of the head. Use the same technique as with the intake valves, except use the 0.12mm blade. Tighten the lock nuts.
Replace the valve adjustment caps, crankshaft and timing caps, and the fuel tank, seat, and side covers.
It is recommended that you adjust the linkage on the automatic (or manual) decompression system after adjusting the valve clearances.
Be careful not to spill gasoline when you remove the fuel tank. Keep all flames and heat sources away.