Engine Checking Valve Clearances - 2005 KX250F RMZ250

James

Staff member
One of the most important maintenance items on a 4 stroke is Proper Valve Clearance. Thankfully it's not that hard to do and no special tools are needed. Consider this Part 1 of 2 (Second being Adjusting Valve Clearances)

If you're wondering what the heck a valve clearance is, this might help.

Valve_Clearance.jpg

Valve clearance is the distance between the cam lobes (A) and the lifters or tappets (B).


Tools Needed:
• 8 mm socket wrench
• 10 mm socket wrench
• 14 mm socket wrench
• 5/8 spark plug wrench
• 6 mm Allen
• Flat blade screwdriver (or a quarter)
• Feeler gauges


Ok first go and thoroughly wash your bike, make sure that you don’t have any dirt in/on/around the engine, because you don’t want dirt falling inside it. After the bike’s all nice and clean, start by taking off the seat and shroud plastics.

Seat & Shrouds Removed.jpg


Next come the radiator guards (if you have them) and the gas tank.


Ignition_Coil-&-Spark-Plug-.jpg

You should now have easy access to the ignition coil (A) and spark plug (B), both of which have to go. Removing the spark plug relieves the compression, making it easier to find TDC (Top Dead Center) later. I always move the wiring harness out of the way, for the extra room it gives, but it isn’t necessary.
Before you take off the valve cover, give it one last wipe down for that last bit of dirt. Then let the fun begin. Using a 6mm Allen wrench, take off the 2 bolts holding on the valve cover. Take it and the rubber seal and put them in a clean area.



Dirt Bike Cams.jpg


Next remove the 2 timing plugs from the left side of the engine. Then use the 14mm socket to rotate the crankshaft counter clockwise until you find TDC. This is achieved by lining up the right mark (there's 2 beside each other) on the flywheel, to the middle of the inspection hole (A). (See images below for more details)


You can barely see it but the right flywheel mark is lined up in this image.


Timing-Plugs.jpg


TDC.jpg


The dots (A & B) on each camshaft should line up with the head (C).



Camshaft-TDC.jpg


Cam-Lobes-TDC.jpg

The cam lobes (A) should also be pointed out, if they aren't you're off by 180 degrees, which is BDC (Bottom Dead Center).



After you have it set to TDC, it's time to use those feeler gauges.

DSCN0260.jpg

Use the Go/No-Go method when you check the clearances. Which means if the clearance is supposed to between .004 and .006 (4 thousands and 6 thousands), then it should take the .004 but not the .006


Stock Valve Clearance:
*confirm these valves with your service manual*
  • Intake - .004 to .006 in or .1 to .15mm
  • Exhaust - .007 to .009 in or .17 to .22mm
If the valves aren't in spec they need to be adjusted, this will be covered in Part 2 (Click Me). However if they are in spec, reassemble the bike and go ride.


Cause and Effect:
Valve too tight - Hard starts, poor and erratic idle, lack of performance, and hesitation at low RPM.
Valve too loose - Noisy, lack of performance, Hard on valve train, causing excessive wear on cam shaft and tappets (lifters).

* Valves typically do not loosen over time and use, they usually tighten up.

 
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