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FIX-IT: How-to adjust your ATV Valves

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October 28, 2016
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Keeping your ATV running strongly requires routine maintenance, and adjusting your valves is a part of that. On most machines, valves will only need to be checked and adjusted every few months, but on high-performance ATVs, you should check them every three to four oil changes. We have compiled a little how-to on adjusting the two most common types of valve trains, so keep this guide handy!

DOHC: SHIM UNDER-BUCKET DESIGN

VALVE-1STEP 1
Start off by removing the plastics necessary to get the tank off. On the KFX, we removed the front fender plastic and radiator covers. Then unbolt the gas tank and unhook the fuel lines and fuel-pump wiring on EFI machines. On carbureted models, turn the petcock to the off position and remove the fuel line from the carburetor.

VALVE-2STEP 2
Fuel-injected ATVs like the KFX450R have press-on fuel line adapters like this one. Be careful when you remove them; if you break the red plastic clip, you’ll have to order a new one before you hook the tank back up!

VALVE-3STEP 3
When the tank is off, remove the spark-plug boot and valve cover. On the KFX, it is held on by three 10mm hex head bolts.

VALVE-4STEP 4
Remove the timing cover and crank the engine to top dead center, ensuring that the cam lobes point away from each other and upward. Here you can see the timing marks: a line on the exhaust cam and a dot on the intake cam. There should be 29 pins between the marks on the cam chain (for the KFX450R).

VALVE-5STEP 5
Using a set of clean feeler gauges and measure valve clearance. Record the measurements on a piece of paper.

VALVE-6STEP 6
The chart is your saving grace, so don’t move it or turn it around while you’re doing the job or you may end up confused!

VALVE-7STEP 7
After all values are recorded, remove the cam chain tensioner and unbolt the cam caps.

VALVE-8STEP 8
Note these small metal clips on the KFX’s cam bearings. DO NOT drop these down into the cam-chain valley or you’ll be disassembling the entire engine to retrieve them!

VALVE-9STEP 9
Using a magnet, remove the buckets. A strong magnet will pick the shim up with the bucket, making the task much easier. Set the shims on the paper in the location that matches the clearance reading for that particular valve.

VALVE-10STEP 10
This is the right-side intake-valve shim; notice how the measurements of the shim thickness are worn off by the valve stem? Make sure you put them number-side up when you install the new ones to avoid this problem. Sometimes the numbers will wear off on the bucket side, but it takes longer to do so.

VALVE-11STEP 11
Using a good set of calipers, measure the thickness of the shims. Here’s where the math comes in. You will have to take your desired measurement (we will use 0.008-inch) and find the difference between the shim size and your recorded valve lash that you have written down. Say you recorded 0.013-inch of valve clearance on exhaust valve 1 and a shim thickness of 0.002-inch. This means that in order to bring that 0.013-inch down to a 0.008-inch measurement, you need to add 0.005-inch of shim to the valve bucket. Add the 0.005-inch to the original shim size of 0.002-inch and you get 0.007-inch. Install the 0.007-inch shim and you’re set! Note that these numbers are theoretical and do not represent actual required adjustment specs.

VALVE-12STEP 12
After measuring all valve clearances, shims and completing the math calculations, install the correct shims in the head and put the buckets back on top of the shims.

VALVE-13STEP 13
Reinstall the cams, ensuring that the timing is correct. Check the valve clearance once more with your feeler gauges to ensure you did everything correctly!

SOHC: ROCKER-ARM STYLE

VALVE-14STEP 1
Strip the quad’s front plastics off, as well as the gas tank and plastic drip pan. You can see the motor from the top now—on the front and rear of the engine; there are two large circular caps with nuts cast into the center. Remove these caps and grab a set of feeler gauges. Set the engine to top dead center using the nut on the end of the crankshaft behind the large cover on the left-side lower case. The valves should be set on the compression stroke, meaning all four are closed and the cam lobes are pointing toward the ground.

VALVE-15STEP 2
Using a 0.08mm feeler gauge, check the intake valve’s clearance. If there is too little or too much clearance, back off the 10mm nut on top of the rocker and thread the adjuster tappet in until it produces a slight drag on the feeler gauge. Tighten the nut. Repeat for the exhaust valve, but set the lash to 0.13mm. All measurements should be done with the engine cold! When you’re done, bolt everything back up and go ride!

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