ATV valve adjustment is often forgotten about, and I understand why. Just getting access to the valves on some bikes is a mission in itself. But if you hear a new sound from the top end, it may be time to bite the bullet.
Adjusting ATV valves is a 6 step process:
- Remove valve covers
- Set engine TDC
- Check lash with feeler gauge
- Loosen adjuster lock-nuts
- Adjust valve lash
- Tighten adjuster lock-nuts
This guide covers the more common rocker-style valve train. If your ATV is fitted with an OHC (Over Head Cam), the procedure is different and a little more complex.
1 Remove Covers
Some bikes make it easy to access the valve covers, and some will be a complete pain in the ass. It’s not unusual to remove side covers, seat gas tank and air-box, and ducting.
Most ATV engines are single cylinders but will likely have four valves. Two inlets and two exhausts.
When you finally gain access, remove the valve caps. Most will be individual valve caps with an O-ring seal. Each valve will likely have its cap.
2 Set Engine To TDC
Your engine is a four-stroke unit, and we want the piston at the very top of the cylinder on the power stroke.
The four defined strokes are:
1 Intake stroke – Intake valve(s) opens as the piston moves down the cylinder and closes again as the piston bottoms out.
2 Compression stroke – Intake valve(s) now closed as the piston moves up the cylinder, compressing the air/fuel mix into the combustion chamber at the top of the cylinder.
3 Power stroke – The spark plug fires and ignites the compressed mix causing the piston to move down the cylinder under power.
4 Exhaust stroke – Exhaust valve(s) open as the piston moves up the cylinder, pushing the spent gases out. The valve is fully closed as the piston reaches the top of the cylinder, and the cycle starts over.
What is TDC (Top Dead Centre)? It’s when the piston is at the very top of its stroke.
Why is this important? When the piston is at TDC, both exhaust and intake valve springs are unloaded, and valves are fully closed. That’s when we can check and adjust valves.
How To Set TDC
Begin by removing the spark plug, and on the flywheel cover, you’ll find a timing inspection cap. Go ahead and remove it. You’ll also find a flywheel nut access cap, go ahead and remove it too.
Using a suitable deep socket and ratchet, turn the engine over while viewing the flywheel through the inspection hole. TDC is commonly marked by a letter “T” or “Dash” stamped into the spinning flywheel.
You’ll need to rotate the flywheel/crankshaft fully to find and familiarize yourself with your marking.
However, the “marking” will pass the viewing window four times in a full four-stroke cycle, but we want to identify the power stroke.
The simplest way to identify the power stroke is to watch the valve springs after the engine passes TDC. When the valves do not compress immediately after TDC – You find the power stroke.
Just turn the engine back anti-clockwise to locate the “Mark” again, and you’re ready to begin checking lash.
Tools You’ll Need
The tools required are primarily basic. But you’ll need a feeler gauge.
What’s a feeler gauge? It’s an inexpensive tool used to measure clearances between mechanical components.
It comprises stainless steel strips of various sizes, measured and marked in standard and metric arranged in a fan configuration.
If you need to buy a set, know that the standard type feeler gauge fingers are straight, which is fine for working on car engines where there’s a lot more room.
On a bike engine, an off-set feeler gauge is easier to handle, might cost a few dollars more but totally worth it.
3 Check Lash
All engines will have at least two valves. Some will have four per cylinder. To check lash, you’ll need to identify which valve is which.
The intake valve is larger, but that can only be seen by removing the cylinder head.
The easiest way to identify the valves is by their location. An ATV engine is small and logical.
The intake valve(s) will be on the same side of the engine as the carburetor and the valve(s) closest to the carburetor. You may have a pair of intake valves, and if so, they’ll be arranged as a pair.
The exhaust valve(s), likewise, will be the valve(s) closest to the exhaust. Bear in mind. You may have two exhaust valves.
At this point, you’ll need to refer to your engine spec to find your intake and exhaust valve lash measurements. It’s given as a range, and so any measurement within the range is to spec.
Using the appropriate size blade, slide the blade between the valve tip and the rocker. You should feel gentle but constant resistance. The blade should neither be too tight or loose. Check all intake and exhaust valves. Note the measurements.
4 Open Lock-nuts
Valve lash adjusters vary in style, but the principle is the same. The lash adjuster is a two-part assembly. A lock-nut (usually a nut) and an adjuster screw (usually a screw head or Torx).
The lock-nut keeps the adjuster screw at the set height and so must be loosened before any adjusting can take place.
Having identified the valves that need adjustment, go ahead and loosen the adjuster lock-nuts.
You’ll need to hold the adjuster screw while loosening the lock-nut, back the lock-nut off a turn.
5 Adjusting Lash
Slide the appropriate feeler blade under the rocker and adjust the screw adjuster (Clockwise to tighten and anti-clockwise to loosen). Feel the change in resistance by moving the gauge as you adjust.
6 Tighten Adjusters
When you find the sweet spot with the adjuster screw, you’ll need to hold the screw stationary as you tighten the lock nut.
If you allow the adjuster screw to move while snugging up the lock-nut – the lash will be off.
Test your gauge feel once again after snugging down the lock-nut. If it’s too tight, repeat the adjusting process.
A common mistake is over-tightening and breaking the lock-nuts.
When reassembling, torque the cap or cam cover to spec, replace the gaskets (lube rubber seals) to prevent oil leaks. Nice work!
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