Valve adjustment determines how far the valves enter the cylinder and how long they stay open and closed. If the valve clearance is too large, part of the lift of the camshaft will be used in removing the excessive clearance. Consequently, the valve will not be opening as far as it should, it will start to open too late and will close too early. This condition has two effects: the valve train components will emit a tapping sound as they take up the excessive clearance and as the valves slam shut, and the engine will perform poorly because the valves don't open fully and allow the proper amount of gases to flow into and out of the engine. If the valve clearance is too small, the intake valves and the exhaust valves will open too far and they will not fully seat on the cylinder head when they close. When a valve seats itself on the cylinder head, it does two things: it seals the combustion chamber so that none of the gasses in the cylinder escape and it cools itself by transferring some of the heat it absorbs from the combustion in the cylinder to the cylinder head and to the engine's cooling system. If the valve clearance is too small, the engine will run poorly because of the gases escaping from the combustion chamber. The valves will also become overheated and will warp, since they cannot transfer heat unless they are touching the valve seat in the cylinder head.
While all valve adjustments must be made as accurately as possible, it is better to have the valve adjustment slightly loose than slightly tight, as a burned valve may result from overly tight adjustments. This holds true for valve adjustments on most engines.
See Figure 1
These engines use hydraulic lash adjusters for the intake and exhaust valves, which require no routine adjustment. However, these engines also incorporate a 3rd valve per cylinder called the jet valve which should be periodically checked and adjusted.
- Run the engine to normal operating temperature. Then, shut it off.
- Remove the valve cover.
- Rotate the engine by hand until the No.1 cylinder is at TDC compression. Both the intake and exhaust valves will feel loose and the timing mark for 0 will be aligned with the pointer.
- Check the jet valve clearance with a flat feeler gauge. Clearance for all engines should be 0.010 inch (0.25mm). If not, loosen the locknut on the rocker arm and turn the adjusting screw until a slight drag is felt on the feeler gauge. The jet valve spring is relatively weak, so don't press down on the valve stem or you'll get an erroneous reading.
- Turn the engine by hand, in the normal direction of rotation, until each cylinder in the firing order reaches TDC of its compression stroke and adjust each jet valve in turn.
- Install the valve cover.
No routine adjustment is necessary or possible.