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 for as long as it should. This condition has two effects: the valve train components will emit a tapping sound as they take up the excessive clearance 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. The carburetion may also be too rich.
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 gases 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. It may also run too lean. 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.
The valve lash should be checked and adjusted, if necessary, after the first 1000 miles (1100 km) of operation and then every 6 months/6000 miles (10,000 km) on 1970-73 models; every 12 months/12,000 miles (19,000 km) on 1974-79 models and every 15 months/15,000 miles (24,000 km) on 1980 and later models.
- Rotate the engine so that the No. 1 piston is at Top Dead Center (TDC) of its compression stroke. To determine TDC, remove the distributor cap and the plastic flywheel housing dust cover (normally aspirated engines only). The No. 1 piston is at top dead center when the distributor rotor is pointing to the No. 1 spark plug lead terminal (as though the distributor cap were in place) and the 0 mark on the flywheel or front pulley is opposite the pointer on the housing or front cover.
- Check the clearance of both the intake and exhaust valves of the No. 1 cylinder by inserting a feeler gauge between each valve stem and rocker arm. See the Tune-Up Specifications chart for the proper stem-to-rocker arm clearance.
- If the clearance is not within specifications, loosen the locknut with the proper size metric box wrench and turn the adjusting stud either in or out until the valve clearance is correct. The stud should just touch the gauge. Don't clamp the gauge tightly between the stud and head of the valve.
- Tighten the locknut and recheck the valve stem-to-rocker clearance.
- The rest of the valves are adjusted in the same way. Bring each piston to TDC of its compression stroke, then check and adjust the valves for that cylinder. The proper valve adjustment sequence is 1-3-2-4, which is the firing order.
- To bring the No. 3 piston to TDC of its compression stroke, rotate the crankshaft 180° and make sure that the distributor rotor is pointing to the No. 3 spark plug terminal. Rotate the crankshaft 180° after each valve adjustment before going on to the next adjustment.
- When the valve adjustment is complete, install the distributor cap, the valve covers with new gaskets and the dust cover on the flywheel housing port.