This applies to the 4-121 engine only.
Valve adjustment is one factor which determines how far the intake and exhaust valves open into the cylinder. If the valve clearance is too large, part of the lift of the camshaft will be used in removing the excessive clearance, therefore the valves will not open far enough. This has two ill effects; one, the valve gear will become noisy as the excess clearance is taken up and, two, the engine will perform poorly. This is because intake valves which don't open the full distance will admit a smaller air/fuel mixture into the cylinders. Exhaust valves which aren't opening the full amount create a greater back pressure in the cylinder which prevents the proper air/fuel mixture from entering the cylinder.
If the valve clearance is too small, the intake and exhaust valves will not fully seat on the cylinder head when they close. When a valve seats on the cylinder head it does two things; it seals the combustion chamber so that none of the gases in the cylinder can escape and it cools itself by transferring some of the heat absorbed from the combustion process through the cylinder head and into the cooling system. Therefore, if the valve clearance is too small, the engine will run poorly (due to gases escaping from the combustion chamber), and the valves will overheat and eventually warp (since they cannot properly transfer heat unless they fully seat on the cylinder head).
While all valve adjustments must be as accurate as possible, it is better to have the valve adjustment slightly loose than tight, as burned valves can result from too tight an adjustment.
The valves are operated by an overhead camshaft, driven by a toothed rubber belt, connected to the crankshaft. The cam lobes contact `bucket' type tappets, which are set over the valve and valve springs, and force the valve and springs to move downward, moving the valve from its seat on the cylinder head. Both intake and exhaust valves are manual adjusted by a wedge type screw angled into the tappet, perpendicular to the valve stem. A flat area is milled onto the screw, which contacts the valve stem end. The threaded area locks to a threaded area within the tappet. Each turn changes the clearance 0.002 in. (0.051mm). When tappet adjustment is done, the flat side of the adjusting screw must be toward the valve stem end at the completion of the adjustment.
See Figures 1, 2, 3 and 4
Valve adjustment must be made with the engine at normal operating temperature.
- Remove the TAC hose, the cylinder head cover, the spark plug wires and distributor cap.
- Rotate the crankshaft to bring the number one cylinder to TDC (the beginning of its firing stroke). The position of the distributor rotor will assist in determining this position.
There is a mark on the edge of the distributor housing at number one terminal position. Do not attempt to rotate the engine by turning the camshaft. Turn the crankshaft in the direction of normal rotation to avoid damage to the timing belt.
- With number one cylinder on TDC of its firing stroke, the clearance of the exhaust valves on cylinders number one and three, and of the intake valves on cylinders number one and two, can be checked.
The front valve in each pair per cylinder is the intake valve. If the clearance requires adjustment, a special too is required to move the adjusting screw.
- Adjust the screw by turning one complete turn until it clicks, and continue until the proper clearance (0.006-0.009 in. [0.15-0.22mm] for intake, and 0.016-0.019 in. [0.40-0.48mm] for exhaust) is obtained.
- After adjusting the clearance, use the special AMC gauge J-26860 to check the position of the screw in the tappet. If the gauge indicates the adjusting screw is turned too far into the tappet, the screw must be replaced. Five sizes of screws are available, identified by grooves on the end of the screws.
If the adjusting screws must be replaced, the tappets must be removed from the head. Note which tappets must be removed, then continue the adjustment procedure. When all eight adjustments are made, remove those tappets requiring screw replacement. Refer to camshaft removal and installation.
- Rotate the crankshaft 360°. The distributor rotor should be 180° opposite the mark on the distributor housing.
- The clearance can now be checked on the exhaust valves for cylinders two and four, and the intake valve on cylinders three and four.
- Reinstall the head cover, using a new gasket.
- Reinstall the distributor cap and spark plug wiring. Reinstall the TAC flexible hose.