Valve Mechanism Troubles

In the course of operation, the rubbing surfaces of the valve mechanism components suffer wear, with the result that the clearances in the movable joints of the mechanism grow larger with time. Moreover, the contact between the valve faces and seats, or the valve seating, gradually deteriorates as a result of the action of hot combustion gases and impact loads and also because of carbon being deposited on the valves and their seats.

Basic valve mechanism defects include worn and burnt valve faces and seats, worn valve stems and guides, worn rocker-arm contact pads and adjusting screws, worn push-rod ends and tappets, worn cams and camshaft bearing journals and bushings, worn timing gear teeth, and weakened valve springs. These defects are evidenced by characteristic knocking noises and result in poor engine performance.

To ensure that the valve mechanism will operate normally, it is necessary to carry out certain tune-up procedures periodically and in strict compliance with the pertinent service regulations. These procedures include the checking and tightening of fasteners holding the cylinder head, rocker-arm shaft pedestals, and hi her components, the checking and adjusting of the valve clearances and the compression if lease mechanism.

The engine will operate normally only if the valves open and close in conformity with the valve timing diagram and, when in the closed position, tightly close their ports in the cylinder head, and it is incorrect valve clearances that may disturb the timely opening and closing of the valves.

There are standard valve clearances for automotive engines. Too small or too large a valve clearance reduces the engine power and increases specific fuel consumption. With the valve clearance too small, the valve poorly fits its seat when hot, this causing premature burning of the valve face and seat. Where the valve clearance is too large, the valve stays open for a shorter period than required and starts knocking, valve knocking being attended by intensive wear of the valve rocker contact pad and the valve stem tip.

The valve adjustment sequence for some engines is as follows. Clean and remove the cylinder head cover. Tighten the rocker-arm shaft pedestal fasteners and engage the compression release mechanism. Crank the engine over to position the piston in No. 1 cylinder at TDC, or slightly before TDC; at the end of its compression stroke, disengage the compression release mechanism, and check the valve clearance with a suitable feeler gauge slipped between the rocker arm and valve stem. Should the clearance prove to be wrong, adjust it using a wrench and a screwdriver. To do this, loosen locknut and turn adjusting screw until the feeler gauge has a slight drag when pushed and pulled between the contact surfaces of the valve rocker and stem. Then tighten the locknut and recheck the clearance.

If the engine uses an adjustable compression release mechanism adjust the compression release in No. 1 cylinder simultaneously with the valves. For this purpose, engage the mechanism, i.e., position compression-release shaft so as to make the axis of adjusting screw assume a vertical position, loosen the locknut securing the screw, and turn the screw counterclockwise until it looses contact with the valve rocker. Then turn the screw back until the contact pad on the valve rocker just touches the valve stem. Add another turn to the screw and tighten the locknut.

Successively turning the crankshaft half a revolution, adjust the valve clearance and compression release in all the other cylinders in their firing order. In multicylinder engines with more than four cylinders, the valve clearance can be adjusted in several cylinders at a time.

The valve clearance and compression release adjustments finished, install the cylinder head cover in place. Then start the engine and make sure that there are no oil leaks from under the cover.