The two simple causes of these phenomena for Old model cars were: one, the fuel had too low an octane rating, and would self-ignite from the heat built up when the fuel mixture was compressed, particularly with the engine under load. Changes in refinery methods and using the lead compounds to increase octane rating improved engine performance and efficiency.
Two as the engine accumulates carbon and lead deposits in the combustion chambers creates spots that get white hot during engine operation. Soon, these “hot spots” cause the ignition of the plugs. The cure for this cause is to remove the cylinder head and scrap and wire brush deposits from the combustion chambers, the top of the pitons, and the valves. Decarbonizing is the common name for this job and it often is combined with valve service.
An engine top cleaner (a specially made solvent) can be run through the fuel periodically, to help reduce the formation of deposits and delay or possibly eliminate the need for decarbonizing. Even the occasional spraying of water into the carburetor air intake, with the engine running, will help remove carbon deposits.
A knock problem from a low octane rating is common today. Most engines are designed with compression ratios as close as possible to the limit posed by the octane rating of the fuel available.
If a higher octane fuel is available to the motorist, he should use it to suppress knock. Or the compression ratio can be reduced by removing the cylinder head and installing a new cylinder head gasket with a shim. For more severe cases, lower compression ratio pistons may be required. in milder cases retarding the ignition timing may be all that is needed.
Engine cooling system problems-
Several other factors related to modern automobile and engine design can cause knock. They include the following :
If the cooling system is poorly maintained, passages get plugged, and hot spots from around the combustion chamber because coolant cannot reach them.
Or the engine simply may be running close to an overheating condition because of a low coolant level or slipping of the fan/water pump belt. With any of these conditions, the engine may run hot and not normally boil over. The reduction in heat transfer results in ut combustion chamber hot spots.
Lean fuel mixtures-
Although many modern engines are designed to run on a fuel mixture for better economy and lower emission of air pollutants into the exhaust, the lean mixture tends to raise combustion chamber temperatures. As a result, some spots get hot enough to cause a knock.
Knock may be severe if air leaks into the intake manifold, further leaning out the mixture. One reason is leakage at the carburetor base and intake manifold gaskets. It may be rectified by tightening the retaining nuts or bolts.
Improper ignition timing is another possible cause of engine knock. If the timing is too far advanced, it will raise combustion chamber temperatures and hot spots will result.
First, check basic tuning with a timing light. Clean the timing marks first and, if necessary, apply a line of chalk to the correct mark. liming should be within two degrees of the manufacturers’ specifications.
It is possible to eliminate most knocking by retarding tuning, even if it is within specifications. But if the timing is retarded too far, the performance of the engine would not be at an acceptable level.
Next, check the distributor centrifugal (speed-controlled) advance. If the timing seems to advance very much with just a moderate increase in engine speed, it is possible that the centrifugal advances effectively perhaps the springs are weak or broken. A simple check is to shut off the engine and unit the distributor upper shaft by the rotor. The shaft and rotor should turn a small amount against moderate spring pressure, then stop. Release it and it should spring back. If there is any weakness in the spring action or any stiffness, remove the distributor plate, and inspect more closely. If found defective, fix a new spring.