There are concepts: "heat rating" and "heat ignition". If, due to carelessness, spark plugs that do not comply with the manufacturer's recommendations, and especially do not comply with the heat rating, were installed on the car, the occurrence of the described defect is guaranteed. After warming up, the engine begins to work accompanied by metallic knocks, similar to detonation. However, in this case, this is heat ignition. Drivers often confuse these two different processes - detonation and self-ignition. Self-ignition (glow ignition) usually occurs with engine overheating, when the mixture compressed in the combustion chamber ignites from high temperature and before a spark appears. "Igniters" can be hot electrodes or carbon particles. Checking glow ignition is simple. Turn off the ignition. When working with detonation, the engine will stop immediately. With glow ignition, it will still work. Naturally, it is necessary to stop the engine (you can disengage the clutch, engage the gear and smoothly release the clutch pedal), remove the spark plugs, make sure that they are indeed installed incorrectly, and install new ones.
It happens, however, that the spark plugs are not to blame, and the engine runs unstably at high crankshaft speed with all the signs of pre-ignition. This happens when a significant layer of carbon deposits has formed on the piston bottoms and combustion chamber surfaces. Carbon deposits are the result of using low-octane gasoline or running the engine for a long time with an enriched mixture. To prevent this from happening, you should only use AI-93 gasoline and monitor the carburetor adjustment. But to eliminate the malfunction that has arisen, i.e., first of all, to destroy the carbon deposits, you will have to contact a service station, since not everyone can competently cope with a whole range of works on their own. It will be necessary to remove and install the cylinder head, adjust the gaps between the levers and camshaft cams, set the ignition timing, adjust the tension (belt) of the timing mechanism, the fan belt. Although almost any car owner can perform each individual operation, the set of such operations and the need for certain tightening torques for a number of connections require skill.
(The text is copied from the online resource VAZBOOK.RU)