shaihulud wrote:It's especially effective if you keep your foot on the accelerator after a rev up and a turn off. Extra cooling petrol and air to cool the hot spot.
Letting the engine idle for a while before you turn it off, as if you have a turbo to cool down first, might also solve the problem.
Ii agree, It is most likely being caused by a hot spot in a cylinder and an excess of unburnt gasses/fuel in the combustion chamber - leading to an explosion through a partly open exhaust valve.
A couple of things that you could check is that the valve clearance is still correct, your timing correct and that your carb(s) are correctly tuned/jetted (ie: not too rich) and are not leaking down the throats when the engine is off.
As a matter of interest - my brother (a VW mechanic) tells me that it is quite common for engines that have had a spark plug helicoiled to do exactly what your engine is doing, as sometimes the end of the helicoil sticks into the combustion chamber creating a sort of 'glow plug' effect. Has your engine has a helicoil fitted?
One last thing - revving an engine as you switch it off causes an amount of fuel to be drawn into the combustion chamber which will not be burnt, which will then wash the oil off the side of the cylinder and dilute the oil in the sump - so this practice should be avoided unless you enjoy re-ringing/re-building your engine on a regular basis
🙅
Hope this helps,
R
🙂