It's been a great couple of years! Comrade Trabi has been tirelessly patrolling the midwest for well over two years and 49,000km with no major issues after my 2nd hand parts motor overhaul in May-June 2012. However…. Lately, over the last couple of thousand kilometers, I've noticed….. Spark Knock! Particularly at high speeds of over 90km/h. Also known as "pinging", "pinking" "pre-ignition" and "detonation", this phenomenon can murder your motor quickly. At first I thought: "aha! those iridium sparkplugs must be too hot!" But no, re-installing the NGK BR6HS had no effect other than less throttle response. Remembering that if 2-cycle motors had a fault, which they don't, it would be the tendency to build up carbon in the combustion areas. So, I concluded that there must be carbon buildup. So, I started using high-octane fuel until I could get the heads off. This only reduced the tendency to ping. So, 3 days ago, at 49,500km since overhaul, I pulled off the heads and guess what? Both heads and pistons had considerable carbon buildup! And, the carbon had a hard, partially melted texture with many sharp, raised points due to the pre-ignition which would tend to encourage even more pre-ignition. Cleaning the carbon off revealed no damage to the underlying aluminum. A set of new head gaskets, torquing the bolts to 42NM and we're back on the road with no more spark knock! Probably good for another 45,000+ km! Some pointers: Spark Knock, which sounds like a light, rapid rattle that varies with throttle application, can damage your motor in the following ways: 1- It can damage your cylinder heads, gaskets, spark plugs and piston crowns, tearing out small chunks of aluminum and breaking spark plug ceramic. 2- It can cause very rapid wear to piston rings 3- It can damage your connecting rod bearings at both ends. It can be caused by the following: 1- low fuel octane for the motor's compression ratio 2- overheating 3- too lean of fuel/air ratio 4- incorrect spark timing 5- Too "hot" of heat range spark plugs 6- carbon build-up, which increases compression, insulates the combustion area from the cooling system and might also provide sharp points that can glow at high load When experiencing spark knock, it is best to: 1- reduce load immediately so that the pinging stops. 2- High-octane fuel should be used to reduce the pinging until the cause can be determined and corrected. When decarbonizing a Trabi motor, best practice is: 1- have a new set of head gaskets on hand 2- Do one cylinder at a time, to prevent debris from one cylinder entering the other 3- Clean the piston crown at TDC so that a minimum of debris gets past the piston. A wire cup brush on a drill works well. Avoid gouging the piston surface with scrapers or excess pressure on the brush. Also avoid solvents on the piston, which will get past the rings, washing off oil and bringing debris downward. 4- Clean the heads in the least abrasive manner possible. Blasting with walnut-shell media is quick, effective and safe on aluminum. Solvents might also work. Avoid scraping if possible. Cylinder head bolt torque is 41.2 Nm or about 31 ft-lbs. it is best done criss-cross, in at least two stages. That's it! don't over-tighten the bolts, as the threads can easily strip out and over-crushing the gaskets will cause leaks, not cure them. Happy Motoring….. PS> Comrade Trabi accumulated enough carbon to cause light detonation after 49,000km using a "low ash" synthetic oil at 50:1 fuel/oil mix, or 2% oil with mostly highway driving. Using non-synthetic oils or a higher oil percentage in the fuel will result in faster carbon build up!