Since the Sigma had been sitting in the garage "hibernating" through winter, I thought now that summer is here it would be a good time to give the car a check up and tune. Once again it started first try, despite being untouched for quite a few months. The engine didn't smoke at all besides a little bit of black smoke under full load and high revs. After tuning the engine and checking everything, I remembered one little thing I forgot to do after building the current high compression (12:1) setup: the spark plugs! I did fit new spark plugs, but they were still the recommended type for the stock (8.2:1 - much lower compression) engine. Basically the recommended plugs were a "5" temperature rating. It is advised that for each full point of compression you go higher than stock (under 12.5:1) you should also go down (or up depending on perspective) one point in the heat range of the spark plug.
Confused? Hopefully my example will shed some light on the matter. The standard plugs in the NGK brand were BP5ES for the stock 8.2:1 compression on the older Astron 2.6 engines (factory pre-unleaded Australian engines). On the NGK part numbers the part of the code that indicates the temperature type of the spark plug is the number between the letters (in this case, 5). The higher this number the colder the spark plug type it is and the lower the number, the hotter the spark plug type it is. So I went up to a code 9 spark plug (could've gone to 10 but these are too hard to find). I also decided to spend a bit extra and select the Iridium NGK plugs, which not only last longer but also (due to their shape and design) offer a much better combustion, which is much more important on a higher compression petrol engine. The correct colder plugs will also stop the plugs running so hot that they end up igniting the air-fuel mixture prematurely due to being too hot.