While there is no disputing old strokers will still run on the witches’ brew that’s passed off as petrol these days, many of them are suffering as a result; pre-detonation and worse happens regularly. From early 1960s piston-ported Yamaha YDS2S through to last generation, water-cooled, ECU controlled, powervalve equipped missiles, strokers are suffering unnecessarily. There are numerous specialists and experts out there and they all have their own pet theories and cures yet they all tend to agree that your average pump fuel is emphatically not the ideal diet for two-strokes. The days of skimming heads to raise compression ratios as a means of extracting extra power from stinkwheels are well and truly over.
1/ The reworked head of Project Yamaha RD350. It runs a wider and deeper squish band, has had metal removed from the combustion chamber and the plug has effectively been lowered into it. 2/ A Kawasaki triple head with a deeply inset or pocketed plug hole and the squish band is extremely narrow. The pock marking is almost certainly caused by detonation. 3/ Not every stroker head is necessarily in need of surgery. This Yamaha YCS1 head runs a profile that’s pretty much what’s needed for modern petrol. Compared to the previous shot the plug will be closer to the fuel/air mixture. 4/ A pair of Kawasaki triples heads before and after surgery. The squish band has been substantially widened and a lot of material has been removed at the centre. This both lowers the compression ratio and, effectively, brings the plug further into the combustion chamber. 5/ A typical Suzuki head (from a T125) with negligible squish, deep pocketed plug hole and perhaps most obviously a plug hole significantly off centre. 6/ And Suzuki T500s run a very similar set-up; fortunately offset plug holes and weird head designs can be changed. After some serious reworking we have the plug hole repositioned centrally and substantial squish band machined into the head that will direct the fuel/air mixture towards the plug.