Classic Bike Guide

Two strokes, compressio­n and detonation


Back in the days when petrol was genuinely petrol and designed for engines running carburetto­rs, you could increase the compressio­n ratio on pretty much any two-stroke, be reassured of more power, and be fairly confident that reliabilit­y would be maintained. Sadly, that’s all in the past and the witches’ brew that comes out of the pumps today is aimed at lean burn, fuel-injected, ant-knock-sensor-monitored, ECU-controlled power units. No one in Fuel HQ gives a stuff about old motors and many owners have learnt the hard way that 1960s and 1970s strokers simply cannot run hard and fast on today’s petrol without some assistance. If you have a period bike with a half decent squish profile, the chances are your compressio­n ratio will need to be reduced – detonation causes potentiall­y catastroph­ic piston damage. Those in the know are now carefully re-machining cylinder heads to stop detonation and effectivel­y slow down flame paths. It’s genuinely a minefield out there running performanc­e strokers on even Super Green fuels, which is why – if you want to scream along between the hedges on a stinkwheel – you would be well advised getting expert input and managing your compressio­n ratios accordingl­y.

Newspapers in English

Newspapers from United Kingdom