On some engines, it is difficult to place the gas injector nozzle close to the petrol injector. On others, where there is a separate tract for idle air, it can be difficult to position the nozzle so that gas is drawn in at idle and open throttle. In either case the answer may be to use an injector extension. These are fitted by drilling a 9mm hole in the inlet manifold and tapping it M10x1.0 to take the gland nut. The copper straw is fed through until the end is close to the inlet valve. The other end of the straw is joined to the injector hose (6mm).