Q: I’ve just taken control of my uncle’s old 1977 Yamaha RD400, after pestering him for many years to let go of his old two-stroke.
I’ve got it running but the pipes were falling to pieces, so I’ve imported a pair of cool looking expansion chambers from Australia, which will make the bike sound better and go faster. I’ve kept the stock air box, but fitted a new air filter and cleaned out the carbs. The bike also had an electronic ignition fitted around 10 years ago and the timing was set correctly, so I’m told, so for now I’ll leave that part of the bike alone.
But my question is about the jetting and how to choose the correct main jet size? Do you have any idea what jet sizes to use and how to get the correct mixture; is the old ‘plug-chop’ method still applicable in today’s world of modern fuels, etc?
Eric Musson Cape Town.
A: The only way to get the jetting correct is indeed to do a ‘plug-chop’. Hold the bike flat out in fourth or fifth gear and immediately kill the motor with the kill switch.
The plug should have a light brown tint to the white center piece, but if you see any sandy deposits or erosion of the centre electrode it can be dangerously weak and therefore cause terrible damage to your engine. If you blow a hole through the piston by running too lean it will spray molten aluminium all over the crank, and that will then become a major problem.
As a golden rule with old air-cooled two-strokes, if you’re not too sure what you’re doing please take it to a professional who does?
