Q: Hi folks. I’ve just acquired, well took it actually, a Yamaha YZ250 MX bike from my brother who lives close to Durban.
Not sure what year or model it is, but it’s blue. It is a bit rough but runs OK and it’s just about having fun around my farm up here in the Gauteng area.
So, what I’d like to know is what jet size should I use because it’s currently jetted for the coast and feels, to me, like it’s running too rich? I run a NGK ‘9’ plug and the air filter is nice and clean so that’s not an issue.
Ian Douglas
A: Take out the main jet and see what size it is, it will be engraved on it somewhere, and if you’re too old you might need glasses to see it. Then, as a rule, we just drop the jet size by around 7% to compensate for our 1,500m altitude. So work that out on your calculator and then find the size you need, or close to because you’re not involved with competition riding, from any off-road type shop.
That should make it ‘cleaner’ and ‘crisper’ throughout the rev range.
