Jetting advise GT380
Posted: Wed Feb 17, 2016 8:47 pm
About to take the leap with Jemco's 3-3. I have a single open element K&N on the stock 1-3 rubber manifold. Any hints as to where to start jetting wise on the early style carbs?
OK, let's try that again shall we....
In my experience, tuned two strokes tend to seize on part throttle rather than when flat out, so I would start rich with one size up pilot jets, raise the needles one clip position and go up two or three sizes on the mains. That should give you a reasonably safe starting point. It's easy to fit way too large main jets to try to get mid range right and then when the throttle is almost shut on cruise, it is too lean.
If you have a dyno shop close by, get them to run it with exhaust gas analysis and jet it for top end on the main jets and have a look at the Air: fuel ratio at low to mid throttle to see what is going on and then move the needle to get that part back in line.
A good trick is to mark the throttle at 1/8, 1/2, 3/4 and see what the fueling looks like at those settings on the dyno after the main jets are right. And you should use main jets one size larger than absolute peak power just to be a little safer.