GPX250 Carbie tune

[jonjon86]

I've recently fabricated and attached an after market exhaust system to my 250.
It has much greater flow out of the engine due to the removal of the stock systems restriction.
Due to this, i've noticed a big improvement in low to mid range torque throughout the rev range but a reduction in top end. Its not as peaky as before.
I'm begining to think this is because the engine is running lean at the top of the rev range.

Is there a way to adjust this on the carbie. I also want to balance the carbs and tune them properly.

Any ideas or help would be appreciated.

[Johnnie5]

if its running great in the bottom end and mid then great for you

top end you are correct likely a little lean , need to go up a main jet size

[MiG]

At a recent dyno day I saw a ZZR250's (same engine) A/F ratios. It was running 11:1 in the bottom end and mid range and 16:1 in the top end. That's really rich and really lean respectively. Assuming your's is similar, and the exhaust has leaned you out across the board, the symptoms you've described are exactly right. In other words, increase the main jet.

[Slow and wobbly]

Try a couple of washers under the needle first. This has the effect of richening up the mixture across the range. Then try the next size up - 108. Contact show and go over in Adelaide they will have the washers and jets out to you next day.
Make sure that the carbs are clean and synched first though and that your mixture screw is 2.5 turns out. Just to make sure that you have at least got a starting point from which to work.

[Z4Kwaka]

Raising needles will affect the mixture across the entire rev-range; you may find it to rich down low? Larger jets and lower needles will yield slightly better results, this should keep your low end fuel curve around what it is now but at WOT the needles will be out of the jet tubes and allow a fatter mixture up high. If this is still not enough you may need a needle change (profile change) to allow the fuel curve to be a little more workable.