Main Jets vs. Slide needle position

[sark]

DOes increasing the size of the mains have the same net effect as raising the slide needle position?

[hampshirehog]

No. There is some overlap in the transition but you can often run a bike without the mains fitted as long as you don't go too deep in to the needle circuit.

[sark]

Ah very interesting. Thank you

[49er]

The needle position affects the air/fuel ratios in the 1/4 to 3/4 throttle positions.

The advantage of dynoing your bike is that you get good feedback from the data. When they do your test for maximum HP and torque, it's always done in 4th gear, at WOT, with a 20% load on. Because of WOT from 2500 rpm, the needle position has limited/no effect on the AFR during the engines climb to its peak. It will have more effect on the lower end rpm range on CV carbs though.

By analysing my 850's data, I found that the main jet was providing an AFR of 13.2 -1 to 13.4 -1 from 7200 to 9,500 rpm and then richened slightly to 12.8 -1, up to 10,500 rpm. This is slightly lean, so a next sized up main was called for. I deliberately went up 2 sizes and dropped 2 HP off the max, but the AFR map was identical, be it in a more richer range than above.

You can do runs that allow AFR's at certain rpms too, but that doesn't tell you what part of the needle shape is affecting the recorded AFR.

Plug chops at marked throttle positions will deliver the most accurate data for your fine tuning. Learn to read your plugs and enjoy the tuning challenge.