Hey guys, I am new to the forum, I have had a good search through older posts but would also appreciate a bit of input with regards to main jet sizes for my bike (a 96 RVF, stock as far as I am aware, except for an unknown carbon race can)

I am used to working on cars mainly (mechanic to trade), but had a good few bikes in my youth so know my way about them, although today was the first time in a while that I have stripped any carbs down.

My problem has been the (i gather rather common) flat spot between around 5k and 7k rpm, so I decided to spend a bit of time and sort it out, since I had new plugs, oil, filter etc to do anyhow.
Before I stripped the bike down I took it out for a run with the wideband AFR meter hooked up to get an idea of how it was fuelling, I guessed lean around the flat spot, but was a bit surprised at just how lean it was throughout the revs at wot. At around 5k it basically ran 15.9:1 which it maintained, only dropping to 15.7:1 up top.
Part throttle is perhaps a tad rich running between 13.5:1 and 10:1.

The main jets are i believe stock - 108s, and the needles look std as far as I am aware (J8VA marked on them) with one small washer under them.

I am going to see about getting some different sized jets tomorrow morn, and figured trying around 115 or 120s? Though am unsure as I do not really have any experience with these motors.

Any input would be appreciated.
Thanks,

Phin

I would try an extra washer under the needle - as you lift it to make it richer in that part of the rev range. 1mm total would do it I reckon..
Main jets do not really come into play at t 5k revs range


Welcome to the site too - new guys with tools..cool

+1

this works ive tried it, you may aswell replace the springs while your there, easy to do.

rick oliver can supply the parts you need

Would I be right in thinking that lifting the needles will also richen me up on part throttle? As I am already fairly rich here I would like to avoid making it worse if possible. I thought I could maybe go up in main jet size, and if required drop the needles by quarter of a mm or whatever their washer size is?

Will the needle height have much of a bearing on fuelling at wot when the sliders will be up at the top of their travel?

What difference will the springs make? are they not just to hold the needle against the main jet when the throttle is closed so that the idle jet controls fuelling?

I will see if I can get some washers and Jets this morn and have a play about.

Loads of questions I know, sorry guys.

Cheers for the input,


Phin

a/f readings at part throttle and below 5k rpm won't tell you much.

Shim the needles up an extra 0.5mm and install 112 main jets all round and it will go hard all day.

This is a tried and proven jetting combo.


What sort of a/f guage were you using and where was it taking readings from?

Stickshift wrote:What sort of a/f guage were you using and where was it taking readings from?

I was using a Dastek lambda meter, with the sensor in a tailpipe clamp. It gets used daily for mapping cars and is reliable, so I am confident the readings I got were good.


Just had it out for a brief run after work, and I think I have it pretty nailed now, got 114 main jets in, needles are back in stock position, and the air screws 2 turns out. At WOT it is running at a pretty smooth afr from 14.4:1 down to 12.9:1 @ redline, also seems leaner at light throttle.
The motor pulls stronger all the way, and the flat spot seems completely gone, so all in all I am pretty stoked, I will get out for a longer run soon just to verify things.

Cheers,

Phin

ram5ay wrote: Just had it out for a brief run after work, and I think I have it pretty nailed now, got 114 main jets in, needles are back in stock position, and the air screws 2 turns out. At WOT it is running at a pretty smooth afr from 14.4:1 down to 12.9:1 @ redline, also seems leaner at light throttle.

Good work.

The mid flat spot afflicts nc30's and 35's, probably having to meet noise and emission tests. Fitting a good half or full exhaust system helps a lot with midrange power over stock pipes.

Do any of you know where the circlip on the needle should be? I dont under stand why you would use a 0.5mm washer to raise the needle if you can just change the circlip position to one lower. Have i missed something? Mine is set to 1 down from the top (lean). should it be set to the richest position??

mylessheehy wrote:Do any of you know where the circlip on the needle should be? I dont under stand why you would use a 0.5mm washer to raise the needle if you can just change the circlip position to one lower. Have i missed something

Tha standard needles don't use circlips/grooves, hence the washer. Yours aren't standard.

That makes alot of sence now. Thanks.


I wonder what they are then, hrc set maybe?? There brass and groved at the bottom, not like a needle, flat bottom, but tappers in just like an ordinary set.