Hi There 1st post on this forum so please excuse if been asked before. ... Have a UK 1989 CBR600 FK street fighter so no fairing currently running 108 mains and 38 pilot jets ... going to be fitting a free flow Scorpion Stubby exhaust but keeping standard filter although air box has had snorkels removed and 8 1/2" holes drilled in the top have throughly stripped and cleaned carbs using 2 cans of cleaner and blown through with air line twice so am confident that they are clean ... has always had a flatspot similar to the symptoms in a previous post (lean Running Cane) and I'm guessing will be worse with a free flowing exhaust can fitted so question is what size mains and pilot jets should I fit as a starting point. ... Many thanks in advance. .. Andy

 

This is a trail and error sort of thing. I usually buy the next 3 sizes of mains and pilots on try them each to see what happens. You may have a flat spot now but that might change with different exhaust.


Flat spots as RPM's are increased are usually caused by the transition fueling being off. This is the point where the mains begin to open and fuel is being fed by the pilot jets and the mains. It can be tricky to get right with mods.


If you weren't changing the exhaust my suggestion to remedy the flat spot would be to increase your pilot jets and shim the main needles...but like I said it will all change with a different pipe.


You also need to be sure your pilot circuit is clean...most people think they cleaned it but really didn't get it clean. You need to be able to see light thru the pilot jets. It takes a keen eye because they are tiny. You need to take the pilot screws out and spray through both directions and see fluid come out the other side(s).


You need to adjust the pilot screws correctly. I grind off the flats on the screw heads so they can be adjusted on the bike because that is the only way to get them set properly. The screws need to be bottomed lightly on the bench and then adjusted to 2 1/4 - 2 1/2 turns out. This is the static setting for start-up. Most people think this is the adjustment, but they need to be adjusted on the bike with the engine at operating temp. The screws should be turned in until the engine begins to stumble then turned out until the highest RPM is obtained. Once they all are set the idle speed should be adjusted to spec with the idle ****.

 

Thank you so much for the reply very helpful

 

i think it was my thread about the lean running cane... it was actually too RICH .. i now have put size 98 dynojets in it and it has no flat spots.

 

That's why an O2 sniffer is the best way to go. Very unusual that your bike was running rich courior. Upgrading exhaust and breathing results in more air and less fuel (lean). It would be nice if you would update your thread and explain how you came to the rich conclusion.

 

In my thread post 52 is where i mentioned it was rich.. i can add a better conclusion if you want tho. In my case the plugs were always black and my friend who was riding behind me said he could just smell fuel really strong, also you mentioning it could be rich. So thats what made me move to leaner jets.

Shed .. Listen to Dennis, he knows his stuff.. In my case it was rich, when as Dennis says it should have been super lean. I havent put any miles on it since i got rid of the flat spot as ive been upgrading the handlebars to supermoto bars and parts take ages to arrive where i live.. I will be fine tuning it as i go but i got a great base to work from now...

My setup is this ... removed snorkels and also COVERED up the drilled holes.. so the airbox has just the 2 original air intake holes on top. I have what looks to be a standard filter. Exhause seems to be the original one thats been gutted and shortened (can see straight through it), and also where the silencer slips on, there is a db killer in the pipe that adds a bit of back pressure and quietens it down a bit.. i wouldnt mess about with the airbox at all, unless your going for mad power and drag racing etc.. keep stuff as standard as possible.. If you can find DynoJet 98 jets give them a try as them seems to work great for me.... also.. DONT BELIEVE whats written on your jets! i had what was written a 107.. but it was bigger than a Dynojet 120 (standard 128) .. the PO had drilled them out and this threw me off considerably, when i compared the "original jets" to the massive stage 2 dynojets i noticed that i could well be rich!

From my experience, if i was you i would:

1) clean carbs as best u can, strip em and spray everywhere.
2) compare your jets to new originals to make sure PO didnt drill them
3) Cover all the extra holes in the air box lid and get it stock

If this doesnt help, and the jets are really the size they should be, go 2 sizes up and work from there.

 

Sorry man. I went back and looked at your thread and you did explain it. My brain sometimes doesn't work very good. I'm getting old and have multiple motorcycle sites that I comment on. I'm really glad things worked out on your bike.

 

Hi all quick update to this thread ....
Finally got engine running much better after a lot of trial and error... The pilot jets I had to go from the standard #38 to a #35 this was relatively easy as I had an old "Colortune" spark plug to set up with so just adjusted air screws until bunson blue flame achieved at approx 2 1/4 turns out ..
The main jets I tried every size Keihin jet from 100 up to 115 without any luck getting rid of the flatspot between 1/8 and 1/2ish throttle I noticed that pulling the choke about 1/2 way on made the flatspot better .... So the only part I hadn't looked at was the needles on the throttle slides although they were clean I placed an extra 0.50 washer shim under the needle and went back to a 110 main jet and result pulls really hard throughout the throttle range .... big big thanks to everyone who helped especially Dennis who really does know his stuff ... Cheers

 

Thanks for the update. Glad to hear you got it sorted.