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Hello i have a 1999 900rr with a slip on and a K&N filter what are the stocks main jets sizes? 120 across? would the stock main jets be ok with the aftermarket parts on my bike? let me know thanks..
 

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The stock main jets for a 99 are 120. Are the parts already on the bike? If so how is it running? Have you ever jetted a bike before? Do you know any thing about carbs and how to fix fueling problems at different rpm ranges? Getting a bike jetted properly takes time and knowledge of how a carb works at different rpms. It is hard to say if the stock jets will be enough for your bike and mods without actually riding your bike to see how it is running.

Usually though cbrs in general are very sensitive to intake or fuel changes and at the least require a base jet kit or the needles shimmed and the idle fuel mixture screws adjusted.
 

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9 times outta 10 a slip-on won't require any adjustments. The filter may need some tuning though. Run it down the road and see how it acts. Do a full throttle roll on at around 4K rpm's and see how it accelerates. If it's jerky or sluggish, slip a washer under the needles. Then do a full throttle plug check, that'll tell you how the main is doing.
 

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The washer needs to go under the needle of each carb to raise it. Thus making the fueling richer. You need to make sure that you have the right main jets though before trying to tune the midrange and idle to off idle settings. If you have the wrong main jets the other settings will never be right.
 

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I tune the exact opposite. I go from bottom up. The pilot and needle will affect A/F ratio on topend but not the other way around. Radio Shack sells a pack of 100 washers that are perfect size for needles. They have 20 each of 5 different sizes.
 

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Put it on a dyno and use a sniffer in exhaust. Change out the mains and check A/F at idle. Then put stock mains back in and back fuel screws out couple of turns and check A/F at wide open. Fuel screw, pilot jet and even needle will affect A/F at wide open. Main jets will not affect A/F at idle. I'm not saying anyone knows or doesn't know anything, but it is what it is. Go with what ya know...
 

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It is true that the main jet wont effect A/F ratio at idle but it does start to effect A/F ratio at just off idle once the needle starts to move. So if your main jet is to small or to large you are going to be trying to compensate for this by moving the needle out of its optimum range for cruise and mid-range power. You will be chasing ghost trying to smooth out the fueling on a street bike. Plus you can run into other problems such as cold start issues if you move the fuel mixture screws and needles to far to compensate for a main jet that is to small or to large.

Now the above problems are in the low to midrange side of things at small to moderate throttle opening. At full throttle you are going to have even more problems in the midrange to redline side of things. If the mains are to small the bike will run lean from around 8000-redline and if they are to big the bike will run to rich in this same range. Now if the mains are only off by a size you maybe able to correct this issue by adjusting the needles and idle mixture screws but not without screwing up the idle, midrange and cruize speeds. If the mains are off more than this though you wont be able to correct it no matter what you do with the fuel mixtue screws or the needles.

This is why I say start with the best main jet at full throttle operation. If you have the correct main jet size the 8000-redline will be spot on and the little amount of adjusting that you will need to make to the needles and fuel mixture screws to get the midrange, low end, and idle correct will have no ill effect on your top end. The only thing you may have to change is the pilot jet. It is very rare but in some cases the pilot jet has to be raised to the next larger size to smooth out lean surging at cruize rpm.

Like you have stated go with what you know. To me dynos are great for tracking changes being made to make sure things are going in the right direction or race bikes. When it comes to real world street riding though I could really care less what a sniffer is telling me about A/F ratio. As long as the bike starts in extreme cold or hot weather, idles smooth, has spot on fueling througout the rpm range, makes enough power to get me thrown in jail in first gear, doesn't buck like a horse at cruise speeds, and is not over heating or running so rich it is fauling plugs or spitting fuel out of the exhaust then I am happy even if the dyno is telling me I am down 2 hp, 1.5 lbs of trq. and that may A/F ratio is off by .5.

Oh and I really didnt think you were trying to say that anyone knows or doesnt know anything I was just trying to avoid a back and forth typing fest between us.:D When the real issue here is that a fellow member needs help. All is good.
 

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I'd never get into a typing fest. I've read replies by you before and respect that you know what you're doing. But that small ill effect needle and fuel screws has on main IS a big deal. One jet size makes a HUGE difference. Back when I had my old 1000F a buddy had an F3 and we ran almost identical times. I started playing with main jetting and 1 single jet size sent me from running side by side to watching him pull away. These things are hellaciously picky and the smallest of adjustments make big difference. We are totally in agreement about dynos. I use them to make sure my tuneup is going in the right direction. For tuning, I use the dragstrip. MPH tells the tale. If you don't have a strip handy, do full throttle roll-ons with a buddy (if his bike is a lil faster, that works even better). Play with the needles...one clip position (or 1 shim) makes a world of difference. Same with the mains. That's why I never recommend jet kits. You're paying over 100 bucks for 3 sets of jets basically. Somebody show me where their aftermarket needles are making more power over stock and I'll quit the business alltogether. They are so generic in their tuning tips it's pathetic. Dynojet used to recommend running bike up to 4Krpm's and opening throttle. If bike pulls smoothly, needles are fine. If you are willing to settle for fine, no problem. But there are a couple of positions where it will pull smoothly but only one that will give you max performance. Anyways, this is turning into a novel so I'm stopping. All is good man! You should've bidded on those FCR's on ebay, they were cleanest you'll ever find for sale. I take care of my stuff.
 

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I figured the flats were a want and I needed some other things like brake lines and riding gear so I had to pass on them. If I would have know they were a fellow members carbs I may have bought them any way. As for this carb tuning debate it just can't be done with typing.:crap: If we were face to face we could sort this out as fellow tuners and then go hit some curves. I mean I agree with you on one jet size or one slot on a needle can be a big difference and that the needle and other settings can make big changes in power but at full throttle in the upper rpm range when the needle is at its highest point the main has to be right. As for Dyno jet I couldn't agree more. Ok thats enough I think I am working on my second chapter now. Catch you in another post CBR. Good luck at the strip this year get some video of a few passes if you can. Would like to see your bike and skills in action. To Zach sorry for jacking your thread. You got the bike running yet?
 

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I think I posted this one on another post but just in case. This was best pass of last weekend. Walked off and left stretched Busa. As for those curves, you can have them. I don't have the nads to run curves, takes more guts than I can muster. I'll stick to my straight lines...

cRRowbuild090.flv video by cbr900racer - Photobucket
 
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