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MUNK
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Discussion Starter · #1 ·
Hi fellow street fighters, I've just converted a 98 Cat into a streetfighter. I'm really pleased how it turned out. It spent many months at the mechanic trying to sort out a problem i had with the carbs. It is still not completely sorted. I bought a replacement 99 set of carbs and after much effort it ran quite well except for the band between 3.0k and 3.5k. Within this range it stutters or judders. Above and below this range it runs nicely. I did the carb mod to the airbox, i.e. ran the vent tube up into it. I am using the stock YZF air box and remove all the plastic tubes that pass through the frame. I just have a home made air scoop with a mesh opening, all of which is bolted to the outside of the frame. I do not know what number of turns the idle screws are set at - that is something i need to check. It might be running a little rich, not too sure. Need to check the plugs and maybe put a colour tune on them. I did not change the jets or needles, the are stock. i don't want more power (scares the crap out of me as it is) but i don't know if i should change them to a dyno kit to solve this annoying problem. The exhaust is very slightly shorter than stock while i was replacing the rusty insides.

Any ideas on the stuttering would be really appreciated.
 

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Ridin Dirty
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if the problem goes away while the choke is on, you may need to raise the needles .5-1mm with the radio shack washers.. or try stacking screens on the intake. This sometimes is an issue with the shape of the cutaway on the slides or the needle taper, the trick is to raise the needle to change that taper, but that will richen most of the throttle range.
of course, you could have dirty air jets, or wrong pilot jets, air screws not adjusted or a ram air-vacuum issue, assuming it had ram air.
 

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3-3500 is right as most bikes are coming "on the needles". An opened up airbox will push the bike toward lean, and in the range right before the needles start allowing fuel in, it's going too lean and stuttering out. I would agree that shimming the needles up a bit would be a good first thing to try. A jet kit could help also (they usually include adjustable needles), but if it runs good everywhere else it's probably not needed.

When it's in this range, how does it react slowly rolling on the throttle vs suddenly whacking it open?
 

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MUNK
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Discussion Starter · #4 ·
Thanks guys,
if i accellerate hard i don't notice any stuttering in this range at all.
Also i noticed that it stutters much less the first 1/2 mile after driving it from cold. I am not sure if the choke in or out when it was still cold made a difference. I need to experiment with this more. Normally i push the choke in as soon as the engine copes without it, which is normally fairly quickly. For sure when it is a little warm pulling out the choke does not improve the stuttering. The rest of the range is great. Dirty air jets, or wrong pilot jets should not be an issue as the carbs were very well cleaned (ultrasonically apparently) Re air screws - is this the pilot screw?? I can only hope i don't have a ram air-vacuum issue. I know it was set up that way but read some where that the 98 model does not rely on them in the same way the later models do. What is stacking screens on the air intake?
 

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Not necessairly. In general, screws downstream (toward engine) of the throttle are fuel screws, and screws upstream are air, but this isn't gospel- I have seen exceptions.

Most 4 stroke CV carbs have one adjustable screw, on the downstream side, it's a low speed fuel mixture screw. It technically adds fuel at all RPM, but it's effect rapidly diminishes once you're on the needles. Before you make any changes, turn them all in until they lightly seat and count the # of rotations, then back them all out the same amount. 2-2.5 is typical.

Regarding your stutter, it might help to back these screws out 1/2 turn. If it helps, go 1/2 turn more.

Often these screws have an anti-tamper plug stuffed over them by EPA nannies. Carefully drill a small hole it it, then use a small pick and they come right out. A small self-tapper screw and vise grips works also.
 

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Ridin Dirty
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First, you make sure your low speed mixture screws are right. If they are letting to much fuel out, the bike will become hard to start after it as warm as it can get. So, if it starts easily when warm, they are close.When the problem gets worse after the engine is warmed, it is to rich, So, it maybe overloading with fuel as it transist from idle mixture to needle. meaning, if the mixture screw is correct, the needle would need to be LOWERED or, the is a vacuum issue not letting the slide raise..
stacking screens is what sometimes the factory does in an airbox to richen the entire range.
you say you have a mesh screen on the inlet, what does removing it do to the issue?
My ram air bike has to have a slight suction on the air line that balances the air inside the float bowls, those lines ran to the end of the ram inlet from the factory, now that end is near the v.stacks..in the airbox.
 

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MUNK
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Discussion Starter · #7 ·
Thanks for the advice guys, really appreciate it.
My carb has one adjustable screw with no tamper plug. The manual refers to it as the pilot screw (but i guess you knew that). I will check what they are set at and adjust as suggested. If that does not improve i will try backing them out a little more. Each step of the way i will check the plug colour and also with the colour tune to keep track of how it responds etc..

To answer the second post – the bike starts nicely when warm, so i suppose the pilot screws are quite close to their correct settings, as you say. I can check for any vacumn leaks with the ‘ starter spray’ to see if that gives any results . The stock needles are not adjustabe so i cannot lower them.

Removing the mesh screen makes no difference. It really seems quite open and unrestricted. The air line on the carbs also runs into my air box at the front (closet to the steering)

got some detective work to do. Don't you just love getting your hands dirty!!!
 

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Ridin Dirty
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no prob, man.
I remember my bike was very tricky deleting the ram air from the front of the fairing.
I tried running that carb balance line to all kinds of places, even tried replicating the factory design, it never would run right.
Finally, I unhooked the 3/8" line that fed the crankcase vapors back into the airbox, hooked my ram air balance line there instead and stepped down the line in size to the size on the carb, I installed a "t" 3-way vacuum splitter in line, tapped and threaded the middle of the "t" so I could screw a bolt in it for a homemade inline shutoff-valve. The fact that the carb balance line has a large end for more vacuum surface area, and I can fine tune the amount of vacuum with the "t" valve really did the trick for me.
 

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MUNK
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Discussion Starter · #9 ·
i think the problem has been solved. I set the pilot screws to 2.5, 2, 2 2.5 as mentioned in the forums and raised the stock needles using 1mm using 0.5mm thick x 3.2mm inside dia brass washers. Interestingly for me; whilst experimenting with the pilot screws at many different turnouts, the plugs always remained white (to lean) after a test drive. Meaning since the test drive took the bike over the 3k range, adjusting them was not solving the problem of engine stutter at 3.0 - 3.5k. Raising the needles (Richening the mix) did as this forum already pointed out.

thanks a lot guys
 
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