You need to richen up your idle circuit mixture some... whether you opt for a full jetkit, a tuning kit from holeshot (enrichens some of the system), or simply richen up the existing idle mixture screws is strictly a matter of personal choice given your set-up. Given your set-up, I'd simply recommend increasing the existing settings.

Here's what's happening: on hard deceleration (release of throttle), your carbs are feeding effectively an idle or close-to-idle mixture through into the cylinders. That mixture under this circumstance is too lean to ignite properly, and the mixture doesn't combust (or doesn't combust completely) in the cylinder. Then it exits into the exhaust header and ignites there instead, causing the tell tale after-fire.

There are a couple paths you can go down. The first is richening up the settings yourself (get a flexible shafted screwdriver to make this easier), or have a shop that has a exhaust gas analyzer do it (they'll get it set perfectly the first time out -- and since you're not rejetting, there's no dyno needed). The bike ships with lean carborization from the factory for EPA reasons, and when combined with slightly lower back-pressure from the D&D end-can, it pushes the mixtures to a too lean state.

Additional Readings:
http://www.factorypro.com/tech/carbt...m_engines.html (see section 4, idle circuit)
http://www.ricter.com/jetkit.html (jet kit install, 98 - 04 Katanas)
http://771doug.netfirms.com/ (doug771's pages for pre-98 Kats)

Cheers
=-= The CyberPoet

right on cyber - it's lean.

we don't know if the carbs have been cleaned b4 - the air/fuel adjustment screws MAY still have the caps on. if so, the carbs need to come out and drill out the caps.

i went by the local bike shop & they didn't charge me anything to give me the reading. mine was WAY lean also. ( i didn't have them adjust it because the plastic needs to be off )

so - i pulled the carbs again ( only about the 10th time )

tim

Are we talking about the pilot adjust screw on the bottom of the carbs?

I cleaned them before summer. I have access to the screws on the bottom of the carbs.

Which way do I want to go to richen the mixture clockwise or counter clockwise?

look at this little writeup i did ( this is a 98+ carb )


the second to last picture shows the pilot screw.
richen it up turns counter-clockwise.

i really went around on my carbs to get it running ( pulling the hardest
throughout the rpm range )

i'm running a K&N jetkit ( dynojet ). the recommended settings are
clip 3rd position & 3 turns out on the pilot screw.
this ran rich & pulled really hard but had a stumble around 2- 3k rpms
and popped on closing the throttle. so this says lean on idle but rich just
off throttle.
i varied the clip position and even installed a larger pilot jet (12.5 stock to a #15 factory ) also adjusting the air/fuel screw (pilot screw).

the best i've gotten right now is 3 -3/4 turns out w/ needle on 2nd clip
position ( from the top ) with the stock 12.5 pilot jet.
this is with a K&N w/ large donut and a slip-on.

i do beleive that the jet kit is NOT needed ( unless you get it cheap - which i did on the K&N closeout sell ).
just buy factory main jets and go to radio shack and buy the washer kit (part # 64-3022A ) and shim the stock needle.


tim

Is there any possiblity that a weak spark could cause the same effect? I have my idle screws set to the point where they don't run too rich or too lean at idle, but I get a slight popping on decell. For a while I thought the coils might be running weak, but if I richen it any more it acts real funny around the 2-4K range and smokes, if I lean it anymore, well, it acts lean.

Doubtful, but possible. Coils are real easy to check anyway to find out... If you're plugs aren't fouling within 3.5k miles, I wouldn't worry about it...

My best guess:
Most likely is that you have most of the cylinders set right, but one is still a bit lean; when you adjust all of them richer, it sets the lean one right and set the others to be too rich, causing the problems you described. It's a hard issue to correct for (mismatched fueling), but there are some solutions, like ColorTune (a specific see-through plug solution to adjust the mix by the color of the fuel explosion), or taking measurements at each exhaust header pipe with an exhaust analyzer. A fuel-system cleaner might also help, in cases where the idle circuit fueling difference is due to a slight blockage from built-up residues (I recommend Chevron Techron as my preferencial choice for this task).

Good Luck!
=-= The CyberPoet

I was mwondering why all the kids around here are always popping like crazy when they put on slipons. Thanks for the info

CyberPoet, you are most likely correct, do you think the crayon on the header test would work the same? IE mark the headers with a crayon, start bike, the one to melt first or last has an issue. I at one point had the mix screw on one carb so far off it sounded like a V-twin.

Turns out I had some a pretty serious exhaust leak where the header met the mid pipe. Once I took care of that the deceleration pops stopped.

I've never heard of the crayon test before, but I would imagine that would work (aside from the smell). Most around here either use water (like in a spray windex bottle) or a laser themal probe ($60, harborfreight).

Cheers
=-= The CyberPoet

That makes great sense -- the loss of back-pressure at the leak was causing changes in the fueling... And because the leak was probably more to one side (left/right/top/bottom), the leak probably affected one cylinder more than the others...

Cheers
=-= The CyberPoet

LOL, you don't use the whole crayon

Vance & Hines SuperSport system?

D&D GSXR 750 Can on a stock 98+ mid pipe