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VTX 1300 Carburetor Adjustment

312404 Views 547 Replies 188 Participants Last post by  Timbanditos
You may have just purchased a new exhaust and/or air intake and are wondering how you should "tune" the carburetor. First off if you just "fire the bike up" with the new parts installed, THE BIKE WILL RUN. Depending on how "free flowing" the aftermarket parts are that you installed the bike will run anywhere from "pretty good" to "pretty lean and very nervous", but it will run. So now the bike has to be TUNED. The easiest, least expensive, quickest, most predictable and fool proof method of rapidly tuning it would be to ADJUST the stock parts that are already in the carb. Some people venturing into the "mods" game want to do them all at the same time... this has OFTEN been proven to cause problems for the neuvo MC "wrench". DO NOT remove the PAIR system during the same session that you are tuning the carb. :nono: , it is common while working with the PAIR removal to "miss" something in the process and have an open vacuum line or "other" air leak. Many who have done both of these mods simultaneously have ended up fiddling with their carb for days/weeks on end chasing their tails looking for what went wrong with the carb tuning while all the time it was an error in the PAIR system removal. There have been some war stories spread about how difficult it is to tune a carburetor.. bull hocky! The most difficult part of tuning a carb on a VTX 1300 is removing the fuel tank for the first time and figuring out how to disconnect the electrical connectors under the tank.

So, what comes first eh? REMOVE THE FUEL TANK. Follow the directions in the service manual.. everyone should have a service manual.. the money you save in THIS carb tuning process alone will pay for a service manual. The money you save doing your first valve adjust using the service manual would buy a nice set of tools that can be used in OTHER money savings projects... EVERYONE needs a service manual..

Next REMOVE the air filter and backing plate. I won't "get into" hooking and unhooking the hoses attached behind the backing plate, they are pretty straight forward and will become part of your "learning process" of the easiest ways to rapidly navigate through assembly/disassembly.

NOW the carburetor is EXPOSED. Sitting on top of the carb you will see the vacuum chamber cover. Remove the cover and then remove the compression spring under the cover. Now gently grab the diaphragm/piston rubber and pull it out.. the Vacuum piston (slider piston is what most old timers call it) is attached to the diaphram and the whole package will come out with it. YOU ARE ALMOST DONE AND READY TO PUT THINGS BACK TOGETHER.. see, I told you.. removing the tank was the most difficult part of the process... unscrewing the 4 vacuum chamber cover screws was no big deal, right?
Look at the slider piston and you will see a tappered "needle" sticking out the bottom of it, that is the jet needle. Look inside the slider piston and you will see the Jet needle holder. The jet needle holder is next to be removed.. you will see in the center of the holder it will accept a Phillips head screwdriver which can be used to "unscrew" the cam locks, BUT the "best" method is to use a small socket and fit over the head of the "cap" and unscrew the cap... I'm thinking it's an 8mm.. difficult to recall those things sometimes at my age, but you'll discover the correct size. Twist the cap counter clockwise just as tho you were unscrewing any "standard" nut or bolt and you will feel a "pop" after making a very small motion.. that was the cams unlocking.. the cap can now be removed, it is "unlocked".

Under the needle holder cap there is a small spring "screwed" onto a plastic "tit" protruding from the bottom of the cap... well actually the "tit" has no threads, so the spring is not "screwed" on, but consider it so... instead of "pulling" the spring off and distorting it's shape, give a "light force" pull on the spring while unscrewing it from the "tit" and set the spring aside. The "tit" does not protrude very far, but it must be "trimmed" for "heavy breather" mods, so go ahead and trim it now even if you are doing a "light breather" tune... Take some TOENAIL clippers (those are the large ones, not to be confused with fingernail clippers) and "snip" the "tit" in half.. that should be enough for heightened needle raising and still provides enough remaining "tit" for the small spring to be screwed back onto... set the needle holder cap aside.

Remove the needle from the slider piston.. place your shims into the needle "point" and make sure they slide all the way to the needle head with the washer/shim hole big enough so free movement along the needle is achieved. Hold the slider piston in your left hand and turn it at an upwards angle and insert the needle INTO the slider piston hole, passing through the CENTER hole in the slider piston (this process will not work for "left handers", you will have to purchase a Yamaha carburetor). Face the slider piston DOWN and the needle should be hanging from the bottom of the slider piston and sitting "on" the shims/washers INSIDE the piston.

Take the SMALL spring that was unscrewed from the underside of the needle holder cap and screw it back onto the shortened "tit" under the needle holder. Put the needle holder cap into the slider piston with the small needle spring tensioner sitting on top of the needle head. (If the 8mm socket is placed onto the top of the needle holder cap "nut" the tension is enough to hold the cap.. the needle cap holder can be easily inserted into the slider piston with this method utilizing a socket extender). Twist the needle holder cap clockwise (even the left handers) as tho "tightening" and you will feel the cams lock into place... that's it, the needle holder is locked. If enough of the "tit" was trimmed, this should be a relatively easy "twist"... Hold the slider piston with one hand and "wiggle" the needle with the fingers on the other hand... the needle should "wiggle" (movement), this assures true alignment into the emulsion tube :) . If the needle is "stiff" and won't "wiggle", then the "tit" was not trimmed enough :nono: ... go back and trim more from the tit until free needle movement is achieved. (If you are using a DJ needle, the tit must also be trimmed for the same reasons)

Replace the diaphragm spring onto the needle holder cap and replace the vacuum chamber cover.. NOTE: There is an irregular "shape" in the cap that must be matched to it's mating surface when remounting, this is what the manual calls the "tab and air passages"... "Snug" the vacuum cap screws and continue reassembly.

Under the carb is the A/F screw (see manual), the stock screw head requires a "D" type tool for adjustment. If you slightly dent the end of an empty .22 shell casing, this will fit into the hole and can be used to unscrew the "D" screw. Take the "D" screw completely OUT. On the top of the "D" screw head will be a spring followed by a washer and then an "O" ring.. save these parts and reassemble in the same order when putting the A/F screw back in.

While the A/F screw is OUT, use a hacksaw blade or some similar device to cut a thin channel into the head of the top of the "D" screw.. this slot or channel should be wide enough for a flathead screwdriver blade to fit into for A/F adjustment. Once the slot has be cut into the screw/needle head and the screwdriver blade fits, replace the spring/washer and "O" ring and replace the A/F needle into the needle jet hole and screw IN. Take your screw driver and screw in the needle until it LIGHTLY seats or stops turning ;) ... don't get anal about this and over tighten or damage could be caused to the needle/jet mating surfaces :( . If you error on "seating", error on the LOOSE side, final tuning will not be affected by a minute discrepancy of "seating" at this stage of tuning. Turn the A/F screw OUT 2 1/2 turns with the screwdriver.

Reassemble the airbox and fuel tank.

Before firing the engine, look at the CHOKE KNOB and push it in all the way... any flexing of the carb while working on it will flex the choke cable and pull the choke out... it will not run properly with the choke pulled out AT ALL and final tuning cannot be achieved.

Shim heights:
"Open" style airbox and aftermarket "free flow" pipes... usually about 2or 3 shims.
Airbox flowing more air than stock design, but not in the "heavy breathing" class and/or pipes either free flow or somewhat restricted... usually about 1or 2 shims
Stock airbox (K&N filter in stock box is still "stock") and after market pipes... usually 0 or 1 shims
Stock airbox and stock pipes... tune the A/F screw for maximum performance...

A/F settings:
While determining proper main jet circuit settings the A/F screw should be set at 2 1/2 turns and NO adjustments made to it until the high speed circuit has been properly tuned -- Turning the screw IN leans the mixture... turning the screw OUT richens the mixture -- Fine tuning the A/F screw should be done in no more than 1/8 turn increments... very fine tuning will be even less... it is highly recommended to "slot" the "D" screw and tune the A/F screw with a screw driver... a 90 degree drive is useful if you have one... improvise.

That's what I call the big spring directly under the vacuum cap and sitting on the needle holder cap. -- Most tuners that have springs of the type found in the VTX carb shorten the spring for faster throttle response... this is not necessary for the bike to operate, but you will shorten the response time if you shorten it... I would not recommend shortening it past 5 1/16"... that is the lenght of the shorter DJ kit supplied spring and is a proven entity... The Keihin spring is of slightly larger diameter wire so a "safe" fudge factor is built in if you cut it to 5 1/16", most do... you can cut it less than that if you are nervous, but you shouldn't have any problems with 5 1/16"... place the cut end DOWN into the slider piston and resting on the needle cap... uncut end UP against the vacuum cover.. that is the "common" practice (and no bannanas on boats either).

Most any needle design or jet design will work with the Keihin carb as long as they are set to the proper height and have the correct diameter... It has been my findings that the stock Keihin needle and jet work more "smoothly" if properly shimmed than some other designs because of the needle design and large diameter jet size... The Keihin design is just more forgiving and not as "touchy" as some other designs and need I say you are not required to remove the float bowl cover to change main jet sizes?... The Keihin 195 main jet suits most all purposes... if you require more than that you are past the "beginning" tuner stages and in that case you might think about a Keihin 200 or 205 (DJ equivalents of 213 and 218)... I don't think more than a 205(/218) would be required by anyone unless cam, piston, valve and porting changes have been made... in which case I would like to communicate with you on your engine mods and findings... you most likely won't require any "advice", but I would like to hear about extensive engine mods... mine will retain the stock configuration... I have a 1200cc street fighter for speed.

Have fun and if I am "not around" to answer any possible questions (I have a somewhat irregular schedule at times), I would recommend asking Retro Rich.. He has a mechanical mind and a healthy grasp on the tuning philosophy.

Radio Shack Shims:
Radio Shack Part # 64-3022
Package of 100
Steel Flat Washers
20 each: #2, #4, #6, #8, #10

They are in the Hardware section of the store -- In the plastic pull out trays that are divided into sections.. Plastic bag... Looking at the bag I would say we are using the #4 washers.. anyway it is the second from the smallest... we "miched" em several years ago and can't recall, they are like 0.019 or 0.020 or pretty close to that neighborhood.. they work! -- Oh yea, they now cost $1.99/pkg -- inflation.

Any "washer/shim" of similar thickness (0.020") with a hole big enough for the needle to comfortably slide through and small enough for the needle head to sit on will work just fine.

I could write another entire page on "tuning techniques", but it has already been done in what I consider a comprehensive and accurate manner... use it as your tuning Bible -- Factory Pro

Don -- AKA "Scars" :cheers:
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What about the needle? What notch should the clip be on?
What about the needle? What notch should the clip be on?
I would start with 4th notch from the blunt end and dial in from there
Hey guys so far this has been helpful a lot of useful information. I just purchased an 03 1300s that has shit exhaust and some custom intake doing scar mod as speak. Just curious if I should use 3 washers as their are no baffles in exhaust and open air cleaner


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It’s has stock head pipes with regular exhaust tubing to extend past the passenger
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Thanks, going to go read it in entirety. I am actually thinking of going to a stage 3 (220 jet) as the pipes are wide open and a buddy said his runs very well. Any thoughts on that?
220 might be too big. Unless you go for aftermarket carb your best bet is 200 or 180. Not to mention fuel... If you're not taking long road trips it'll be fine.
bak breather, corbra slips ons , had dj 195 and fp1.. changed to fp3 = 220, 58, 1 7/8-2 turns, 4 clip with spacer, 2nd gear to limiter [email protected]@king fast , ran 4th clip at 50 mph 4 gear some bog added spacer less bog for now still testing , will continue to ride some more ..
I have a velocity stack (tj brutal) and I set it up 220, main 60 pilot, 3rd clip and 2.25 A/F screw. Yet to test this set up. Extended A/F screw, not Glenn's extention. Also Pair removed and coolant lines removed.
Just ordered the PAIR valve plates from

Should have the bike back from the shop Friday or Saturday, hopefully get this done Sunday.
I made my own plates with a bar of aluminum from Lowes/Hdepot, trace the old covers and cut out with jig saw. Save $10 or more, plus plenty of aluminum left for petcock vacuum adjustment.
I’m a little late to the game, but I wanted to say thanks for all info. I decapped the stock airbox added two shims and turned the A/F screw out one full turn and it’s running a lot better! Once my extended A/F screw from Glens Garage arrives I will be able to fine tune it. Already feeling a little more throttle response.
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