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  1. #1
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    Default Shadow Carb Tune Help

    Hey everyone-

    Just got to the point where I need to tune my carbs. I have switched the stock air box with velocity stacks and am now running straight pipes. I bought a rejet kit from TJ Brutal Customs that came with 3 sizes of Idle jets, and 2 sizes of main jets. I currently have the smallest size of each in the carb, here is what is happening:

    Will not idle (shuts off) unless choke is pulled (slightly less than halfway choked gives it a solid idle). Revs like a champ when in neutral with no hesitation at all (choke still partially pulled).

    I took her for a ride to see how it went while in gear...Severe performance dropoff at mid-higher RPMS.

    My conclusion (this is my first time working on carbs and my experience and knowledge has been limited to my research online)
    -needs more fuel at idle, so I need to put in larger size idle jet.
    -needs more air at high rpms so the choke needs to be disengaged completely (main jet should be fine at current size)

    Anybody able to weigh in on my logic? Any help or info would be awesome! Thanks!

    Bike - 2002 VT750 (Spirit) / Dual Carb

    I should also mention I have adjusted the air/fuel mixture screws fro m .5-4 turns with no luck on this current jet size setup. Currently 1.5-2 is optimal with choke partially pulled
    Last edited by TheYounger; 03-21-2014 at 8:56 PM.

  2. #2
    Jetblack
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    Ok, well critically thinking... what happens when you pull the choke? Not what it does but what is the chokes function? It restricts air right? So this means the Pilot is too big for the current Main to feed fuel in, so that means bump the main... I'd try the largest one it shipped with to balance out having too much air. This post should help you dial it in on that end: http://www.chopcult.com/forum/showthread.php?t=29958

    Order for tweaking: Largest main without stumble to red line, Needle jet height, Float height, A/F screws, then pilot if need be... most people run straight for the mixture screws, gotta work backwards from redline to idle not idle to redline.
    Last edited by Jetblack; 03-22-2014 at 10:34 AM.

  3. #3
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    Hey Jetblack, nice to hear from you...you've helped me through 2 of the most difficult troubles with my build already! Is it correct to assume : since the bike will not even hold idle without the choke engaged (air restricted), that I need a larger idle jet? I disengaged the choke at higher rpm while riding (full air flow) and the bike seemed to run fine until I reached lower rpm, which the bike then died.

    Also, getting ton of decel pop right now.

  4. #4
    Jetblack
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    Idle jet is the pilot jet... choke is the same as people restricts air think of choking someone, a larger pilot means more even air requiring more choking... so logically to balance having too much air means too little fuel so if you have to little fuel in the mixture your main is too small. You could back the A/F out in the mean time I wouldn't go over four turns warm the bike up set the idle, then start at largest main in see how it pulls, then set needle height to kill mid range bugs, then float level to fine tune the mid range, then choose the pilot and set A/F and idle.

    Glad to help man before I get into the tools, I sit and think about the whole chain of events involved in whatever is going on, sort of a mental if, than, else. Using logic and what is supposed to occur in the chain of events between components usually troubleshoots it down to one or two things. More than one because some systems can create the same issue... like valves can act like a carb or electrical issue but the last fucking thing anyone would expect, so there's those few random systems that over lap symptoms, but most are a direct finger pointing right at the issue... Occam's razor put to use exactly like in Zen and the Art of Motorcycle Maintenance.

    So drop the largest main in see how it pulls if great done main is out of the way, now run steady mid range for about 3 miles no throttling just wrist locked listen to the pipes for fluttering inconsistency if so either the slide springs are worn causing flutter or the float level needs raising 1mm over stock if the sound is more inconsistent and rhythmic, from mid range whack it, do you get a bog? If so your mid range needle is rich meaning the fuel is too fat and needs better atomization...

    Click image for larger version. 

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    When you get back bike will be nice and warm then use the link I gave to determine lean or rich A/F screws and set the idle, then you're done.

  5. #5
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    Excellent, off to work then...I'll let you know how it goes! Thanks!

  6. #6
    Jetblack
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    word

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    All right, here's an update. Been test riding the bike as I felt I was close in tuning. However, I've determined my front cylinder is way off...

    Symptoms (idle):
    -Misfire at idle...I can hear the cylinder fire sporatically, no rhythm, and definitely inconsistent....
    1.75 turns (pilot jet) - firing begins, but lots of popping...
    2 turns - slightly more firing, bit less pops
    2.5 turns - more firing, less apparent/not as loud pops]
    3 turns - less firing, not much pop
    -Sometimes black smoke (not much) from front exhaust at ignition

    Symptoms (riding):
    -Occasional flutter at locked-wrist - cruise (not much and not very concerned about it, but might help diagnose)
    -Big Problem...in 3rd gear and up...Bike has hesitation when I "gun it". The hesitation gets worse the higher the gear (possibly also the lower RPM i start the roll-on...will confirm this soon). My last ride, I had the bike in 5th going about 40-45mph, and I maxed the throttle...I got absolutely NO acceleration, just pops and flutter for about 2 full seconds...I did not try it again
    -Also, the plugs read very lean on front cylinder

    Additional info:
    -Carbs rejetted to highest jets that came in kit, raised float level 1-1.5mm, raised needle 1 notch
    -Spark is good (new wires and caps read 4.7k & 5.2k)

    I'm at a loss and could use any help/thoughts. I've searched up and down. It's weird that the plugs read lean, because I basically adjusted the carbs to allow the most fuel possible. Meanwhile, the rear cylinder is running like a bat out of hell (which is great)

    Thanks for any thoughts!
    Last edited by TheYounger; 04-29-2014 at 6:49 PM.

  8. #8
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    on my VLX im in the middle of fine tuning mine with Velocity stacks also... My bike has cut stock exhaust and velocity stacks. I had 42 pilots and 142mj with A/F 2 turns to 3.5 turns.... started great revved like a bat out of hell but under load if I cruised like it was a perfect world the bike ran ok... Some black smoke (Rich smoke)... in the freeway if I wanted to pass someone and I WOT the bike it would sputter and just eventually pick up speed...

    I have the "Dont push the red button Syndrome" so I kept going WOT and shit would just make me curse like a sailor..... Decided it was time to play with jets...

    I have a wide range of jets from 132 to 220 from my racing days... lol

    have 142mj in there and black smoke just told me it was rich... but I was told that I needed more fuel at WOT and it was starving for fuel... so I slapped in some 145mj an immediately noticed the bike running like crap and black smoke galore.... turned it off after it sat for a couple minutes.. hey maybe boals where not full.... nope... FOULED the plugs...

    Changed the plugs and went down to a 132mj and immediately noticed the tone of the bike change and just sounded alive... Took it out for a ride and of course WOT and very little sputter and just shot out like a raped ape... checked the plugs and I can see they where black... so I had to order more jets for the collection.. I order 125's and 128's....

    I will let you know how it goes....
    I have a 93 VT600 Dual carb also... I run same jets on both carbs...

  9. #9
    Jetblack
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    Well, you say intermittent and clean plug, makes me think the front cylinder is either starving for fuel... check routings so fuel hoses off T are the correct length if so equipt. a good way to determine quick flow, is crack a float bowl drain with a spray can lid as a fuel catch and see how the fuel runs out, or it's not firing when it should... timing check/valve adjust or plug cap or plug wire or coil. If it runs fine then as it warms up starts this shit prolly coil, if it's shitty from the word go, pull the plug boot always by the boot not the wire, then untwist the plug boot trim off about 1/4 inch and twist that shit back on there clock wise when it feels firm stop or you're gonna jut go hulk and start snapping those lil fucking wires inside making contact worse. Note some plugs even though new are bad right out of the fucking box, swap cylinders with it and check the gap when you you do adjust as necessary... if the problem follows the plug well it's the plug.

    Yeah thats a jumble but start with what looks easier as check/fix and go from there to more complex.

    And when adjusting carbs make one adjustment at a time so you know exactly what the change is, hey it runs a bit better, was it the A/F turn or the float height or etc etc. hard to know which then you're adjusting around other adjustments making things even worse Popping on decel means you're a bit rich on the needle jet. What happens with that thing is the fatter the needle the fatter the fuel when being sucked into the cylinder, and well gasoline needs to be mixed with air to burn when it hits the thin part of the needle jet the mist is must finer and ignites much easier than fat globs of gas beading up on the intake walls, this is why polishing an intake port IS a good thing clumps of fuel beading up on rough shit instead of mixing with air and going into the cylinder is a bad thing.

    Sorry for the ramble, so take what yah need and toss what yah don't.

  10. #10
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    All right...I checked the timing/gaps/swapped plugs. Everything is in order.

    The bike is running great with just an occasional bog when accelerating through the lower rpms at WOT. My main concern now, is simply the plugs reading lean. I'm sure it's a carb/jet issue at this point, but like I said I used the biggest jets in the kit, but still running lean. Should I buy some bigger ones? Current size 148 Main on both carbs.

  11. #11

    Default

    Put some lollipops in the pipes to give you a bit of back pressure which will give you better torque in the lower end.

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