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Thread: Needle Shim Mod

  1. #1
    Junior Member AZTDUB's Avatar
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    Needle Shim Mod

    First Post, been reading everyone else's posts and learning as I go, with no prior experience. I've started a hollow mod and put a cone filter on when I took off the airbox. I thought it was running lean as it would sputter once I reached about 30mph. So I successfully completed the needle shim mod and used 2 washers. Now I can't accelerate at all before it sputters. Was I too rich all along?

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  3. #3
    Senior Member lbxr's Avatar
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    Your post is a little short on information...What year TW? Carb all stock before mods? Did you modify the idle circuit as well? And exactly how thick are the 2 washers you added? Eliminating the air box or swiss cheesing it radically alters the airflow to the carb and probably requires a complete retuning of the carb, possibly including a different taper on the needle....just shimming the needle may not solve a flat spot in the midrange, it may require a different taper(which are probably not available for these TWs)....

    I would delete my stock airbox and run a pod filter in a heartbeat if I was confident i could get the carb dialed in properly, as in perfect. I suspect most of these hollow mod TWs with pod filters just do not carburate crisply. Some folk may get the main jet right and get close shimming the needle...but I bet most of these have flat spots or hesitation problems somewhere...In short, a cone filter will probably require total carb retuning....likely a different Main Jet and Pilot jet along with fuel screw adjustments. Possibly a different taper needle and clip position changes or shims. Possibly a richer or leaner slide even...In short, A LOT of work and testing to get it right....Most folk are not capable of this....

    In my experience, shimming a stock carb, only requires a .015 to .030 shim to make a big improvement over the stock lean setting, especially in conjunction with richening up the idle circuit. Usually the main jet is pretty much spot on for a otherwise stock engine and stock exhaust system....

    YMMV.....

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  5. #4
    Senior Member DougDotyTW200's Avatar
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    It is almost a given that any airflow increase mods to a TW will put you in a lean condition since stock is pretty lean. I have a 2016 and a 2003 and both of my bikes are back to stock jetting now and have the pilot screws set at 2.5 to 2.75 turns out and both are absolutely spot on as of yesterdays riding. I bought both bikes, each with less than 500 miles and carb issues, ne was dirt and varnish and the other was previous owner tune up mods gone bad screwed up. After cleaning both thoroughly and returning to no shims and stock oem factory jet settings they run perfect with just the opening of the pilot screws. They now start very easy with half choke in cold or no choke in warmer weather. Both accelerate clean and neither has any decel popping on the downhill 300' of elevation change leading to my driveway turn in . I live at around 500 to 900 feet normal riding elevation. Regardless of all that i still plan to switch to #34 pilot jets yet since I feel my pilot screws are both adjusted beyond their realistic range at 2.5 or a tinge more and I would rather they were back in to a normal range of 1 turn out or so and the low speed fuel was coming from the pilot jet instead. I bit of up coming testing will tell the tale if I am right on this.

    In short I have a different set up and mostly stock, but from an immense amount of reading on TW tuning and also personally tuning many different things over a lifetime, I would find it hard to believe yours is not lean with that breather mod, however since those cone type breathers can vary wildly based on who made it and how they are made, does it have a pre-filter, how heavy is it oiled is a big deal on air flow too. I think it is easily believable that a heavily or improperly oiled cone filter with a prefilter on it could be more restrictive, but i am not real experienced with them !! Just make an incremental change in one direction and let the bike tell you what it needs. Not having alternate needle options makes it harder on these bikes.
    lbxr and admiral like this.
    2016, stock'ish and a 2003 Starting on a 225 or bigger 6 speed build as a spare for now and install later when done and ready on this bike.

  6. #5
    Senior Member lbxr's Avatar
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    Dougdotytw200, I could not agree more with your post. And I had not even considered that a cone filter could be more restrictive than the stock airbox, which they may very well be, especially considering that the airbox design/tuning is so critical to airflow/Volumetric efficiency....
    Last edited by lbxr; 03-20-2018 at 09:57 AM.

  7. #6
    Senior Member DougDotyTW200's Avatar
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    Ya, I hear so many people talk about loosing their tune up with air box mods so it is sensitive to changes. I have heard of good mx racers that are real specific on how they clean, oil and store their filters and foam pre filters in order to keep the tuning spot on as set and expected.
    lbxr likes this.
    2016, stock'ish and a 2003 Starting on a 225 or bigger 6 speed build as a spare for now and install later when done and ready on this bike.

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