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Thread: Mikuni TM28-1 Intake Boot Options

  1. #21
    Senior Member renny10n's Avatar
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    TM33-1 pumper

    These are viewed shots/ obsticals of the Mikuni TM33-1pumper/ 33-8012 in relationship to the TW200's intake manifold & frame/ gas tank back bone. It appears the tab on the bottom of the back bone, needs to be clearanced (grinded down) to clear the taller pumper top. The rubber intake flange is the stock oem 200 item, but it gives you an idea of how much clearance is needed above (not much really) After that, .....?????? If I recall offhand, Tumbleweed had first flipped the rubber intake flange to gain the clearance? Just curious, other than the bigger slide, etc. , does the TM 28 & 33 pumpers share the same outside dimensions, i.e., top cover to intake. Since this topic of the mikuni pumper keeps rearing it's head from time to time, perhaps those in the know can chime in and we can do a sticky on an install???
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  2. #22
    Senior Member Cornelis's Avatar
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    Yes renny10n, it looks like you are correct, that is a pumper carb for a two-stroke engine, here is a photo of TM28-1 pumper for a four-stroke Click Here - Monkeybiker Sweden - 28mm Mikuni TM28-1 pumper carb

    Last edited by Cornelis; 01-22-2017 at 11:46 PM.
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    1990 Yamaha TW200Hidden Content _____________ Come to Me, all you who are weary and burdened, and I will give you rest. - Matthew 11:28

  3. #23
    Junior Member squirrel's Avatar
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    So time for a little update! I managed to get the TM28-1 on the tdub without any permanent modification. Using the straight intake flange the carb clears the frame, although the airbox boot took a lot of wriggling and rubber grease to get on, it kept slipping off but once secured with the strap it stays on tight. The bike starts and runs, but unfortunately there seems to be a vacuum leak at the intake manifold on the head. WD40 around the area makes the bike rev up so next i will have to figure out a way to seal it. Liquid gasket perhaps? Its not an original Mikuni flange but a copy manufactured here in UK by MB, the mating surface is just rubber so there isnt a slot for an o-ring like on the original TW manifold... My fone is playing up so i couldnt get any photos before it got dark but ill take a few during the next round of tinkering. Think i will also try to source an XT225 airbox to carb boot to see if its a more comfortable fit as any adjustments that require the carb tilted are near enough impossible without the boot slipping off the carb.

    Renny10n and Cornelis thanks for the photos! You can compare the dimension of the Mikuni carbs on Sebastian's website,
    TW-Parts.com - Tuning-, Zubehör- und Ersatzteile für Yamaha TW125, 200 und 225
    As far as i can tell not many of them are exactly identical and at least one dimension seems to always be off.
    Last edited by squirrel; 01-23-2017 at 12:14 PM.

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  5. #24
    Junior Member squirrel's Avatar
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    Some photos of the install, cant wait to get the leak sorted n take her for a spin. Think thursday will be the day...
    IMG_9465.jpeg
    IMG_9466.jpeg
    IMG_9467.jpeg
    IMG_9469.jpeg
    IMG_9471.jpeg
    Tweaker, Ken, Cornelis and 2 others like this.

  6. #25
    Ken
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    Senior Member Ken's Avatar
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    My carb to airbox tube never mated correctly. I used 2" rubber tube from Sears hardware. I cut the original tube some to fit the angle better. I have a couple of pics under my album 1993 TW. Not enough memory here to post it. It was actually a little too thick in my opinion. I would measure yours since it is not stock. I recommend a radiator hose of the correct diameter. You could get the correct angle perhaps.
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  7. #26
    Junior Member squirrel's Avatar
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    Yup im having difficulties with the angle of the airbox boot and the tight fit isnt helping... Thought i sorted the intake manifold leak with some permatex, put everything back together but the dam thing just wouldnt go. Really hard to start, then once it started up it would rev up and die few seconds later. I could keep it going by pumping the throttle but would still stall out eventually. The battery died before i got a chance to fiddle with the idle screw, as now its even more of a PITA to get to, and im hoping that its just needs adjusting, although a vacuum leak somewhere wouldnt surprise me either as the awkward angle of the airbox might be twisting the intake manifold. But im thinking since it started way easier with the obvious vacuum leak the idle mix screw must be set way too far out. Well, its set 2.5 turns out but i mean it must be too far out for it to idle... Battery on charge, will try again after work tomorrow but have the old carb reassembled and sitting on the bench incase i cant get the TM28-1 going.

    On a side note, in anticipation of jetting gremlins i've ordered an open source wideband o2 sensor kit from 14point7. Being a resourceful squirrel i went for the SLC Free kit, $45 without the sensor as i didnt want to pay extra uk import duty and can get the bosch sensor over here. Comes with the PCB, components and an LCD display which gives you a read out of the lambda value and the sensor temperature. Really dont enjoy having to basically guess the jetting so pretty excited to get my hands on this little gadget.
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  8. #27
    Junior Member squirrel's Avatar
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    Nae luck. It didnt run yesterday cause the permatex didnt work, couldnt see the gap during the day but spotted it tonight when i was using a torch. Turns out the more i tighten the bolts the more the manifold gets twisted, penny washers didnt help, 5p with a hole drilled through it for larger surface area didnt work... On closer inspection i noticed that the metal plate inside the mount doesnt extend all the way to the mounting holes but stops just short so the bolts compress the rubber and distort it. Pretty gutted, back to square one.
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  9. #28
    Junior Member squirrel's Avatar
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    Quote Originally Posted by elime View Post
    I have a TM33-8012 carburetor and was at one time considering trying it out. I had actually gone "carburetor crazy" and bought several different ones thinking the new one was better than the last one and I would use that one. I haven't got around to trying any of them yet. Figures. Anyway, a couple of intake manifolds I found that fit the TM33 carb but are a little large for the head. They head port will need to be opened up a little bit.

    Pictures cuz I like them:
    Attachment 69386

    Attachment 69394

    Attachment 69410

    Attachment 69426

    Attachment 69442
    Elime any chance you could tell me if that original VM34 mount has the inner metal plate extending to the mounts or are they just rubber?
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  10. #29
    Junior Member squirrel's Avatar
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    I have a little update for anyone keeping an eye on the mod. I finally found an 1998 XT225 airbox to carb boot and it does indeed fit the 50mm throat on the TM28-1, im hoping it will just fit right in the hole in the airbox in place of the original boot but with the project's track record of straightforwardness i have my doubts... ill make it work though!

    Also have the AFR meter up and running... Jetting gremlins be gone!! Interesting results with the standard carb. Here is a short video of it warming up https://www.facebook.com/sqrly/video...2618175068924/ as you can see when i pulled the choke out a little the lambda value drops. I couldnt record a video of it while riding but ill try to figure something out. Basically im running fine at idle and on pilot jet, too lean on the needle, and too rich at WOT so on the main. Will try to get the standard carb dialed in tomorrow n will have another go at installing the pumper carb next week. Will keep yous posted!
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  11. #30
    Junior Member squirrel's Avatar
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    SHES ALIVE!!! Thanks for all the info folks!

    After one final push over the weekend i finally managed to get the pumper carb on and jetted. What a mission that was... The set up isnt perfect as the XT225 airbox boot isnt the right shape, so it gets pinched a little which in theory decreases the airflow somewhat but i have only noticed a major improvement in performance. Way more pull lower down, the pump really does a great job of waking the engine up from low rpm and i can almost do a power wheelie in 1st. Im not great with wheelies so havent tried very hard, but pretty sure its doable... Keep in mind that my Tdub is a 125cc. My top speed has also increased and it now pulls hard all the way to 8500, where as before it would only go over 7000 downhill or whilst slip streaming a vehicle ahead.

    Jetting wasnt too much of a bother with an AFR gauge, but i can see how it would get tiring very quickly if i was just going on guess work and had to keep taking the thing apart to try different settings. Stock carb is waaaaaay easier to work on i must say, but if there is a will there is a way! All in all i would only recommend this mod if you're a glutton for punishment, love a challenge and problem soliving, and dont give up easily... Im still to take the bike out on a proper long test run, but feeling very optimistic and pretty sure that this was all worth the hassle.

    Here is a video from my last ride with the stock carb, and ill get one of the new carb in action as soon as i can. Hopefully the difference will be quite noticeable!

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