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TB108, race head, 20mm carb. Initial jetting

This is a discussion on TB108, race head, 20mm carb. Initial jetting within the Z50, CT70 and JDM Monkeybikes forums, part of the General Talk category; I am planning on running without the spacer at all on my 108 with V2 head, just a plain old gasket. No fitment issues at ...

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Thread: TB108, race head, 20mm carb. Initial jetting

  1. #16
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    Tweakin's Avatar

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    I am planning on running without the spacer at all on my 108 with V2 head, just a plain old gasket. No fitment issues at all for me and have been told this is the way to go by people I trust.

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    Why is it the way to go?

    I could fit mine without the spacer as well on TB's standard latest race head. The bowl drain is just really close to the cylinder but doesn't hit.
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    Honestly I do not remember the specifics to why. Something about the spacer doesn't allow the fuel to get hot enough, but I can't remember. I have the spacer on my 88 with race head and no issues, but the engine builder I trust said to leave it off. He is running it that way on 108 and more recently on a 117 kit. My plan is to run it with no spacer at first and if I need to I will add it later.

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    Well, I ended up flipping my spacer, trimmed the little bit off needed and fitting it so that the spacer now "blocks" the heat under the intake. Pretty sure that is how it was intended to be used (not saying it does anything) but maybe TB's intake is a touch shorter then whatever that spacer heat shield came off of.

    FINALLY got the bike together and took it for a few miles. Runs great and the 108 kit really rips. Big improvement over the 88. The 20mm carb seems to be a perfect match for it. I have zero bog off the throttle and it's very responsive. Ended up going with my gut and using the smaller recommended jet sizes of 92/38. It feels good. Plug is actually rich but with the way I ride this, I don't mind a little rich. Need to get a new plug and then I'm going to fine tune.
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    When I get my 108 with VM26 in my bike, I'll use this thread to update what I end up going with for tuning the carb at 6,000ft elevation.

    Anyone ever use a flex jet? A lot of mountain trails go up in elevation real quick out here..

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    Here's my two cents. I don't like using these spacers for the reason that someone above mentioned...it doesn't allow the fuel to get warm enough. Warmer fuel atomizes better....that's a fact.

    I'll give you an example from a street bike setup. Coming down a long, large hill in the smokey mountains on built up CT70...two of us were riding CT70's...very similar setups...one particular difference was one of us had a spacer on the intake and one of us didn't. When we got to the bottom of the hill (this is like a 5-6 mile hill) we pulled over at a particular point to take a picture by a creek. I noticed that my intake had a lot of moisture on it. I felt it...it was ice cold...and actually had some ice on it. I can only think that the evaporative nature of the fuel through my intake manifold made it colder and colder and the moisture in the air condensed on the manifold causing ice. I'll admit i didn't really notice any issues with the way the bike was running, but it was surprising that an engine that had been running for more than an hour straight had an intake manifold that had ice on it

    I guess you can probably surmise which intake had the spacer on it.

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    Interesting. Easy enough to remove but it does put the car bowl really close to the cylinder.

    So, from that I assume it is the fact that the spacer is plastic and isolating the heat from the manifold? If the spacer was metal, would this all ring true?

    What about intake lengths? Is there anything in that? Longer or shorter making more power?
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