Airflow-constricting flange inside my carb...

Discussion in 'Motorized Bicycle General Discussion' started by Cannonfish, Mar 1, 2010.

  1. Cannonfish

    Cannonfish New Member

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    Hi, I was busy doing some port-matching on my air intake tonight, when I looked inside my carb - and I noticed this flange for the first time (the red arrow is pointing to it in the picture below).

    After matching the intake manifold to my jug, the narrowest part of the air intake pathway is now definitely this flange inside the carb. The little airway in the middle of this round flange is so small that I wonder if it was even worth my time to port-match the intake manifold...

    Is this airway what people are talking about when they mention getting a bigger carb for these engines (I've read about folks upgrading to an "18mm carb", for example)?

    Has anyone tried to dremel this flange down to make the airway bigger on the stock carb? Or would that just destroy it?
     

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  2. 2door

    2door Moderator
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    That flange is where the intake manifold seats. It is the same inside diameter (I.D) as your intake manifold. Removing it will accomplish nothing but allow the carburetor to slide further onto the intake manifold. Some people use an 'o' ring and some a flat gasket to help seal the carb to the intake and either will seat against that flange. I prefer silicone gasket sealer. Hope this answers your question.
    Tom
     
  3. Cabinfever1977

    Cabinfever1977 New Member

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    If you removed that flange nothing would stop your intake pipe from hitting the carb slide.
     
  4. bairdco

    bairdco a guy who makes cool bikes

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    if you look at the front of the carb, there's the same thing, more or less, like a little "step up."

    i grind that smooth with the bore and polish it. seems to work great for me...
     
  5. Nashville Kat

    Nashville Kat Active Member

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    The new NTS 66 carb boasts a millimeter (I think) wider flange, all the way around the ring- I'm wondering if anyone is shipping it stock with the kit yet- just a matter of time I guess.

    but you are absolutely right- I think too that this is the real limiter to mixture flow- it wouldn't make sense to dremel the ports on the motor and leave the carb the same-

    On the other hand, the billet intake from Pirates and Creative Engineering puts the carb right up next to the motor, and really improves performance for less than $15.

    I reamed the billet intake out further, the alloy goes quickly.

    the danger with over-reaming a stock carb is to create an air leak I guess.

    I like the bilet intake and new NTS carb- (although I still have the stock on for now)- It's the cheaper fix than the CNS racing carb- the jet is bigger too they say-and there's less clearance issues in the frame- it's shorter-

    but still a bit tricky because you have to lower the cable guide by cutting a bit off-
     

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    #5 Nashville Kat, Mar 2, 2010
    Last edited: Mar 2, 2010
  6. Cannonfish

    Cannonfish New Member

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    Thanks everyone for the input. When I can find enough spare time to do it without interruption, I going to try dremel-tooling this flange down. I don't want to get rid of it entirely, I just don't want it blocking the airway any more than the inner diameter of my intake manifold.

    I'll update this post if that ends up destroying my carb :)
     
  7. Topkick40

    Topkick40 New Member

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    DO NOT REMOVE THAT FLANGE !!!!!!!!! That flange is what is called the venturi of the carburetor , as air flows through the venturi it speeds up causing an area of low pressure, which at high engine speeds causes fuel to be sucked through the main jet. Engine vacuum only sucks fuel from the carb at low speeds because at high speeds there is very little intake vacuum. Your engine will run seriously lean at high speeds if u remove this flange. Hope I am not to late responding here , hate to see ya ruin a perfectly good carburetor , or even worse an engine from running too lean. The best thing to do would be to get a larger carburetor if seeking more power , the carburetor throat size and venturi size are matched to make the carb work .
     
  8. Cannonfish

    Cannonfish New Member

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    Hey it's been several months since I was on this website last, so I never saw the last post on this thread, from Topkick40. As it turns out, he was totally correct - I destroyed the venturi effect within the carburetor, and the engine ran horribly afterwards. Had to buy a new carburetor :(

    But the story gets worse... after I got the new carb set up and runnning, then I decided to port my engine cylinder (dangit - somebody needs to take away my dremel-tool). As you could probably guess, that experience didn't go very well either.

    Now I'm engine-less, and it'll be a few more months before I can do anything about it. Next time I will limit my dremel-tooling to simple port-matching, and not attempt to hollow out the entire contents of the engine case!!

    Perhaps this sorrowful tale will keep some of you guys from the same mistakes :)
     
  9. 2door

    2door Moderator
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    Cannon,
    We're sorry you had to learn the hard way but at least you did learn something. Modifications to these little 2 strokes are something that needs to be done by someone who fully realizes the effects of what they are doing. I'm not putting you down, sir, not at all but when changes are made to a design that has been proven there needs to be a full understanding of the principles involved. This is why we warn against engine modifications of any kind unless you're an experienced engine builder or you have unlimited funds and can afford to replace an engine if things don't work out as expected.
    Thank you for posting this here for others to see and read and maybe it will save someone else from making an expensive mistake.
    One thing to remember when attempting alterations is to do one thing at a time and then testing to see what the results are. Doing several mods at one time confuses the issue and if something doesn't work you won't know what to avoid next time. Just my thoughts.
    Tom
     

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