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Thread: Design and build Side-shaft hit and miss from barstock

  1. #251
    Join Date
    Mar 2008
    Location
    Barrie, Ontario, Canada
    Posts
    9,065

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    I've spent the last three hours going over everything I knew or thought I knew about small i.c. engines and carburetors. I knew I was missing something, but couldn't remember what it was. Okay kiddies, here it is. The speed of air flowing thru the carburetor is directly proportional to how many times the piston goes from top dead center to bottom dead center in a given period of time. Small high rpm engines can get away with large bore carburetors because at the rpm range they run, the air must flow very fast through the carburetor throat. The venturi effect that creates the vacuum to pull gas up from the tank is created in direct relationship to how fast the air is flowing through the carburetor. On small SLOW rpm engines as I have designed here, the air flowing through a large bore carburetor doesn't have to go nearly as fast, so the venturi effect is much less, and consequently it won't lift the gas from the tank as quickly and efficiently as we would like. To compensate for that, low rpm engines use a smaller bore carburetor, which makes the air flow faster. This in turn makes the venturi effect greater, and allows the engine to lift gas up from the tank as efficiently as we want it to. There is no doubt in my mind that the cracked fitting in my intake manifold was allowing air to come in thru the crack, so as a consequence the fuel wasn't being lifted up from the tank at all unless I manually choked the carb with my finger. The part I couldn't understand was why, after I had replaced the cracked fitting, didn't the engine run and keep running. The Traxxas carburetor has a 6 mm bore--that is 0.236". My engine would only run with the throttle almost completely closed, and even then would die out within 15 to 20 seconds after the drill was disengaged. My plan for tomorrow is to make a new carburetor with a smaller bore, similar to the carb that was used on the Kerzel and Upshur hit and miss engines, with a 3/16" bore.
    Brian Rupnow

  2. #252
    Join Date
    Jun 2007
    Location
    Anderson SC
    Posts
    1,231

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    Can't you just sleeve the homemade carb you had on the engine at first?

  3. #253
    Join Date
    Mar 2013
    Location
    West coast of Canada
    Posts
    619

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    Hi Brian, I have been sitting on the sidelines watching this build with interest. On the subject of the carburetor bore, I just happen to be starting to build an Atkinson differential engine. It has a bore of something a little less than 1 1/4". Because I don't have the necessary bar stock yet to build the larger parts I decided to make the tiny carburetor because I have the material available. This one uses a very simple carb with a 1/4" bore having an inlet air orifice of .078". This is quit a different animal than the one you are building because it has two pistons in the same bore, so double the possible compression leakage. Just thought I would throw this out there more for interest than anything. Apparently after you get this one broken in it will start with a flip of the flywheel. Time will tell.
    Larry

  4. #254
    Join Date
    Jan 2004
    Location
    Missouri
    Posts
    30,078

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    That's very reasonable.

    Maybe you should start with a smaller orifice/bore, and open it up to make it better. Much easier to take away material than to add it back in.........
    1601

    Keep eye on ball.
    Hashim Khan

  5. #255
    Join Date
    Dec 2016
    Posts
    241

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    For a while I was modifying RC carbs for 4 stroke model airplane engines. Control Line models do not need or use a throttle and this made the engines run the way we needed. I did a write up on our Clubs web site that might give you some ideas on how to make your carb venture adjustable.

    http://www.tulsacl.com/SaitoCarb.html

  6. #256
    Join Date
    Mar 2008
    Location
    Barrie, Ontario, Canada
    Posts
    9,065

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    All things cometh to he who waiteth---If he worketh like Hell while he waiteth!! I put the smaller bore non throttled carburetor on this morning, fiddled with the ignition timing a little bit, and away we go. I think my heart may explode!!! I may fill the reservoir with water, start the engine, and do nothing else today except set and watch the engine run.
    https://www.youtube.com/watch?v=O3A0...ature=youtu.be
    Brian Rupnow

  7. #257
    Join Date
    Oct 2011
    Location
    Niagara-on-the-lake Ontario Canada
    Posts
    82

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    Congratulations Brian and well done. Looks great.

    Harold

  8. #258
    Join Date
    Mar 2013
    Location
    West coast of Canada
    Posts
    619

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    Great job Brian. Another sweet runner. When you get time, could you tell us about the final carburetor and it's bore and venturi sizes?
    Larry

  9. #259
    Join Date
    Apr 2009
    Location
    Ashburton, near Christchurch New Zealand
    Posts
    6,577

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    Congratulation Brian, another excellent job! When you get it settled just sit in your favourite chair, close your eyes and listen to the beautiful music!
    Last edited by The Artful Bodger; 01-13-2019 at 02:56 PM.

  10. #260
    Join Date
    Jan 2004
    Location
    Missouri
    Posts
    30,078

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    Looks good!

    After it is run in a bit, you will have to see how slow it will run, and if it will do the hit and miss at that speed.
    1601

    Keep eye on ball.
    Hashim Khan

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