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Thread: rejetting a motorcycle carburetor

  1. #1 rejetting a motorcycle carburetor 
    Forum Ph.D. william's Avatar
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    Hi all,
    I've got a 1972 motorcycle that I want to put pod filters on instead of the stock air filter. To do this, I'll have to rejet the carburetor.

    Has anyone done this? Any advice, or "do's and don't's"?

    Thanks,
    william


    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  3. #2  
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    Hi William.

    Interesting post in a science forum, but I guess motorcycle maintenance is a science too. Advice... if you have never taken apart a set of carbs before I would recommend taking pictures as you go. It can get kind of tricky remembering where everything goes when you button it back up. As for the actual jetting, it's can be a trial and error sort of thing.

    Generally you will want to go up one or two sizes in main jets and one size in you idle jet. If you end up having a rivet style idle jet (one that can't be removed) you may want to put a shim at the top of the jet needle to raise it up a little. I had to do this on one of mine and it worked alright.

    Once you get everything back together, fire up the bike and rev it about half way for about 30 seconds and then check the spark plug. I should be a nice chocolate color. If it's lighter, go bigger with the jets, if it's darker, go smaller. Btw, what kind of bike is it? I love the old 70's ones.


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  4. #3  
    Forum Ph.D. william's Avatar
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    Hi Feifer!
    Thanks! I wasn't too sure I'd get a reply, let alone a fast one!

    The bike is a 1972 Honda CB750 K2. I like the old ones because it seems the new ones have abandoned the kickstart.

    I'm totally restoring the bike and turning it into a cafe racer. It's completely in pieces now, but will be finished before Spring. (Actually, reassembly will take place pretty soon.) I must say, it is an extremely rewarding project! I'm learning a lot.

    I've hardly driven the bike before I ripped it apart. So the plan is to restore everything except the engine. Then I'll ride it for the Summer, and next Winter I'll consider rebuilding the engine (after I learn all the quirks from riding it).

    Do you have experience rebuilding an engine? I have another bike (a '66 Honda Dream 150) that doesn't seem to have compression. I think that one will need major surgery.

    Thanks again Feifer!
    Cheers,
    william
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  5. #4  
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    ahhh the old hondas are my favorite. I also have a cb750 that I'm going cafe with but its a 1977 F. It's still in pieces and been sitting for a while now because I opted to leave a moderately well paying blue collar job to be a starving student. Oh well... some day. But I have worked on a set of K carbs before and heed my warning with taking pictures... those things are complex. Lots of vac hoses, linkages, springs, etc. Keep everything documented and you'll be fine. And don't forget to balance them when you put them back on, but thats a whole nother can of worms.
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  6. #5  
    Forum Ph.D. william's Avatar
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    Hi Feifer,

    And don't forget to balance them when you put them back on....
    What all does balancing entail?

    Thanks,
    william
    "... the polhode rolls without slipping on the herpolhode lying in the invariable plane."
    ~Footnote in Goldstein's Mechanics, 3rd ed. p. 202
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  7. #6  
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    When you start fiddling with the carbs you will notice that by the out flow side of each one there is a little screw that doesn't seem to do anything. It's just opens a little passage way to the inside chamber.

    Once you get everything back together you will need to take out these screws and put in adapters that go to a carb balancer. This is just a device that measures the pressure being drawn into each cylinder while the bike is running. Each carb on the rack must be adjusted to the same pressure otherwise the bike just wont run right. You will notice when you hit the sweet spot because the idle revs will jump way up.

    That's the basic concept anyway, although you will get much better how tos in the owners manual or carb balancer instructions. They're not too expensive and you can get them at any motorcycle shop.
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  8. #7  
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    William
    If you havn't already, subscribe to Motorcycle Rider magazine. or check them out online. Good luck with your bike.

    Last year I picked up an '86 Kawasaki Vulcan 750A2, had to replace the stator, it was in cherry shape. With only 16 K miles on her.
    (black and chrome)

    Feifer,
    good advice !

    Ride safe!
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