Really weird problem with the carb on my 1971 CB100
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  1. #1
    Junior Member DrQuackerz's Avatar
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    Really weird problem with the carb on my 1971 CB100

    I picked up a 1971 CB100 recently to make into a street scrambler/mini adventure bike. This is my first project bike so I'm learning as I go. Bought the bike as a non runner but after consulting the original manual found the main issue was the timing and the points were in contact with eachover and got it started. Anyhow the problem I found when tuning the carb is that the slide seems to stick, the weird part is when I tighten the carb onto the engine the slide becomes even more resistant to movement to the point where it will hold the throttle open. I've checked the throttle and it isn't the culprit and the spring in the carb certainly isn't weak.

    Anyone else ran into this problem? Is it fixable or is this a case of replacing the carb?

  2. #2
    Supporting Member ctrider's Avatar
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    Sorry but we need pictures of that nice CB100 before we can help!!
    Just kidding , lol but we would love to see it!

    Someone will be along to help.
    Where abouts in England?
    Life should not be a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside in a cloud of smoke, thoroughly used up, totally worn out, and loudly proclaiming "Wow! What a Ride!”

    Hunter S. Thompson

    1970 CB350
    1974 CT70
    1977 XL75
    2005 PW80

  3. #3
    Sensei 66Sprint's Avatar
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    Over-tightening has been known to bend the carb flanges and "warp" the bodies.... Sometimes fixable, sometimes not.....
    If the slide still binds when carb is unbolted and removed from engine, it is replacement time........
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

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  5. #4
    Junior Member DrQuackerz's Avatar
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    Quote Originally Posted by ctrider View Post
    Sorry but we need pictures of that nice CB100 before we can help!!
    Just kidding , lol but we would love to see it!

    Someone will be along to help.
    Where abouts in England?
    Near Liverpool, I'll try get some pics up soon for you

    Quote Originally Posted by 66Sprint View Post
    Over-tightening has been known to bend the carb flanges and "warp" the bodies.... Sometimes fixable, sometimes not.....
    If the slide still binds when carb is unbolted and removed from engine, it is replacement time........
    The weird thing is that when its not attached to the engine the slide moves freely which is what puzzles me. I'll have a look this afternoon though to see if you're right hopefully I'll be able to fix it, I'd rather avoid having to find another original carb the ones on ebay are stupidly overpriced or not the same as the one I currently have.

  6. #5
    Senior Member middletons's Avatar
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    Sand the flange flat using a piece of glass or other known flat surface. If there is metal intake, sand that flat as well.
    1972 CL450
    1972 CL350
    1975 CB360

  7. #6
    Sensei 66Sprint's Avatar
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    Quote Originally Posted by DrQuackerz View Post


    The weird thing is that when its not attached to the engine the slide moves freely which is what puzzles me. I'll have a look this afternoon though to see if you're right hopefully I'll be able to fix it, I'd rather avoid having to find another original carb the ones on ebay are stupidly overpriced or not the same as the one I currently have.
    Remove the O-rings at the joints, Cut full flange gaskets that extend to and fit between the stud/bolt holes so the entire flanges are supported (instead of just around the air passage tract) as this helps to prevent flange bowing and carb warping, and gently tighten until snug.... IF flange does have any curvature when checked against glass, it can be carefully sanded until flat..... IF you find additional sealing necessary, a thin film of HYLOMAR is what I'd use......
    "I have a mind like a steel trap.....Old and rusty, of antiquated design, and hard to get stuff back out of...."
    Contact info: E-mail; [email protected] Phone; 540-525-5199

  8. #7
    Junior Member DrQuackerz's Avatar
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    Quote Originally Posted by 66Sprint View Post
    Remove the O-rings at the joints, Cut full flange gaskets that extend to and fit between the stud/bolt holes so the entire flanges are supported (instead of just around the air passage tract) as this helps to prevent flange bowing and carb warping, and gently tighten until snug.... IF flange does have any curvature when checked against glass, it can be carefully sanded until flat..... IF you find additional sealing necessary, a thin film of HYLOMAR is what I'd use......
    You where completely right about the bowed flanges, checked against a mini spirit level and they where indeed curved about a mm out of square at the center, when I got to checking was actually worse than I remembered and wouldnt even slide with it off the engine. I fixed it no problem and the slide now moves freely though I didnt sand it mainly because my thinking was if the slide cylinder was warped sanding the flanges wouldn't do much. My fix was a bit more crude, I placed a plywood sheet down and hammered it back into shape using a offcut of 4x1" to soften the blows and protect the carb, carefully checking after each blow. Probaly not the best way to go about it but it worked a charm, thanks for all the advice guys appreciate it specially since this is my fist time.

    I'll upload some pictures of the bike once its back together. I may even create a build log thread if anyone would be interested.
    Last edited by DrQuackerz; 04-23-2019 at 08:39 AM.

  9. #8
    Junior Member DrQuackerz's Avatar
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    Really weird problem with the carb on my 1971 CB100-img_20190423_174927.jpg

    Here's a pic as promised, as to be expected with a 48 year old bike some pervert's had their hands on it and made an abomination of the exhaust and a few other parts. Thankfully that will all be getting swapped out.
    Attached Thumbnails Attached Thumbnails Really weird problem with the carb on my 1971 CB100-img_20190423_174809.jpg  

  10. #9
    Senior Member Richard_Pitman's Avatar
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    I remember reading a road test of the CB100 single back in the day. I think it was in 'Motorcycle Sport', defunct now, and the tester was, from memory, John Robinson. They managed to get some ridiculous top speed out of it, rider in race leathers, head on tank.

    I had a go on its later variant, the CB125S, and was impressed at how well that went, compared against my then CB175, very torquey little engine.

    Regarding your sticking throttle slide, I recently had the exact same problem with a CB175 carb. After ultrasonic cleaning, slide was completely free in carb body, but jammed solid once carb was attached to inlet manifold. Loosening the two screws attaching the carb to manifold frees off the slide, so clearly some distortion present, probably been overtightened in the past.
    1970 Honda CB175K4
    1972 Honda CL175K7
    1999 Honda CB600 Hornet

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