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Discussion Starter · #1 ·
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?
 

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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?
 

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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........
 

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Discussion Starter · #4 ·
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

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.
 

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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.
 

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Sensei
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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......
 

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Discussion Starter · #7 · (Edited)
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. :D :D

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.
 

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Discussion Starter · #8 ·
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.
 

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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.
 
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