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Discussion Starter #1
Howdy, folks. I'm new to this bike and these carburetors, so I'm hoping one of you can help me. Here are the particulars for my 1973 CL175:

  • The primary issue is that the bike idles briefly then stalls.
  • If I hold the throttle so that engine runs at 2K RPM, it stays running; if I let go of the throttle, the speed decreases gradually until it chugs, then stalls.
  • Bike runs fine when engine is above 2K RPM; engine chugs (almost like it's missing) below 2K RPM.
  • Spark plugs are dark and a bit sooty.
  • Idle mixture screws are at 1 1/4 turns (I have experimented with them at 7/8 and 1 turn as well), but I can't get the engine to idle reliably or long enough to know whether changing the mixture with these screws does anything for the idle.
  • Needle clip is at 4th groove from the top.
  • Jets are stock sizes.
  • Idle speed screw (the one that adjusts the speed, not the mixture screw) does absolutely nothing. Turning the screw in, turning the screw out, makes no change to the idle speed at all.

And some other details:
  • Timing is correct.
  • Cam chain is adjusted.
  • Tappets are correctly adjusted.
  • Carbs were
ultrasonically cleaned (4 cycles, with heat) today and reassembled. This did not improve things one lick.

Anyone care to offer some suggestions?
 

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66Sprint
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Are the slides in the correct carbs?......Phone to discuss....


And Mike won the race to answer by seconds.....LOL.....;):lol::lol::lol:
 

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Discussion Starter #5
Sloped/cutout bottom of slide faces airbox, right? Which then positions the sloping side groove against the idle speed adjustment screw?

But a question occurs to me about the u-shaped retaining spring that holds the needle/clip mechanism in. Positioned with the open legs of the “u” toward the channel where the throttle cable end nestles seems to make sense—the end of the throttle cable will sit between those two legs and seat against the top of the slide piston. Pivoted around so that the bottom part of the “u” sits where that channel is means that the end of the throttle cable would sit in its channel but seat against the top the “u” spring, not a bit further down against the top of the slide piston. Which position is correct?

And I’d be honored to call and discuss this with you 66–are you available sometime tomorrow?
 

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Although I am new to this forum and the Honda twins I am not new to old bikes and carburators, so here are my thoughts:
I don't think the needle retainer springs position does make any difference. But, are the cables on both carbs adjusted long enough (adjuster screw quite screwed in carb top cover) so that the slides have a chance to reach their lowest (=close) position? Otherwise the idle screw won't touch the slides even...
 

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It's the throttle cable!

I came here to ask this same question! My idle was too high and no matter how unscrewed the throttle stop screws were, idle would not come down enough. I did a little reading here and saw that the throttle cable could be too tight, causing the slides to not go down far enough, therefore the throttle stop screws aren't even touching the slides.

I tugged on the throttle cable just a little and the idle dropped all the way down to where it was supposed to be! I found the highest idle with the air mixture screws and adjusted the throttle stop screws one last time to get the perfect idle (1.2k) and the bike is now perfect.
 

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Discussion Starter #9
Interesting, only my problem is the opposite—I can’t get the idle speed to increase, so the bike ultimately stalls.

But I found something interesting—the previous owner had obviously done some work on the carbs and in so doing, replaced the mixture screws with new ones. Only the new screws don’t have a hole cut through the center like the OEM screws. Luckily, the PO had saved the original stuff. I’ll be swapping those out today and am hopeful this solves the problem.

I’ll keep you posted.
 

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Discussion Starter #10
Okay, folks, here are the preliminary results (I'll provide a final analysis after a much longer ride): replacing the aftermarket mixture screws--the ones that had no hole in the middle--with the original mixture screws did the trick. I now have an idle that responds both to the idle speed screw AND the mixture screw, and I now have an idle at 1200 RPM. WOOT!

So this is kind of a minor big deal, but something that apparently shouldn't be unsaid: the aftermarket carb kit purchased by this bike's previous owner had all sorts of goodies that fit, and even an idle mixture screw that fit. It was the right length, the right diameter, and a nice shiny brass color. But it lacked the little hole that's drilled through the width (that is, sideways, not lengthwise) that the original mixture screws have. I'd upload a picture of my OEM screws if they weren't already installed, but I know I saw a drawing of the OEM screws--with the appropriate hole--on this site last night, because that's how I knew to look for one in my new mixture screws.

So, maybe I'm nuts, and maybe this is a red herring, but for me, putting the OEM mixture screws back in made the difference.

Thanks to all of those who weighed in with their thoughtful and learned suggestions. And thanks to the original owner for saving the old hardware!
 

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I've got two sets of CL175 carbs, stamped 653A on the manifold, and two sets of CB175 carbs, stamped 306B on the manifold.

The CL carbs have the hole in the mixture screw that you describe, whereas the CB ones are solid. The ones found in the aftermarket kits are also solid.

I'm afraid to say that I've also battled with getting a stable idle on my CL, and swapping between the two types of air screw hasn't made much difference. However, I'm running CB carbs with CL jetting on my CL, so mixing and matching parts probably doesn't help.
 

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Discussion Starter #12
Nevertheless, it’s good information to know that the CB and CL screws differ. I wonder why the difference exists.
 

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66Sprint
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I believe the difference is within the carbs, not necessarily which bike it is mounted to, although because of other differences (air jets, slide needles), that does seem to apply.....AND, the carbs specific to a bike are designed to balance flow, etc to the entire SYSTEMS' demands, including the exhaust......
The CL exhaust is much more restrictive than the CB or even the SL.....
IIRC, the 653 A carbs use the perforated screw, the 306 B use the solid screw......

This generally only becomes problematic IF you employ aftermarket brass bits, or have spare carbs and have removed them from their identifying (numbered) manifolds......
 
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