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Discussion Starter · #1 ·
I recently acquired a 1971 CB175 from a family member who wasn't getting much use out of it. I helped buy it about 5 years ago and the PO kept it in really amazing condition, but it'd sat around for about 2 years with little TLC. I got it home and it ran just fine despite not being stored properly, and rode it around for about a week and maybe 300 miles with no problems, however it always idled high (3000 rpm). During a break on a ride I decided to adjust the idle screws to set it to 1200, which it did without complaint. I rode the bike for about an hour, including turning the engine off and restarting, without any real issue, before I noticed it was idling slower and rougher at a stop light, and then a block later suddenly died while accelerating. It wouldn't hold idle at all, but I found it I kept the throttle twisted open the bike would run at ~5000rpm and I managed to limp it home a mile or two this way.

I already cleaned the carbs - like the rest of the bike, they seemed in really good condition - and put new gaskets in along with new float needles (one didn't have much 'spring' to it so I replaced both). Since then, my dad and I have been troubleshooting, and are positive the issue is carburation (electrics are all good and to spec). We screwed around with it a bunch today to no avail. With the air screws set at 1 turn each, the bike will start but immediately leaps to 7000 rpm and stays there despite tuning the throttle screws. At best it seems to increase the idle. Also, and weirder, when messing around my dad discovered that covering the air intake (we're running the bike with the air filters off while working on it) on the left carb with his hand caused the bike to rev even harder, while covering the right carb in the same way will cause the bike to immediately die. The needle and throttle cable look good. About the only thing we haven't touched is what I've read around here is the 'needle clip', but that doesn't seem like it should be a factor for idling?

No combination of screw turns seems to get it to calm down. I'm mad at myself for touching the idle screws at all now, since I seemed to open pandora's box - but maybe the bike was revving hard to cover some kind of issue the PO had and couldn't fix? We're stumped and would love ideas.
 

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Sensei
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Did you check/replace the O-rings on the carb to manifold and/or manifold to head joints?..... Sounds like an air leak....
 

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Discussion Starter · #3 ·
Replaced the manifold to head gaskets during the rebuild, never opened the carb to manifold joints since they looked pretty solid. In the last day of tinkering, we adjusted the float level on the left carb down, which fixed the high revving and wet-with-fuel issue, but now it's back to the original issue I had when it died: it'll start fine while choked, and continue to run, but as soon as you remove choke it dies out. So it's running too lean, but we can't figure out what to actually DO about that issue.
 

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Sensei
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Just because the carb to manifold joints "looked" OK, does not mean that the rubber o-rings have not lost their resiliance after sitting for years compressed (after being wet with fuel)...
OLD rubber gets hardened-up and does not seal properly.......Overtightening the joints also is a possible cause of air leaks.....

AND, it does NOT have to be the carbs...... ANY air leak can be partially compensated for by running choked...... Valve adjustment/seal, head gasket, cross threaded spark plug hole, etc.......
 

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I'm having very similar issues on my bike, thanks again to Steve for helping me chase down my no spark issue, now I am dealing with this redline idle and i mean that thing really is wide open... i have taken apart and cleaned my carbs idk how many times now. I'm starting to think its an air leak, Ive taken the bike out for a few rides around the neighborhood and has zero throttle response when its moving. Steve, is there any way to use a smoke machine to pin point the leak?
 

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Sensei
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When it comes to the 175's and 200's, the first thing I check when there is a high-revving problem is to verify that the carb slides are not swapped.....
I also replace the o-ring manifold joints with a thick, flat paper gasket.....This has been discussed in detail before, but you may phone......
 

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CB175/CB200 are very sensitive to throttle cable routing.
Many times the act of putting the tank back on will pull on one of the "legs", keeps the slide from fully closing.
The cable is just barely long enough, routing is critical.
I speak from brutal, humiliating experience on this..........
 

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CB175/CB200 are very sensitive to throttle cable routing.
Many times the act of putting the tank back on will pull on one of the "legs", keeps the slide from fully closing.
The cable is just barely long enough, routing is critical.
I speak from brutal, humiliating experience on this..........
Yes, that caused me a lot of grief, running the cable the 'proper' way around the headstock. I've now replaced the cable, and rerouted it so that it runs directly from the twist grip to the carbs, following the line of the tacho cable. Carb slides now return fully, AND I can get full throttle, slides fully open. I've routed the secondary cables over the top of the fuel tank crossover pipe, running underneath it causes them to bind.
 
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