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Discussion Starter · #1 ·
HI, folks.

A couple months ago, I purchased a 1971 CB175 that was a resurrection in progress. For the most part, the bike is in pretty good shape, and I have just been working through various minor issues in preparation for putting it on the road. However, while the bike always starts and runs, one constant has been an apparent imbalance between left and right cylinders. I'm in the process of diagnosing the issue, but thought I would take a moment out to seek some input from those more experienced with issues like this. So, here is what I am experiencing:

When idling (very high -- more on that later), the right exhaust is quite warm, while the left is cooler. If I block the carb airflow on the right cylinder, the bike dies immediately, but if I do the same on the left, the revs only drop by about 150 RPM at 3000 RPM. When covering the right carb, I can feel strong vacuum, but not on the left. Interestingly, the bike will run on either cylinder alone, just not as well on the left as the right, so the left cylinder is not completely dead, just down on performance. Oh, and closing the mixture adjustment on the right cylinder has a large impact, but the left doesn't even register. So here is what I have done so far:

  • Valve clearances checked and set
  • Spark at both cylinders confirmed
  • Compression checked (warm, throttle and choke open). 135/135 (No leak down available)
  • Left carb gone through in detail, cleaned very carefully, all components checked against spec (new carb kit installed anyway).
  • Spark plugs reversed (no effect)
I know that compression is lower than ideal, but I honestly don't know how accurate this gauge is. From what I understand, around 170 is ideal on these bikes, with above 150 being considered acceptable. So, 135 might be an issue, but I would think that this should impact both cylinders equally?

In addition to the cylinder differential, the bike seems to want to idle at 3000 RPM once warm, and no change to the idle adjustment screws on the carb, or the adjustment at the throttle make any difference (there is slack at the throttle grip).

At this point, I am suspecting either a vacuum leak on the left cylinder, or possibly something internal. I do notice an slight idle speed increase when spraying WD40 around the left cylinder, but it's not really easy to monitor when the engine is idling at 3000 RPM. The carb kit I ordered didn't come with replacement o-rings for the carb-->manifold and manifold-->head, and those are very old/flat, so I'm planning to replace them to ensure that there is a good seal there.

The other thing that I am thinking of doing is getting a leak down tester to get a better picture of what is happening in that cylinder.

Last thing is that I should describe what the bike does when under power on the road. It will accelerate reasonably well in first and second gear, but it's obviously not where it should be, and it really struggles to exceed 6000 RPM. I can get it to exceed 30 MPH, but it's not happy. I've never ridden one of these before, but I'm assuming that it should be a little better than that? ;)

That's it for now. Any input would be very much appreciated.

Cheers,
Shaun
 

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Shaun,
Which "idle" screws are you adjusting?.....Idle MIXTURE screws or throttle stop (slide height at idle) screws?
Are the slides in the correct carbs with cut-aways facing rearwards?....(They will install "backwards" if put in wrong carb, and throttle stop adjustment to get it to continue to run will result in high idle speeds of perhaps 3K).......
It may be worth the cost of an international phone call to solve this......Feel free to phone while at bike....number below....
 

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Discussion Starter · #3 ·
Thanks for the response. Funny, last thing before bed last night, I read a post of yours that talked about the possibility of the slides being installed reversed. I didn't get how that could be possible until I saw your comment about swapping the slides between the left and right carbs. Then, the light bulb came on!

The idle screws that I mentioned in relation to the high idle are the slide height adjustments. Even when fully backed out, the bike still idles at 3000RPM.

I'll head out as soon as I can to have a look at the slides, and will plan to give you a call.

Thanks again.

Shaun
 

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Discussion Starter · #4 ·
Well, I don't think this is the issue with the left carb, as I think the slide is oriented correctly (see pic). I'll check the other side (in case someone was mixing and matching multiple carbs), and give you a call.

Cheers,
Shaun
 

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Discussion Starter · #6 · (Edited)
Steve was kind enough to step me through the issues that tend to happen with old o-rings, causing air leaks. That makes perfect sense, so I took the step of making paper gaskets for both the carb to manifold connection, and the manifold to insulator connection. Unfortunately, I'm still experiencing the high-idle issue (and the cylinder imbalance problem). I'll try to upload a video, or will provide a link

One question: I didn't look to see if there was a gasket between the insulator and the head, so that might be the next thing I check. Gotta go out for a bit of a ride first, though (on a different bike, of course). Can't miss nice spring days like this!

Sincere thanks to Steve!

Cheers,
Shaun
 

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Nice spring day like this huh. I mowed the grass Friday and I have 5 inches of snow in the driveway today!🎿🛷👀
I didn't think grass grew during the winter... you should get out of "Tea" and come to "Orange Juice" :p I haven't mowed in 5 months and it's already almost too warm to ride comfortably
 

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Have you checked the throttle cable adjustments? There should be about 1 mm of slack in the upper cable at the handlebar and a similar amount at the top of the carbs. Back the idle speed screws out fully and check that both slides go all the way down with the throttle off. Work the throttle a few times(engine off) and listen for the click of the slides when they hit bottom. The cable adjusters at the carb tops need to be adjusted so both sides click at exactly the same time, then both carbs will be synced to open evenly, very important for good off-idle response and an even cruising mixture.
 

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Discussion Starter · #11 ·
Nice spring day like this huh. I mowed the grass Friday and I have 5 inches of snow in the driveway today!🎿🛷👀
Yup, sunny and about 60F at the moment. I'll take it! 😎 Sorry to hear about the snow...
 

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Discussion Starter · #12 ·
Have you checked the throttle cable adjustments? There should be about 1 mm of slack in the upper cable at the handlebar and a similar amount at the top of the carbs. Back the idle speed screws out fully and check that both slides go all the way down with the throttle off. Work the throttle a few times(engine off) and listen for the click of the slides when they hit bottom. The cable adjusters at the carb tops need to be adjusted so both sides click at exactly the same time, then both carbs will be synced to open evenly, very important for good off-idle response and an even cruising mixture.
Hi, Mike.

Yes, I have backed off the adjuster at the throttle grip completely, so they're is lots of slack at both ends. Also, both idle screws are completely backed off, so there is no external influence on the throttle at all, and the slides are both all the way down. Still, the bike wants to rev at about 3000 RPM instead of idling, so there must be an air leak somewhere. The only thing remaining that I can think of is between the insulator and the head, so I'm going to have a look at that now.

Thanks for the info on balancing. I usually do that on my Guzzis with a homemade manometer, but this bike doesn't have vacuum ports. Will need to sort something else out.

Cheers,
Shaun
 

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Discussion Starter · #13 ·
Well, I removed both carbs (again 🙄), cleaned everything up carefully, used a straight-edge to check the manifolds for flatness, inspected the insulators to ensure that they weren't damaged, put new gaskets on either side of the insulators, and... not much change. 😑 There is some minor improvement, in that the bike now idles at 2700 RPM instead of 3000 RPM, but it hasn't solved the problem.

Once again, the left cylinder is not running properly, and I can completely block the intake with hardly any impact to revs, so obviously something is not right there.

Taking a break for the evening, and will have a go at it again tomorrow evening. Any additional thoughts would be gratefully received.

Cheers,
Shaun
 

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Advance unit stuck?
Ignition spark occurring EXACTLY at F line to index line alignment?
Correctly sized brass in carbs and float height on factory shut-off needle correct?
Mixture screws set to factory specs?
Carb throttle cables correctly inserted into splitter body?
 

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Discussion Starter · #15 ·
A wise man once told me that most carburetor problems are in the ignition system. 🤣 Other than checking for spark and swapping the plugs, I haven't spent a lot of time on ignition, yet. This really feels like a carb issue, especially as the ignition system is shared between cylinders on these bikes, but I'll plan to dig in and have a closer look at ignition, tomorrow.

I've only been through the left carb, so far, and everything was exactly right (even measured the jets as I've been surprised by jets that have been drilled out before!). The only thing that was out of spec was the float height, and that was only slightly off. I'll have a go at the right carb tomorrow and will report back.

Thanks again for your help, Steve.

Shaun
 

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Discussion Starter · #16 ·
Oh, forgot to address the throttle cables and splitter. That's something that I haven't looked at yet, but it does look like the throttle is acting equally on both sides. Will have a closer look, though.

Thanks again for the suggestions!

Cheers,
Shaun
 

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Discussion Starter · #17 · (Edited)
Okay, so came back out to work on the other carb, and found gas all over the shop floor. Forgot to turn off the petcock, and I guess the new needle and seat leaked. 🙄 The original never did, so I guess I'll probably go back to that.

Now, have the bowl on the right carb open, and found a likely cause for the imbalance issue. Where the left carb had float height set to about 20mm, this one is set to 28mm! Both were measured without the needle plunger compressed, and on the correct angle as specified in the manual. Speaking of the manual, it shows that the float height for the 175 should be set to 19.5mm, which is what I did on the left, but when searching the forum, I see suggestions of 19.5mm, 21mm and 28mm. Can anyone confirm what the setting should be? For reference, the number on the manifolds is 306B.

For my own education, is it possible for different carb models to attach to the 306B manifolds? I suspect not, from what I have seen in searching, but thought I should ask.

CORRECTION:. I just checked again, and the float height in the right carb is actually 18mm. The needle must not have been seated properly. Can't believe I didn't notice. Anyway, if you can confirm the correct height, I'd appreciate it.

Thanks!

Shaun
 

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Okay, so came back out to work on the other carb, and found gas all over the shop floor. Forgot to turn off the petcock, and I guess the new needle and seat leaked. 🙄 The original never did, so I guess I'll probably go back to that.

(...)
Thanks!

Shaun
Please, check if gas has not dripped into the engine oil....
 

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Discussion Starter · #19 ·
Last post of the night. I went completely through the right carb, and came to the further realization that the carb kits I bought are crap. Having now read many more posts on the forum, I see frequent recommendations to use either Honda or Keihin kits only, and I can understand why. As mentioned earlier, the new float valve leaked, the clips that hold the needles down in the slide were too large to fit properly, and the top gaskets that seal the where the throttle cable passes through were improperly cut. Crazy! So, most of the original parts were put back in, and I'll plan to get the proper kits for future installation.

I can see that David Silver Spares has individual OEM pieces, but not an entire carb kit all together. Is there a better place to order the proper kits from? Maybe just from a dealer?

In the end, I set the float height to 21mm from the carb surface where the gasket sits as that seemed to be the most common recommendation that I found. I then carefully inspected all the mating surfaces of the carbs and manifolds, and took the time to sand each of them on a makeshift surface plate to ensure that each was completely flat and smooth. I was surprised by how uneven they were, and how much sanding it took to get a completely smooth, polished surface.

Finally, I reassembled everything with the old top gaskets, and I'm now reasonably confident that the carbs are properly sealed. Unfortunately, I wasn't able to test run the engine tonight as I had drained the oil to make sure the leaking float valve hadn't caused fuel dilution, and I'd like to properly check the timing before refilling the oil. I'll get to that tomorrow, and will report back.

Cheers,
Shaun
 
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