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Running rich at idle (cb175 k5)


Well-known Member
Jul 26, 2023
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Lawrence, Kansas, USA
Hi all,

I've been having rich symptoms from my bike at idle and low throttle I think.
- Backfires when starting and sometimes when idleing
- sputtering at low throttle
- only starts and runs without choke
- smell of gas at idle
- using more gas than I feel like it should (intuition)
- air screws are more than two turns out
- have a 35 idle jet in there instead of a 38
- checked plugs after a little idle and they are completely sooty and black

- note: it holds idle well when warmed up. Does not hold idle when cold without a little throttle help.

I put the 35 jet in there after I had the air screw more than 2 turns out with the 38 jets. Note, these jets are 4-1 aftermarket. I know they are not ideal. One of the original keihin 38 jets that came with the bike looks a little chewed up (when I look through it, it the hole looks warped). Is it worth buying one 38 keihin jet from jetsrus and using it with the one I have that is in good condition? Is there any scenario when a properly tuned carb should have idle jets that are not factory size. Like having 35 instead of 38? Or does this indicate something else is wrong. It is possible my jet needle and or housing are leaking fuel at idle? The slides bottom out with no throttle. Do jet needles and housings wear out?

Please help me diagnose this. Thanks.
When I say more than two turns out on the air screw, I mean I keep turning them out and the idle keeps rising. To me this indicates that the engine needs more air.
I believe you have a mixture screw rather than an air screw.
It sounds like you have a vacuum leak with the idle and starting backfire plus turning the mixture screw out adds more fuel to compensate for the leak.
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Pretty sure I have the right manual. It says it is a "pilot air screw" and turning it in make it richer. Am I misunderstanding this?
How about sharing a picture of the carb? That should clear up the type of adjuster it has. Generally, if the screw is behind the main jet (air filter side) it's an air screw and if the screw is in front of the main jet (engine side) it's a mixture screw, but not necessarily in all cases.

I downloaded a factory service manual from CMC for CB175 K0–K4 and it says what you say: turning the screw in richens the mixture and turning it out leans the mixture. (sounds like an air screw)

The effect of the screw is diminished when the idle screws are in far enough that fuel is being drawn through the main jet. The slides are in the correct carbs, i.e., oriented correctly?
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I suspect the float level is too high, possibly the needle clip is set too low.
just checked the float levels. They are 20-21 mm above the carb body. The manual says 21 so if anything i'm a little low. Do you think I should experiment with the needle clip settings? It has been on the middle slot as per the manual.
Lower float height equates to a higher fuel level in the bowl, which richens the mixture. Maybe you could try 22mm to see if it benefits your plug fouling situation.

What air cleaner is being used?
How is the static timing?
I adjusted the floats to 22 mm in height. I am using the stock air boxes. I adjusted the timing but didn't balance out the discrepency to make it slightly early on one cylinder and slightly late on the other. So it is on time on one, and slightly late on the other I think. I want to fix it but I just put oil in it. Will report back if the float height changes anything.
Sounds good. I've heard about others with the CB/CL175's trying to split the difference on the timing rather than have one spot on and the other off. That could be another thing to consider after an experiment or two with the float height.

Since you mentioned that the slides are resting at the bottom with no throttle, I wouldn't expect the needle to make a difference at idle and I'd hold off on tinkering with that for now.
The adjustment in float height didn't seem to change anything. Still idles but all of the rich symptoms are there (have not checked plugs yet tho). The main thing that concerns me is that the air screw is out at least 2 screws and I can keep going out and it won't change the idle speed. If I go in from there the idle get weak and dies. It seems like the air screw can't provide enough air even when fully out. Shouldn't I get to to a point where If I turn the idle screw in or out the idle will get slower either way showing that I have found the optimal mixture?

I still have aftermarket 35 jets in there.
I've used similar carbs on other bikes and often the idle stop screws must be turned in to help achieve a stable idle. You mention needing to give throttle while warming the bike up, but not after. You also mention the idle dropping when you turn the air screw in from its current 2+ turns position.

Although it might not work, what I would do is put the air screws at the stock setting, put the original 38 pilot jets in and use the idle stop screws to get it to idle cold by raising the slides. If that works you can fine tune the air screw after it's warm.

Raising the slides will allow more air to flow through the venturi and might improve the air/fuel ratio.

Basically, my thought is that I'd want to know that the stock settings do not work before trying to correct something from a non-stock configuration. Your bike sounds mostly stock, so I don't understand why it would run so rich with stock settings.
On my CB160 I have 38’s installed, my needle is lowered by one notch and my air screw is only 1/2 turn out. I still have a super low idle until the bike is fully warmed up and using choke makes difference.
I would go to standard (38) slow jets and then see where you stand.
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