CB175 tuning issues

My wife and I picked up a 72 CB175 for her to ride and we've been trying to get into riding condition but are have some trouble with the carb tuning. 
 

It will start with no choke and will run with the throttle open but there is white smoke and it smells rich. If I let the throttle close it will sit around 2k rom and slowly work drop until it stalls. The idle fuel adjustment is set to the spec in the manual and the slide stop needles seem to have little effect. If I engage the choke fully the bike stalls. 

We put on new foam air filters and have cleaned out the carbs (jets and all) and set the float bowl and main needle to spec. There's no fuel leaks that I can see. 
 

Once it's moving it runs fine above about 3k rpm but I think there is more in it. I have never ridden the bike in full working order so I have no idea how much is still on the table. 

One other note: the idle climbs steadily as the carbs run out of fuel. Petcocks are a new things for me too haha

I have not had a chance to check the valves, timing or adjust the timing chain tension but they're on my list. 
 

Learn me. I really want to get this riding for her but my knowledge is sorely lacking when it comes to carburetors. From what I can tell it's running pig rich at cold idle but I'm not sure what to do from here

A lot of carb rebuild kits contain needles that aren't shaped properly. They're tapered, but not like the stock ones, to the extent that some are completely useless.

I would take care of the valve adjustment and timing adjustment; they have a "correct" and it removes a variable.

Do you have a decent manual? In part to make sure you get the things above correct, in part to make sure you weren't setting up the carb to a sheet that may have come with a dodgy rebuild kit... Also because the timing chain adjustment wasn't intuitive to me.

It should idle smoothly and pull cleanly. There should be no need to be at 3K to run smoothly.

Do you have the slides in the correct carbs? IIRC you can put the right in the left and vice versa, but they won't work right. Moreover, that might jibe with the adjusters having no effect.

Gah, sorry, need to get ready to go out; I'll check back in if I can pick some more thoughts out of the memory bank...

Thanks for responding! 
 

We didn't use a rebuild kit, we just cleaned out the factory parts. I took some tiny drill bits from work and cleaned out the jets. The rest was carb cleaning and elbow grease. Everything seemed to be open and clean when we were done. 
 

I have a pdf of the factory manual. The external adjustments are to that spec. The rest is forum information specific to the carbs on the bike. Slides are in the correct carbs as I was aware of that issue but I should double check probably. 

Im hoping to pick feeler gauges tomorrow for the valves and points. Im new to all of this. 

I've messed with a bunch of old bikes with carbs, and still have a bunch of them...

Based on the symptoms - as you said, it's running pig rich (otherwise it wouldn't start easily without a choke) and you also mentioned "foam filters". Do you still have the original airbox that came with the bike? If so, I'd try to get the correct air filter(s). I've seen too many old carb'd bike running badly because of some sort of performance aftermarket air filter. Pull the air filter(s) off and see if it runs any better.

If it does, that's also why the idle adjustment doesn't make any difference.

The reason your idle is going up as the float bowl empties is because the fuel level going down leans out the mixture. I'd pull the plugs - my guess is that they're sooty black from the bike running really rich.

I don't think it's an ignition or valve clearance issue, although at least checking valve clearances is a good idea. IIRC these run points on the same backing plate so adjusting points and trying to get the ignition advance halfway right tends to be a fight between both cylinders. Pretty much all Honda twins of that era have that issue.

Oh, and as an aside - you might be lucky with your bike being a little earlier, but mid-70s Honda carbs don't always clean up well even with an ultrasonic cleaner. Some of the smog related small orifices still may remain blocked. A friend of mine had a Honda CB500T with that problem, and nothing short of a set of known good carbs would fix it. That said, I think your bike is too early for that bit of fun.

BTW, all of the above is under the assumption that these are the correct carbs for the bike. Unfortunately, Honda bikes are also a tad Lego-like, so you there is a chance you may have the wrong carbs for the bike. Check the jets and needle sizes against the manual to make sure.

If you're anywhere near the WV panhandle drop me a line and maybe I can come over and have a look at the bike to see if I spot anything.

Thanks for the offer Tim but I'm out in Los Angeles. 
 

The air filters are stock but the paper elements cut out and replaced with foam. I came across multiple forum posts saying those are necessary and since they are impossible to find we used foam. I'll try running it without the filters tomorrow and see what happens. 
 

There isn't any smog stuff from what I can tell so that's a good thing. thanks for the tip on the timing. I'll keep that in mind. 
 

The carbs I believe are stock as the number matches up as one of the possible factory carbs it could have. I guess they had a few different models based on year and spec. Technically this is a CL175 but the pipes are CB and that's the look we are going for. The only other thing that isn't stock is the mufflers. We have straight through mufflers on the back that we're hacked on by the PO. They're on the to-do list. 
 

I did notice that once I get moving the smoke goes away and I can't tell if that's because of the RPM or if it's actually running better since it's on a different jet. Does my logic check out? 

In reply to spandak :

Yeah, LA is a bit out of the way, although with my job you never know.

If you didn't do the filter replacement with foam, I'd have a close look at what the person who did it used. Friend of mine and I were trying to get a r Suzuki GT380 he purchased to flip going a few years back, and the person who "fixed" the air filters had used carpet underlay. No wonder he couldn't get it started. We're still laughing about that one today.

Straight thru mufflers should, if anything, help lean out the mixture a bit. My money for now is on the filters.

It might actually be running a little bit better once it gets off the idle jet. When you cleaned the carbs, did you replace the O-rings? There are some carbs out there that are very sensitive to worn O-rings. Not sure if the small CBs ones are amongst them, though.

We did the filters ourselves. I do have a set of filthy paper filters I could try for kicks too. 

Carpet underlay? Lol that's something! 
 

We didn't replace any o rings or gaskets. The ones we pulled looked good so we cleaned them and put them back. After responding last I realizing we didn't pull the idle adjuster screws and clean that area out so I think I'll try that too. 

Alright, today I got back out there and tried again. 
No change that I could tell with the filters on or off so I think I can rule those out.  
I had the bike idling for a second around 2k so I took it around the block. As soon as I got moving it flat out refused to idle. Not even an attempt. No power below 3k then it comes on strong and it while it revs all the way to 9.5-10k it runs out of breath past about 8k. 
Some homework says the pilot jet is probably the issue. When we cleaned the carbs they were plugged. I think I need to replace them or try cleaning them again. 

Thr idle adjustment (fuel not the slide stop) doesn't seem to do much. That might be borked too. The needles were far from sharp and pointed. I think someone rammed them in sometime on the past. 

Wouldn't be the first jet in a Honda carb that was plugged beyond cleaning I'm afraid. I think the two-smoke delivery (nothing ... nothing ... weeeeeh) is pretty normal for these, but it shouldn't run out of puff before the redline.

I'd also check the seat of the fuel adjuster if someone gave the idle adjustments a few to many proverbial ugga-duggas.

Wow has it really been a month? Oye

We pulled the carbs and I soaked them in a chemical dip for a couple of hours and tried again. Much better! But not quite there. 
So now the bike will hold an idle, mostly. It's not dying on me as soon as I clutch in, so, progress. It will run up to redline now after adjusting the carb settings to factory spec. There's still tuning to be done but we aren't there yet. 

The big problem is now when pulling away from a stop the bike hiccups really bad. If I hold the clutch in the engagement zone and let it sit after a second or two the bike will find its breathe and it's off we go. If I rush it at all it will stall. The other option is to let the revs drift up and around 3k rpm it picks up again. 
 

What I have been able to find is that the low speed jet is either too rich or lean, kinda obvious I guess.

I found a suggestion that if it burbles it's rich and if it chokes and suddenly catches its breathe it's lean. Does that sound right?
Is this adjustable? 

You might try running some fresh fuel with a dose of BG 44K in it.  I have had several carb engines that were cured of misbehavior with that mix and a good run.

Once, it didn't work after running - came put the next day after the carb soaked overnight and all was well.

When I do this, I mix it at about twice what the BG-44K can recommends. 

Haven't tried it but would guess that SeaFoam might work as well.

Funny you mention that, it's been on my list of things to pick up but for another vehicle. My wife's Subaru has some minor predetonation and the dealer recommended the BG as part a process to clean out the upper end of the engine. I'll see if I can find some. 

Update on this:

We bought a CB360T that just works so this bike will probably be sold soon. 
 

That said the fact I can't figure this out has been killing me and I don't like being beat. I have gone through everything I can think of except the valves and as I learned today, there is a separate idle circuit I'm going to look for. It still won't idle, at all, but it rips once it's moving. It's a fun bike and I'm hoping to keep it for myself but only if I can make it work. Updates coming soon I hope. 

Result!

Moving the needle one notch leaner has me idling. It's not pretty, but it idles. It bounces a little between 1500 and 2200 rpm. Lower and it dies. Additionally the hesitation off idle has returned. 
 

I believe that was the needle setting when we bought the bike and resetting it to factory specs had it dying. This is curious because everything I've seen says the needle has nothing to do with idle. 
 

More homework tonight to see if I can get answers. If anyone has insight I'm all ears 

Interesting. Once you leaned out the mixture, can you then use the idle control screws to get the idle further down, or is the 1500-2200 it?

With the foam air filters you do get different airflow than with the stock airfilters, so that may affect the settings. Although usually you don't have to lean the mixture out as the aftermarket air filters usually flow more, not less air.

Does the idle mixture screw do anything now that you've leaned out the mixture?

Airbox is stock with foam replacing paper. The difference between them on and off is noticeable but small. 
 

The air screws never made a difference before and the throttle stop screw didn't do anything for idle unless run way in and then the idle would occasionally shoot to 5-6k or something. 
Now it seems like they do actually make a difference. It's hard to tell honestly because I don't entirely know what I'm doing with carbs and the idle fluctuation masks small changes. But yes, half a rotation would raise or lower the idle some amount I could notice. I could bring it down under 2k but when it would bog it was low enough to stall. 
Too rich? 

My best guess is the bike was pulling fuel from the main jet somehow and running pig rich. Lowering the needle forced the pilot jet online. 

Today if I have time I'll drop the needle one more notch and see what it does. 

Also I physically cleared the pilot jet passages as best as I could with a guitar string and carb cleaner. I have no indications there's a problem there. 

Best and most complete walk-through of bike carbs I've seen. IIRC, there's both full explanation and a step-by-step method in there...

https://www.cycleworld.com/story/bikes/kevin-cameron-explains-how-to-tune-carburetors/

(Though I may be mixing up this article and the related chapter in his sportbike tuning book)

In reply to Jesse Ransom :

Thanks for that! I've been looking for something in depth. 

In reply to spandak :

Do you have a set of clean plugs or can you ride around the block a few times to get them hot and clean first? If so, I'd try to do a plug chop at idle to see if it's still running rich.

Some of the "advent of emissions requirements" bike carbs are notorious for causing issues if any of the small passages end up being gunked up, even after an ultrasonic clean. Friend of mine ended uphaving to chuck away a set of CB500T carbs because they couldn't clean out all of the passages.

I only have one set of plugs. I've torn around the block a few times though. I'll look into that

Maybe just clean them up with a brash brush so you can see if you get the typical rich mixture deposits or not?

Waiting for the bike to cool to pull the plugs and check. 
I rode the bike around a bit today and messed with the needles and idle air screw. I can get it to hold a decent idle at about 1400. Lower and the odd miss will stall it. It seems my Idle problem was in the air screw. The 1.25 turns per factory was way too little. I'm at 2.5ish and it's working fine. I even put the needle back to the middle clip and it's still fine so I must have done the needle and screw adjustment at the same time. 
Good news I think, but the hesitation is still there. Letting out the clutch below 3000 has it bogging really bad. For a solid one potato it jerks and coughs before finally finding its breathe and away we go. I can't tell if it's running right in the rest of the range but I think so. I don't feel anything weird once I'm moving. 
If I leave it in gear and take it down to idle speed and then try to accelerate it's even worse. This is all with me gently rolling into the throttle, I know a sudden WOT will cause these problems. 
Once the heads cool I'll pull the plugs and see how they look. I honestly can't tell if the bike is rich or lean. Gut says rich and that I should find a smaller low speed jet. 
I'll update when I pull the plugs

Playing with the idle screws some more seems to help the hesitation. Leaning out the idle is making the stumble a little better but it's still incredibly easy to stall. Far easier than the XL70 I have recently ridden. 
Im going to try and find a smaller low speed jet to lean things out a bit separate from the air screw. 
I have a pretty decent history as a wrench but this bike has humbled me. I'm always learning to slow down and check the basics. 

It really sounds like you've still got some obstruction in one of the air bleed circuits.  The fact that your second cleaning made it better makes me even more suspicious of that possibility.  Are you pushing the emulsion tubes out of the carbs when you clean them?  That's an often overlooked step when car guys rebuild motorcycle carbs.