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1972 CB350K4 Won't Idle

7250 Views 53 Replies 9 Participants Last post by  barab63
I've been searching this forum for a while and haven't found a solution for my particular problem. So this post. Cold engine starts easily and idles at about 1200 rpm. However, after riding for a short time (maybe 5 minutes) engine won't idle when I stop unless the revs are kept at about 2000 rpm or higher. It hasn't stalled unless it goes below that, but if it does stall, one push on the starter button while goosing the throttle gets it going. Won't start unless you give it some throttle. Cruises fine above 3000 rpm and seems to have enough power and acceleration for a 350 twin. Below 3000 rpm, bike becomes a bit sluggish and hesitates. Three days ago the carbs and timing were adjusted and set properly by a professional mechanic whom I trust. During the rebuild (some of you might recall the "chirping issue I had) I completely disassembled the carbs and soaked all the parts in the gallon Gunk carb cleaner for about 24 hours, and then used wire from a wire brush, small diameter drill bits, welding tip cleaners, carb cleaner, and finished with canned compressed air.

Opening the gas cap or turning off lights doesn't affect anything. Battery on volt meter at 3000 rpm reads a little over 13 volts. Send help. Thanks
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Hmm, does one of your float have a hole?
What color are your plugs? You still have an air leak?
You have the felt seals on your carb butterfly shaft?
Paper gasket between the intake manifold and head?
Did you set your idle speed with the engine warm?
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Gaskets in place. I'll check other items.
How long has it been since you set the points and timing? Points gaps that have closed up some from wear cause retarded timing and that will drastically affect low rpm performance
idle circuit still dirty in carbs or not properly adjusted when warm. Lots of threads about carbs thoroughly cleaned and several posts later, carbs cleaned properly fixes the problem.
How long has it been since you set the points and timing? Points gaps that have closed up some from wear cause retarded timing and that will drastically affect low rpm performance
Points, timing and carb settings were done four days ago by a professional mechanic that I have used many times.
idle circuit still dirty in carbs or not properly adjusted when warm. Lots of threads about carbs thoroughly cleaned and several posts later, carbs cleaned properly fixes the problem.
I had disassembled the carbs and cleaned all the components a couple of months ago, and the bike sat with empty fuel tank since then. It was my first ever carb cleaning. So far, I have two opinions suggesting the idle circuit needs a second look. Thanks
The passages are pretty small, letting them soak won't work on a fully clogged port since the cleaner needs to flow through. A lot of us have bought an ultrasonic cleaner from harbor freight, fairly cheap and the agitation helps but isn't 100%.

Using drill bits, steel wire, etc can damage the ports, the carb bodies are a softer metal.
The part about 24 gunk soak and drill bits and wires has me wondering if the felt parts are good and if the jets might be scratched.
It's been a couple of months since I cleaned the carbs. I don't recall seeing any felt pads, but this doesn't mean they weren't there. I could not locate them on the parts diagrams. I did buy rebuild kits, but there weren't any included. Can someone direct me to their exact location on the carbs by providing a pic or a reference in a drawing? Also, the carbs are marked 722A. Did all the carb models have the felt pads? Thanks
The felt pads seal the butterfly shaft on each side of the carb body. Not normally a replaceable thing so they are not shown in an IPC but some on the forum have made their own and replaced them.
Attached pics are of left carb bits. In first picture, what is the brass(?) circle on the right? Second pic shows two tubes with holes, which reside under (above?) the 68 and 115 jets. These are emulsifier tubes? Is the threaded tube on the right, marked with "38", the pilot Jet? Is the Air Jet any of these three or is it something else?

BTW, I had bought a rebuild kit including the press fit tubes with the holes in them (emulsifier tubes?) and installed the new tubes. When I took the carb apart tonight, both tubes (the replacements) practically fell out on their own. But I remember pressing them in required a bit of effort. So I tried the old tubes again and they seem to have a much tighter fit. Is the tighter fit critical or do the jets hold the tubes in place?

Third pic shows the primary and secondary jets' sizes - 68 and 115 respectively. The manual (copyright 1971) calls for 60 and 115. The bike is a 1972, but could larger primary create any issues?

To jd50i, in fourth pic of the butterfly are the felt pads located close in next to the throat, or further out, nearer the spring and the nylon bushing? Is it easy to disassemble the butterfly shaft or are there "hidden dangers"? (Tiny springs, easily broken plastic bits)



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Update: A couple of responders suggested the idle circuit, so I took carbs apart again and cleaned everything (hopefully) with a 24 hour soak in Gunk carb cleaner, followed by spray carb cleaner and canned air into as many orifices as I could find. Also readjusted the throttle cables. Now the bike starts (with some effort and goosing) and seems to idle fine (in the garage) after it warms up. I'll road test it this week and post a final verdict.

With the bike running, an electrical issue has shown itself. With engine running, head light switch off, the tail light filament (not the brake light) burns out because its brightness increases with engine speed. Hadn't noticed it until now. I've put all the details under electrical, continuing a thread I started when I first got the bike over a year ago.

The "FELT" Parts are on the ends of the Throttle shaft.
They are held in place on one side by a press in Cap retainer and on the other side sandwiched between the end of the shaft and the Plastic/Nylon bushing.

They get Greased or Oiled with a heavy weight gear oil.
Their purpose is to seal the ends of the throttle shaft to prevent an air leak from behind the throttle plates whcih is after all the fuel mixing occurs.
If they have been dried out due to soaking in cleaner (the cleaner basically washes them clean) the seal will be weak on non-existant.

You will need to remove the throttle shafts to re-oil them.
The flet seal don't actually have to be removed themselves but it is next to impossible to get a heavy weight oil into them without removing the shaft.
NOTE: If you do this you MUST have a JIS Screwdriver Set or you "WILL" ruin the heads on the screws that secure the thtottle shaft plates.
They LOOK like a philips head but they are NOT. They are Japaness Industrial Standard which has a VERY different angle to the cross tip.
A Phillips WILL strip the head of those small screws.

Also the ends were punched and expanded to prevent them from backing out and being swallowed by the intakes.
This is not something you will be able to duplicate and will cause you to need to WORK them back and forth while removing them or they will bind.
When replacing them use RED permanant Locktite.
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Got the JIS early on, work well on Phillips too. Guess I'll be using them to take carbs apart one more time.

Your bike resume seems to make you well versed on CB350. If you haven't already, would you be able to take a look at my post under "Electrical". It's got me stymied. Thanks

Update: I have not removed the carbs yet to get at the felt pads, but temps finally got above freezing today. So rode the 350 around the neighborhood for about 20 minutes to see how it ran with the throttle cables readjusted. Both cylinders were firing. Engine runs strong above 2000 rpm, pulls nicely through all he gears. Throttle response seemed good. Idle is better but still an issue.

I let it idle for about five minutes before riding and it seemed good. Rode a bit, keeping the rpm between 1500 and 5000. When stopped, idle still dropped but not as quickly. I increased the idle speed on both carbs and it seemed to improve at about 1100 rpm. This seemed high so I reduced it to 900. After a few more minutes of brisk riding, pulled into my driveway to watch and listen. Idle speed dropped but then recovered, swinging between about 900 and 200 a couple of times based on the tach readings. It did this for about a minute before it stalled out.

What might be causing the idle swing and stall? The dried out felt pads? I'll try to get to them this week, but I'm wondering what else could be a root cause? And why does the idle speed need to be adjusted when the engine is hot? What is the correct idle speed? Thanks.

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Correct idle speed is 1250 to 1300 RPM...... You are set TOO low.......
Seems high, but I'll try it. I've listened to videos of 350's which seem to be idling much slower, with a "put-put-put" sound. And if the idle speed needs to be set with a warm engine, I'm curious why that is? Thanks

Adjustments should be set by, and to, the factory service manual specifications... Not by a video...

My question is: IF the professional mechanic (whom you obviously trusted) set the idle "about 1200" why would you change it from where it "Cold starts easily and idles"?

In a warm engine the parts are/have expanded, and meeting/sealing/functioning properly together, compression is better, etc......
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