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1972 CB350K4 Won't Idle

7251 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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Thank you for the explanation. Regarding my mechanic: he came recommended and has tuned up my CB750 and CX500, both still running great. But we are human. After I got the 350 home and realized it wouldn't idle properly, I examined the bike and found the throttle cables out of sync. I didn't mess with the idle speed until I ran the bike again yesterday and still had the issue. We have another nice day here so I'll change the idle speed and see how it goes. Thanks again for your help. I know you've shared your knowledge with many people on this forum, which I think speaks to your character.

As Steve mentioned, 1,200 is correct for idle.
Another thing to consider is if the oil pump can work efficiently below 1,200. Lower rpm may compromise flow to the top end.
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Good point. Thank you for the feedback. Today I upped the idle, but hard to say to what. Before starting the engine, I turned both idle screws CW about 1/8 turn. Rode the bike for 10 minutes, got everything hot. Checked the idle and it was about 1000. (I keep saying "about" because no matter where I set the idle, the rpm swings down and up.). I turned the screws CW again another 1/8 to about 1200 and it idled there a bit. Afterwards pulled the plugs and they were carbon black. I think this means the fuel/air mixture is too rich?

The carb has an air or fuel adjustment screw? What effect does Turning it in CW have?
Fuel screw. Turning it CW (in) leans it.
I set mine around 1 full turn out as a starting point and adjust from there
It's been some months since I did anything with the CB350, but I finally got back to it this week. It was difficult to start, and when it did, only the right cylinder was firing (cold left header), and the engine wouldn't idle. So we've gone from both cylinders firing with a fluctuating idle speed to this, with the bike sitting on its center stand. I still have to check for spark and make sure the left carb is getting gas. The carbs are both the Keihin 722A, and I've rebuilt them using Honda OEM kits. In another thread, Sprint66 made a good point about the density of modern vs 70's gas. I use only non-ethanol gas that I purchase from a gas station chain named Stewart's in NY state, where the pump is clearly labeled non-ethanol, and priced accordingly.

Before I do anything, I would like to get feedback on the correct float height setting. Thanks
722A carb use a Factory Float height of 26 mm.

Setting the float height is one of the easiest thing to get WRONG.
The TIP of the NEEDLE/SEAT Valve is Spring loaded.
If that tip is compressed at all when you are setting the float height it will be wrong.

The FSM recommend holding the Carb Body at a slight off 90 Deg Angle where the Float Tang just barely dangles down to touch the Needle/Saet Valve Tip.
Getting ther Fuel Height correct in the Float Bowl is a MUST.

The Venturi (Low Pressure effect) draws Fuel UP thru the Emulsifier Tubes.
The Starting Point of the Fuel level within those tubes is a factor in how Lean/Rich the mixture will be.
The higher the fuel level the less low pressure drop is required to pull the fuel up the tubes and vice versa.
The Jets constrict the flow to a maximum amount of fuel that can pass but fuel level determines where in the throttle transistion the mixing occurs

On your Jets:
Violet Purple Product Water bottle Bottle

Right Click on the Pic and OPEN in a NEW Window.
This will give you the factory spec's for your carbs.
Which I belive are:
Primary #68
Secondary #105

If you are running 115 as the Secondary then that may be the cause of your Rich running Black Sooty Plugs.
Did you ever do the Oiling of the Felt Seals?

Also the O-Ring & Washer on the Air Bleed Screw are critical components for a consistant Idle.
They are the ONLY thing controlling Fuel Mixture at Idle.


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Excellent feedback, thank you. I need some time to process some of it to get a fuller understanding. I did replace the felt pads with oiled new ones. Thanks
The engine SN starts with 404XXXX. The first chart calls out sizing for the std main jet and the secondary jet as 68 and 105, respectively. The second chart calls out 70 and 105. I would think engine performance will be affected by the size of the std main jet, so which is correct for the 722A - 68 or 70? Is the std main jet part of the idle circuit? Sorry but I don't fully understand how these carbs operate.
There some really good posts over in the 450 fuel section about how the idle, mid-range, and high speed circuits function on these carbs.
While there are some differences the basic circuit functions are the same.

I have seen the same difference in spec's for the main jet #68 Vs #70 in many different spec extracts so I can't tell you which is 100% right.

They are extermely close in size and I suspect you can use either one but if you have OEM jets in your carbs that have not been abused I'd stick with them.
BTW: OEM Jet will have the size stamped on the side not the top and will have a Stylized STAR Logo (Slantintg Star)

Do a site search for Carb Jets and Idle problems you should have many of them pop up.
I do have the original jets as you described them. I may be overthinking this a bit, but I was wondering if it matters which end of the primary and secondary main jets face the bowl. In another thread, I found this response: "I believe the jets have a specific orientation. if you look at the jet, from one side the hole just appears to be straight through, but if you look at the other side, you can see the actual bore of the jet. I think the larger diameter end of the jet is supposed to be the end that is in the bowl, and the smaller end (bore) should be the end that has the o-ring on it and is pushed into the carb...made sense to me, because the larger port would suck up the fuel and the smaller port would help to spray it into the venturi…". Does anyone or agree with this logic? It makes sense to me, and the OEM Honda jets, 68/105, when installed this way, make seeing the jet size (star and all) possible without removing them. Not easy, but the sizes show outside the carb body.
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Update: Went ahead and installed 68 and 105 jets with the starred end out side the carb body. Floats set at 26 mm or darn close, maximum of <1 mm off). Plugs had come out with carbon coating, gapped at approx. .024". I think the smallish gap causes inefficient fuel burn and carbon fouling. Pictures of plugs attached. Auto part Lens Camera lens
Auto part

Cleaned the plugs with dremel wire wheel and 800 grit paper and set gap to 0.031". Fuel/air screws reset to one full turn out (ccw). Bike started w/o choke and settled into 1000 rpm idle. Because previous plugs showed rich mixture, I turned CW both fuel/air screws about 1/8 turn. Idle dropped to about 950 rpm, and used idle screws CW to up the idle to 1200 to 1400 rpm. Bike now revs without hesitation. I'm no expert but so far I like it. Blipping the throttle 4 to 5000 rpm, got no pops or backfires. Bike idled for about 5 minutes fluctuating just a bit between 1300 and 1500 rpm. I'm not sure I followed the correct procedure getting to this point, but it seems to be running much better than February. Can't road test. Comments, suggestions are welcome.
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Plugs still a bit dark down on porcelain, so perhaps still a smidge rich......However if reused you may be fine......
I would reset throttle stops to 1250 to 1300 once engine is warmed, and verify cable pull sync afterwards.......
Not sure I follow. Plug pics were taken between post #30 and #31 before latest settings. TS screws were turned CW about 1/8 turn per step until rpm reached 1100. Very linear. Then using same increments, rpm jumped to about 1500 rpm. These rpm readings are coming off the tach, not a dwell meter. Very hard to maintain a steady 1300 rpm. If I continue to turn in the fuel air screws (CW), at what point will I know it's the correct setting?
MIXTURE screws are turned to point of highest idle and left there.....
You then adjust THROTTLE STOP screws to get idle at 1250-1300......
IF still unclear, ...Phone........
I was going to call yesterday. Before I did I wanted to make sure I could duplicate the "success" I had the previous day (Sunday). Monday very hard to start, would not idle. Got it running by turning the throttle stop screw up (CW) to keep it going. Noticed left pipe not nearly as hot as the right. Put a timing light on the left plug wire - no strobe. Put it on the right and got the strobe. Previous day both cylinders fired every time I started the bike. So suddenly there is no spark on the left?? While running just on the right cylinder, I tried adjusting the fuel/air mixture screw, turning it all the way in (CW), and the bike speed increased. (Shouldn't the engine stall?) Turned the screw CCW out to one full rotation and the rpm decreased. If you think we should talk at this point I'll call, just let me know when you're available.
Always available .... Phone anytime, even just to generally discuss/BS.... Check plug connections on both sides of coil primary for non-firing side, and make sure points haven't slipped (screws vibrated loose) on the points plate...
After talking with 66Sprint today, I realized that I may have a blocked idle circuit passage in at least one carb. This diagram is found in a Honda shop manual dated 4/71.

Plan Technical drawing Drawing Diagram Engineering

I marked in blue ink the two passages affected by adjusting the fuel/air mixture screw; this is how I understood your description today. I removed the diaphragm, float bowl, primary jet (68), and the fuel/air mixture screw with washer and o-ring, then blew canned compressed air into the F/A mixture port. It came out only where you see the #11, but not out the primary jet passage. I blocked the exit at #11, shot air and then carb cleaner with still nothing coming out through the primary jet passage. I tried looking into the primary passage to see the port to figure out how to clean it. Previous efforts to "clean" the carb included 24 to 48 hour soaks in the Gunk cleaner. Thinking about getting an ultrasonic cleaner. Can you suggest a way to get to the clogged passage?
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I'm currently trying to clean the blocked passages marked in the above drawing, without much success. I can only use canned compressed air and carb cleaner, and a very small wire from a cleaning kit. Can anyone suggest how to get at these blocked passages? If an ultrasonic cleaner is the only option, can someone suggest one they have used with good success?

While cleaning, I noticed there are markings on the underside of each carb, in the area normally covered by the float bowl. It's not obvious in these pics, but both carbs have identical markings. In particular, the letter "A" can be seen towards the right side of the carb in the second pic.

Metal Auto part Cylinder Steel Ammunition

Auto part

The carbs are clearly marked on the outer side 722A. Any thoughts on the marks in the pics? Thanks
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NO!!!.... Phone and I'll tell you where to find the setting number (carb designation)
Okay. I'll call you mid morning tomorrow. Thanks
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