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Old 09-29-2014, 09:03 AM   #1
tundra61 OP
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CB350 - one cylinder firing

Hey all,

I have a 73 CB350 and just noticed only one cylinder is firing. If you are sitting on the bike - its the left side that is not firing.

Right side seems to be running fine.

Fuel seems good and I am going to check timing but if anybody else has dealt with a similar problem please share how you fixed it.

Thx
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Old 09-29-2014, 10:11 AM   #2
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This is general info for any bike not firing. Set valve lash, set ignition timing, clean the carbs, sync the carbs. But there's other stuff to look at before even bothering with by the book tune-up stuff.

Look at the spark plugs. If the one not running is fouled from being covered in oil you may have a mechanical engine problem with oil leaking past the rings or into the chamber from the head at the valve guide seals. That requires taking the engine apart which may or may not be a fun thing for you to do.

Check to make sure the ignition coil is firing. If not, address all that fun stuff with points, condensers, coil. The OEM wires on Hondas are not service-able and are glued into the ignition coils. A design feature I hated. I always replaced my ignition coils with something else from a different bike so I could run new plug wires. I liked Yamaha XS650 coils since they fit easily in the same space as the Honda ones.

Once spark is established you can move onto the carburetors.

Mine always had problems with the petcocks plugging up and/or not flowing gas fast enough.. one or both sides not flowing freely enough.

Pull the float bowl from the left carb to see if there's gas in it.

If there is gas in it, start doing all of the work to clean the carb, verify the float is working and that float height is set properly.

Then when both carbs are clean and mechanically in tune you need to set the air mixture screws and then sync them as best as you can by making sure both throttle cables 'come on' from full stop at the same time. There aren't any vacuum ports so you can't sync them with a tool.

That's just generalized info about the carbs for any bike though.

The CV29 carbs in 350s are pretty easy to keep running right if the diaphragms are good and the rubber o-rings on the jets are good. Mine always had dead spots at about 4000 RPM in the rev range which seemed to just be a thing that CB350s do for me in stock form.

Without being present to look at someone's bike 'not firing on one cylinder' has about 1000 possible reasons.
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Old 09-29-2014, 10:44 AM   #3
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great info - thx
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Old 09-29-2014, 11:21 AM   #4
Harry94025
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Originally Posted by tundra61 View Post
Hey all,

I have a 73 CB350 and just noticed only one cylinder is firing. If you are sitting on the bike - its the left side that is not firing.

Right side seems to be running fine.

Fuel seems good and I am going to check timing but if anybody else has dealt with a similar problem please share how you fixed it.

Thx
Check the points for the left side. As you know, the CB350 has two sets of contact points.

My 350 used to start on one cylinder, then I'd run it down the street and the second cylinder would kick in after a few seconds. Normal Operating Procedure for that bike...
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Old 09-29-2014, 04:29 PM   #5
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Originally Posted by Harry94025 View Post
Check the points for the left side. As you know, the CB350 has two sets of contact points.

My 350 used to start on one cylinder, then I'd run it down the street and the second cylinder would kick in after a few seconds. Normal Operating Procedure for that bike...
...but not normal operating procedure for a CB350 that is in good working order.
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Old 09-29-2014, 05:22 PM   #6
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If your battery isn't above 12.4V or whatever that can an issue, these old twins drop a cylinder when they get flat
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Old 09-29-2014, 06:36 PM   #7
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Still not working. Here's what I checked so far:

- points gap : good
- timing : good
- spark on both plugs: good
- fuel from petcock to both carbs: good
- compression : seems ok by feel

What I have done to get it running:

- shot starter fluid in both carbs and the big starts and runs on right cylinder, but even with all of the above the left side still isn't running.

How can I check if fuel is making it through the carb? That seems to be the only thing left - but then again - even when shooting starter fluid into the left carb it still doesn't run.

Thoughts?
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Old 09-29-2014, 06:42 PM   #8
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Diaphragm has a hole in it so the plunger in the carb isn't rising?

Physically blocked pilot jet circuit?

Valve lash is not set right on that side?

I couldn't tell you the difference between 80psi and 120psi by sticking my pinkie in the sparkplug hole, but a bike can, by refusing to run ;)

Try all the easy stuff.. Switch spark plugs to see if the problem shifts sides.

These bikes can be stroke prone where they only run on one side for multiple reasons. Out of sync badly enough, the aforementioned diaphragms or obstructions of fuel passages, and the fact they run two sets of breaker points, two coils, and two condensers. Condensers usually don't prevent starting if they're bad though.
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Old 09-29-2014, 07:18 PM   #9
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But shouldn't the cylinder get at least 1 or 2 fires off the starter fluid?
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Old 09-29-2014, 07:53 PM   #10
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But shouldn't the cylinder get at least 1 or 2 fires off the starter fluid?
Yes, if it is getting spark at the correct time, and has enough compression.

Get back to basics. Start with a compression test. Check valve lash on both cylinders. Check ignition timing and valve timing. Rule out those things and then overhaul the carbs. Once you have it running you need to get a service manual and catch up on all of the service items, exactly zero of which will have been done in the last 20 years. You will probably find at least one item that will make you thankful that you can't just hop on and ride it as-is.
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Old 09-30-2014, 02:05 AM   #11
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I had a 350 many years ago....I remember a similar problem. It turned out that on one set of points, spark was arcing to the points case from the spot where the wire attaches to the points. I bent it inward a bit and all was good.....
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Old 09-30-2014, 07:53 PM   #12
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Updated check list

- compression : 170 psi in both cylinders
- timing : off but reset with nearly perfect alignment
- gap : set to .oo14 per manual
- spark: external check looks strong
- fuel from petcock : good
- battery: fully charged

Sill only firing on right cylinder !

Assuming timing and gap are correct - I think this points to a carb issue- y/n?
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Old 10-01-2014, 09:48 AM   #13
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The only spark plugs I'd ever use on a CB350 are NGK B8ES. If you need something hotter like a B7 or a B6 you still have tuning or mechanical work to do to the engine to get it running right.

Ignition timing is a function of your timing chain, breaker point gap, and valve adjustments. You have to verify that all three things are set right.

Since it runs on the one side, you're down to a coil or breaker points possibly being messed up, but probably not.

Have you pulled the carbs off of the bike? If so, focus on the pilot jet circuit. The problem if you take them apart is you need new tiny o-rings for the jets and i'm not sure if you wanna do that or not. It's up to you.

If you're looking for the cheap/fast try, pull the left carb off and make sure the pilot jet circuit is cleared out enough you can spray carb cleaner into the brass jet and have it come out in the throat of the carb. Remove the air mix screw and do the same thing.. spray carb cleaner in and make sure it's coming out inside of the carb by the butterfly.

Pilot circuits are the first to plug and are a big road block in making a bike start or idle.
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Old 10-02-2014, 01:09 PM   #14
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On the side that is not firing, pull off the spark plug cap and then set it back on the sparkplug, but don't click it fully home.

See if that does the trick.

Guessing plug wire or cap issue
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Old 10-06-2014, 01:08 PM   #15
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O my son's CB360 I had a problem with the points on that side grounding out against the points cover when I put it on. 360's and 350's are pretty similar but having never worked on a 350 I don't know if that could be the culprit or not.
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