Honda CB350 Running on 1. Has spark, compression and fuel

I've recently finished building a CB350 and have ridden it over 100km since completion with no problems at all, revs out clean, pulls well through all the gears, etc. Last week i was riding home and it stalled at a set of lights, not unusual as i was playing the with the idle speed screws before leaving. Managed to get back home without any issues and everything appeared fine.

Once home i attempted to start it again which proved very difficult, always started on 1 or 2 kicks. Once i managed to get it started after about 20 kicks i noticed that it was running on only the right cylinder only and nothing out the left. Over the last week i have been chasing this issue and have done the following with still no success in getting the left cylinder to fire. It will run fine on the right cylinder with an occasional huge backfire from the left side out the exhaust.

Compression check measures 170psi on both cylinders
Pulled apart left carby, cleaned it out with carby cleaner and checked diaphragm and jets all ok
Checked timing, its spot on
Sprayed aerostart down the carby while running, still doesn't fire
Checked the spark plug is firing under compression with a color tune plug, no problems
Checked exhaust to make sure there was no blockage
Checked battery for full voltage, reads 12.6v
Pulled off the valve inspection covers to make sure the valves are opening and the clearance was correct

At this point i have no idea what else to try and can't see any reason why it shouldn't run on both cylinder but it just doesn't. No matter what i try it will only run on the right with the occasional huge backfire out the left.

Any advice would be much appreciated. Thanks in advance.

Re-check for spark. You may be shorting out to the points cover, a common problem. Also, you may have a bad condenser. Even with a bad condenser, you might get what appears to be a spark, but it will be a shitty, weak, yellow spark that will not fire reliably. Swap the two condensers and see if the problem moves to the other cylinder.

Thanks for the reply. I've had it running with the points cover off after checking the points gap so i don't think that will be the issue. The CB350 coils share one condenser which i replaced maybe a month ago when i replaced the points. I'll put the original back in and give it a go though.

Was that voltage reading taken with the battery sitting overnight (and not hooked up to a charger)?

12.6v was the reading after being charged. When I got it home and struggled to start it the first thing I did was put the battery on the charger without testing what the voltage was.

Sounds like you've found your problem
Take it down to auto zone and have them load test it.

350s do NOT like to run on a partially dead battery.

J.K.L.P said:

The CB350 coils share one condenser

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No they don't. There are two separate condensers. They are both attached to a single mounting bracket, but they are electrically separate components. The mount is ground, common to both condensers, and each wire goes to a separate condenser.

You are talking to an experienced and knowledgeable motorcycle mechanic that used to work on these bikes when they were new. This ain't my first rodeo, and I am not making this shit up as I go.

I agree with the battery. I dealt with only running on one cylinder on startup when I couldn't electric start it. I'd start it with the kicker then let it idle on one cylinder till the other started working. Once I replaced the battery it hasn't done it since, but since an experienced and knowledgeable motorcycle mechanic that used to work on these bikes when they were new is stating that it is a condenser, he is definitely right and you shouldn't weigh any other options.

You might note that "noobs" who waltz into the forum with a chip on their shoulder and have a proclivity for stepping on their dick with long established members are not likely to get much help in the future.

AlphaDogChoppers said:

You are talking to an experienced and knowledgeable motorcycle mechanic that used to work on these bikes when they were new. This ain't my first rodeo, and I am not making this shit up as I go.

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My apologies. My lack of understanding led me to believe that they shared a condenser. I appreciate your assistance and will be checking it. Thanks

J.K.L.P said:

My apologies. My lack of understanding led me to believe that they shared a condenser. I appreciate your assistance and will be checking it. Thanks

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No, No. I apologize.
My "noobs" remark was not directed at you. My mistake.

Good stuff. I'm just curious to see what the issue really is. I apologize too. I wear my flame suit at all times so I'm not really affected by it. Just hate all the internet ego. This is a level playing field. No need to flex your years of experience if you're already so right about it right?

hello i was wondering you said you need to condensers or coil? because i only have 1 condenser and 2 coils i included a picture and i haven't started it up yet and was hoping for some help maybe?

that is actually 2 condensors, but it is almost impossible to tell from the shit quality image

I think it's a good idea to make sure your coil bracket is solidly grounded to where it mounts to the frame(bare steel on frame to unpainted metal on the coil bracket)so that your condensers will be properly grounded.The condensers should be replaced if they're orig. CB350 42+ years old.I don't think I would run any condenser that's been left idle for more than 5 years;you can pick up a new pr. from Autozone,etc.
I purchased a new pr. meant for a Chevy Biscayne(they were in Autozone boxes that actually had a date on them !)and mounted them to my coil bracket on my CB350K4 after making sure it was properly grounded to the frame and they work very well.
The condensers are one fix but you should check the offending cylinders main coil also;it doesn't take too much to burn out a coil if you leave the ign. key in the 'On' position w/o the engine running for more than 5 mins...
How do your contact surfaces of your points look? you may need to remove both points and carefully clean/smooth them off so they make solid contact w/ clean contact surfaces;I had a cond. go bad on me a few times and that will usually burn the points crispy especially if you have a Hi Perf. aftermarket coil installed.

Okay thanks and it was a crappy phone pic. grcamna5 you are suggesting that i change my coils and condensers? (the bike was sitting for 25 years when i picked it up) I heard that condensers never go bad and the coils you just test to make sure they act as a capacitor should? and the points are spotless and clean.

I think you should make sure your coil bracket is grounded to the frame which'll ground the condensers.I'm not sure because I'm not in front of your bike but you may have a bad coil..;the main thing is don't leave your key on w/o running your engine,it can burn out the coil.
The bike can(not for long) run w/ a bad condenser but will cause lots of sparks to foul/burn your point set in short order.We used to install car coils(in the 70's)on bikes for a hotter spark(RD350's,etc.) but we needed to upgrade to car condensers also otherwise the cycle condensers wouldn't last long.
Condensers have a certain 'shelf life'..the insulation inside breaks down over time.I was told by a long time vintage M/C mech. that if you run the bike regularly they'll be OK even if they get old(but not always),but if they sit idle for more than 5 yrs. it's better to replace them w/ a fresh pair.

do a full ignition tuneup,make sure the battery is fully charged a low bat will drop a cylinder,get new sparkplugs and new plug caps
the FSM explains the tuneup
while you are in there make sure the advancer is in good shape and free to snap back

grcamna5 said:

How do your contact surfaces of your points look? you may need to remove both points and carefully clean/smooth them off so they make solid contact w/ clean contact surfaces;I had a cond. go bad on me a few times and that will usually burn the points crispy especially if you have a Hi Perf. aftermarket coil installed.

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This.

Surprised nobody else has chimed in with this, and it's the easiest thing to check. The oil seal behind the advance and points plate can go bad, allowing oil to foul one of the points. Or they're old and worn and need replacing. Or they're not gapped correctly. Rule this out first.

he says in the first post that they are spotless and clean