1970 CB350 - Loss of power on left cylinder

I’m fixing up a barn find that’s been sitting for a long time, since 83 I believe. Spent the last 2 months cleaning and redoing the gas tank (3 step process), new battery, ''new'' plugs, condenser, points, coils in working shape, new fuel lines, soaked and cleaned carbs spotless. All the electrical works.

It starts and will idle great, but only the right cylinder seems to be getting full power. The points are within spec, and both cylinders are getting fuel and spark. It sometimes will backfire through the left carb, and the left exhaust will make an almost whistle sound, like its choking. There is some pressure exiting the left exhaust, but nothing like the right exhaust. Both exhaust manifolds are hot. I tried switching mufflers, still no effect. Last time after running it for a while, when I turn it off, smoke exits from the left carb. Not a crazy amount, but still some smoke.

I’m thinking maybe the timing is off and/or the valves are out of spec? Anything else I should check? I can only work on the bike two days a week because of work, so trying to plan in advance anything I should do.

Thanks for your help on this matter , I have found myself totally stomped this time around! ???

check if its getting fuel take of the air cleaner and spray carb cleaner in there and see if it fires. I know on my cl350 the right carb sometimes has a hard time sucking up fuel

Thx for your quick reply Foundation,

When I take off the carb(s), there is sign of fuel in the lower carb bowl, so I am not sure that it is fuel related necessarily, what do you think?!?

Clean an sync the carbs. Make sure timing is set. Check valve clearances (this is VERY IMPORTANT). Check compression.

I'm thinking its a fuel problem. mine will back fire through the carb when I get low on fuel. my tank has to be at least 1/3rd full for gas to run to both carbs Not sure why.

It could be fuel, but it could be other things as well.

You really need to go through and set EVERYTHING to stock. Timing, valves, make sure the carbs are EXTREMELY clean and go from there.

Are you 100% sure that the coils and ignition wiring are good? A weak spark could cause this situation also.

Theres a whole slew of reasons that you could be having issues.

Good luck man. Hope ya figure it out soon!!!

yeah if its not fuel I would lean toward weak spark. I don't see why the valve timings would be any different but Better be safe than sorry getting smashed valves and a destroyed piston would be no fun :[

Thats what hes looking for... Bent/worn/broken valves or guides. Worn valve seat, etc...

When you pull a bike out of a barn or garage you have no idea what the POs have done to it. Before you start firing her up and revving away, you need to go over the motor from top to bottom to make sure everything will stay together.

Something simple, that wouldve been an early fix can quickly turn into a VERY expensive problem at 6k RPM.

Thanks Guys!

Von, I'll get right on it, well, first chance I get that I am not working(Thursday)! ;D
Hope I find some answers on the bike, but I'll be sure to report back as soon as I can.

Ok,
Adjusted the valves to spec as well as the timing. Spark on both plugs but the weird thing is now there is power out of the left side but the right side is not getting hot now ??? :-\ . Crazy thing I say, starting to think if it's not coil or fuel related, although there is fuel going to both carbs and as I said before, I soaked them both for 2.5 days and cleaned them.
Aaaaaaaaaaaaah, geeeeeez.........

Purchased 2 new universal single lead coils as well as new wires and plugs.
Everything is on the bike and tested it getting a spark on both sides.
Problem now is that I keep blowing the fuses!!

Weak connection to ground may cause you to blow a fuse. Shorts will do it, too.

Quick update. Found the coils were grounding out, and fixed it. Great spark now, but still the loss of power. Going to recheck the timing, and post some results. I switched my carbs with my buddy's running cb350, and had no effect on the power. With the new coils, I can rev to about 5000, which is a bit higher than before,but still hits a brick wall at 5000.

Also did a compression test and got almost perfect #160 on both cylinders, so I'm happy about that

Thanks guys ! Back to work

May be a dumb thing to say, but have you checked the cam-chain/tensioner? I have a cb125twin and it would'nt go over 4000rpm. Problem was a worn cam-chain, causing a small delay on the valves at high rpms. Also, check your spark-advancer (if a 350 has one) new springs will do good to it! good luck!

Hey Bert,
Turns out the advancer was stuck, so thx for the info!
;D

Sweet! Glad i could help, now do the ton!

Bike is running better and better. Got her running somewhere around 7,000rpm and got her up to 60 in a hurry down a nice stretch of road here in MI. Only last little issue that I can think of is that I'm not getting power from my right carb until I'm pulling over 2,500-3,000rpm, and then it kicks in. Strange......... Carbs are clean, coils are new, sooooooo............yeah.
Will update again when I climb the next hurdle , but for now I will try to riiiiide as much as I can ;D !!!

check if your battery is fully charged, if it isn't they usually will only run on 1 cylinder because the charging system can't provide enough power to both

Or it still has a partial blockage on the pilot circuit in that carb. Sometimes soaking is not enough. I'd pull the carbs again and use a can of carb cleaner to blow through every jet and orifice. Do one circuit at a time on one carb and repeat on the other to compare "spray patterns". Going back and forth from one carb to the other is the only quick and easy way to see if there are any significant differences between carbs.

+1 for what teazer said. If the battery is causing one side to die out, it's usually at higher RPM, not lower.