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Hey All,

I have a 1972 CB350 that I'm reviving. So far it's been a fairly straight forward project but I've been struggling to resolve this issue.

When I accelerate or even go up a steep hill, both cylinders run great and pull strongly. Once I get up to speed, (regardless of speed or RMP) the left cylinder kind of sputters like it's only firing every other stroke or every 3rd stroke. Any thoughts on what might be causing this?

L&R Cylinder: ~180 PSI (warm engine, carb bowl empty, plugs out, throttle wide open, no choke, not very expensive gauge)
Ignition Control: Common Motor Collective's Shockwave Electronic Ignition System (I've double checked the static timing 3X since installation and it's still spot on after 70 miles)
Carbs: Factory Stock - I've cleaned them out quite well and gotten a carb rebuild kit with stock sized Jets to go with stock airbox and similar to stock muffler. I inspected the diaphragms and did not notice any cracks or tears and found them to be quite pliable, however, I know how temperamental these things can be.
Float Height: 23mm with brand new floats
Ignition Coils: Left side came with the bike and has a good spark when out of the engine. Right side was replaced by me
Valve Clearance: Recently checked and adjusted
Air Box: Stock
Spark Plugs: NKGB8ES - (Since I have you all here, my plugs seem to be darker and more sooty than I would expect and think it's running rich but everything is at stock settings. Should I try to mess with it to try and make it run a little leaner?)

Should my float height be different? Do you think I should get a new ignition coil? Should I take another pass at really REALLY making sure the carbs are clean? Does this sound like a diaphragm issue?

Let me know if you would like any other info when trying to help me solve this issue.

Thank you!
 

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Sounds like you have done everything. one thing you didn't mention is the airfilters. If they are old that may contribute to the dark plugs. Doing the foam conversion to stock airfilters is a very worthwhile modification. I believe there are after market airfilters for the CB350 but they are $$$. The other thing is balancing the carbs. Did you bench sinc before they were installed and then synchronized them once the engine is running? If it was me I would replace the other coil. I just don't trust 40 YO electrical components.
 

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You might check for an air leak in the carb mounting area. Intake air leaks are more noticeable under high vacuum conditions(idle, cruising under light load) and tend to have little or no effect under low vacuum (high load, wfo) conditions. The fact it pulls well under high load is a good indication the ignition system is working well.
 
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