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Discussion Starter #1
I've been rebuilding a 71 CB350. I just rebuilt the internals of the engine (piston rings, cam chain, gaskets, valves lapped)... Carbs cleaned and adjusted, valves set properly, timing is showing spot on both cylinders with a strobe while running. Can't find an air leak around the carbs/intakes. New coils and condenser. Plugs and boots are pretty new as well. Main ground wire has a good connection to the frame.

The bike pulls hard, but backfires/pops (not super loud--maybe through the carbs?) on deceleration. It happens worse while engine braking than just coasting, but it does happen just coasting too. Yesterday, It started doing it on a deceleration and then slowly died completely. After coasting off the road, The ignition seemed to be totally dead--no headlight, starter motor wouldn't click or spin, etc.... Turning the key off and on a few times got it to start, but after a few seconds of idle, it died again with dead electricals. It happened a few times, before it came back to life and ran fine for the mile home. Battery tested at 12.6 just sitting with the bike off. I didn't have a volt meter to test while riding. What should I check next? It's possible the popping on decel is unrelated to the ignition-dead issue.
Thanks!
Stephen
 

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The most common cause of popping or backfiring when closing the throttle is an air leak in the exhaust system. It could be at the pipe to cylinder head or where the head pipes connect to the mufflers, presuming that you have separate mufflers or tail pipes. If you place you hand near either of these joints and feel even slight puffs of air there are leaks. The ignition problem could be related if something is triggering a random spark plug fire for some reason. You indicate that the timing is correct but you don't mention the points gap if you still have points.
 

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Charge the Battery.

12.6 is a DEAD Battery as far as these bikes are concerned.

They need a fully charged system to function.
It is a Battery excited Ignition System.
The charging system on these machines is weak and really runs a a deficit until approc 3000 - 3500 RPM's
 

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Discussion Starter #5 (Edited)
I'll check, but the battery is less than 4 months old and seems to hold a charge with no issues. I thought 12.6v sitting voltage was pretty standard for one of these batteries that's been sitting overnight and not running. The voltage rises when it's running and revs as it should.

I think that I found the source of the dead ignition issue. The wire running to the main fuse was mostly broken through, and came completely apart when I wiggled it. I since reconnected it and it ran without dying for 20 minutes last night no worries. On the popping, I'm starting to suspect something wrong with the idle circuit of one of the carbs. It doesn't like to idle, and seems like no amount of throttle or AF adjustment will make it happy. When I let off the throttle while riding, it starts to gurgle and pop, which I feels is likely just an amplified condition of what's happening when it's trying to idle. I didn't fully inspect everything inside the carbs, as the P/O had just rebuilt them and they looked clean. Time to dig in there and see what I see.

Thanks!

Charge the Battery.

12.6 is a DEAD Battery as far as these bikes are concerned.

They need a fully charged system to function.
It is a Battery excited Ignition System.
The charging system on these machines is weak and really runs a a deficit until approc 3000 - 3500 RPM's
 

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Discussion Starter #6
final update. They WERE unrelated issues. The fuse wire repair fixed the dead ignition system. Re-sealing the felt washers on the butterfly valves and properly syncing the carbs fixed the backfiring issue. Only posting this because I like to report resolutions. Cheers!
Stephen
 
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