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I'm not sure if this is an engine or carb related question, but I just put it here anyway

I tried to start my 1974 cb450 and I noticed that when the cylinders start to fire, there is smoke that comes out of the carb on the right cylinder. The bike wasn't starting for a while, just a couple pops of it trying, but not actually running.
There was one point where the bike did start to run and I'm not sure if there was smoke coming out at that point, but I noticed that the right cylinder wasn't always running. I could hear the difference when both cylinders started firing and when only the left one is. (I can confirm with the heat of the pipes). The right side was coming on and off, and I wasn't changing the throttle position. When it was running, there were also a lot of pops, but maybe that was just from the right side not running all the time. I assume this is a fuel supply issue, because the engine has spark, but I'm not sure, as the bike was running fine before.
Any thoughts on what this could be?
 

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'Smoke' blowing back from a carb is not a good sign. I would start with a compression tester, just to compare one side to the other, and verify the valves are closing. Cold, the numbers will be low, but still give some indication.
 

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How long ago was it parked after "running fine"? sounds like carbs need cleaning. Smoke could be fuel vapor.
 

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Sorry for all the questions, but this sounds like tune up stuff. Are you trying to start up with the air filters off? Do the valves need adjusting? How are the points and timing? Is the gas old?
 
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