Black soot in rear cylinder tail pipe of my 1971harley.

that's only a problem if you have a separate exhaust pipe for each cylinder. if you have a Y pipe on the rear head where the exhaust splits, the left side pipe is pretty much just for looks. the HD designed system acts like a 2-into-1 with a left side pipe for symmetry and cosmetic appearance. that's why you get a good exhaust pulse out of the right pipe and a wimpy exhaust pulse out of the left side. but if you do have a separate pipe for each head there can be several causes for those symptoms. - since the front cylinder runs leaner than the rear, your jetting could be a little off. ok for the front, too rich for the rear. - you may have low compression on the rear cylinder. you need to verify the rear and front are within 10%. - it can also be caused by a bad plug wire or spark plug. - you could have a bad exhaust valve seal in the rear head. check for anything that could result in low compression pressure, poor spark, too much fuel or a way for oil to get into the exhaust port. let me know if you need more help. Steve

I do have separate pipes, drag pipes. W onder between rings or valve seal which would more than likely be cause.Wouldn't rings or seals cause smoke?Could it be one or other of jets being too big, main or intermediate?Thanks

that's why I suggest you perform a compression check. if compression is good, it's more likely a valve stem seal. if compression is low it could be rings or valves. a leak down test will tell you which. S&S carbs come with rich jetting. if you didn't jet the carb, it's too rich. follow the S&S tuning instructions that came with the carb OR get them off the S&S web site at [https://www.sscycle.com/](https://www.sscycle.com/) you need to tune the main air bleed, intermediate and main jets. let me know if you need more help. Steve

What do you think compression should be on old 74 cu? Will check jetting again. Thanks

it depends on the condition of the cylinder, piston and rings. I'd like to see it over 100 psi, closer to 120. if it's 90 or less you need to address it. Steve