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Have a 68 A65L with 750 top end. Starts right up and runs great. I was told ( by riders behind me) that it puffs a little black smoke when I downshift. Other than that there is no other smoking issues. Runs great . Wondering if I should try lowering the carb needles a notch. (932 Concentrics). Plugs a little black but dry. Any thoughts?

When you downshift your throttle will be closed, so it’s quite likely that the inlet guides are pulling oil past them. I had this same issue when I first fitted my Ed V pistons with total seal rings… it’s a good indication that the rings are doing their job properly (if it is oil coming past the guides that is)

Lowering the needle would be my first move as well. If the needle jets have some mileage on them that could well be the reason for the black puffs, either way, lowering the needle should give you a clue.

I think Allan was too quick on the draw this time, as he didn't notice that the puffs were black. He knows better. ;-)

SR

Your right SR, oil should be blue, I was working on the thought that the person following who pointed it out was not good with their diagnosis. And the plug condition being a big clue… black and dry is too rich.

The only thing I would add is to mark the throttle off at closed then every 1/4 of the slide position being open and run the bike at the various throttle under a moderate load. Needle position would be 1/2 to 3/4. Needle jet size would be 1/4-1/2 and slide cutaway off idle to 1/4. There is always some overlap and you might find that if you fit a smaller needle jet, you might also have to raise the needle.

I tend to bring the rpm's up a bit prior to downshifting for smoother transition, and oil smoke is whiteish blue not black. ill try not throttling up before shifting and see if there is a difference, but never had this issue on other bikes. Thanks.

Without knowing the jetting etc on the carbs it's rather difficult to know whether you
are running a rich mixture and finding that it goes even richer at higher rpm or that
the carbs are worn etc.
In general the 750 kitted engine ran identical carb setups to the 650 on street bikes,
but you have 32mm carbs so that changes everything. They will come into their own
at around 6000rpm, below that they will tend to spoil everything.

Do blip the throttle when changing down , the shift change is much better for it, dropping the needle is worth a try, black smoke is fuel, what size of carbs ? if stock 30 mm the 750 big bore needs a smaller MJ, than the 650, somewhere around 190 -180 with filters , a size more without, if its Conc 32 mm carbs around 220 works for me.When the needle is too high the bike will accelerate well from a handful but blubber a bit at cruising steady.

Generally BLACK smoke is caused by mixture being to rich .. so lean it down .

One thing i do find amusing is how we all ask anyone following for a smoke report. The Norton or Triumph owners following BSAs are usualy happy to enviuosly obligue

i went on a camping trip once with a friend who rode his H2.

i rode behind him and about choked