Puff of dark smoke from exhaust?

[Petrolhead]

When I have the Sport on its stand and its warmed up, a blip on the throttle and then snapping it shut to enjoy that lovely single blat blat, I get a puff off dark smoke from the can. Is this something to worry about or is it normal. I did wind the pilot screw out about 7-8 turns.

It does not smoke at any other time.....I was wondering if it was due to such a large piston drawing up oil when the carb slides are closed due to the large vacuum, is that logical?

[mincehead]

Oil usually burns a misty blue, dark / black smoke is indicative of a rich mixture.

[dandywarhol]

Why did you wind out the mixture screw so rich? 5 turns further in might sort your problem.......................

[DAVID THOMPSON]

these bikes are bad about backfiring on overrun
and some have found that 4 to 6 turn out helps bring it to a level where it can be lived with,,

my ideal is get rid of the cv carb for some thing else
i rode BMW from 1969 to 1999 and still have some
the bing cv on the r75/5 i had need to be pitched in the junk behind the shed

the r90s i owned for 28 years had a dell'orto and was a winner

cv,s will work but finding some one that can work out the bugs is not worth the effort

the big problem is the gasoline is CRAP

[Petrolhead]

It was the back firing and popping on a sharply closed throttle on the over run which made me turn the screw out so much. I got out to the garage and turned it in to 3 1/2 turns , it still runs OK on the stand, a bit less easy on the tick over so I had to turn the tick over screw in.....the tick over was best when it was out further though

But the black smoke is almost eliminated on blipping the throttle, just a whiff of vapour and the end of the can is dry , so a result there, thanks all

[dandywarhol]

There's a diaphragm on the side of the CV carb which is designed to stop the overrun popping. If it has perished, you need to adjust the mixture screw to compensate for a weakness caused by the diaphragm. Worth a look, especially with ethanol in the fuel

[Petrolhead]

Thanks for the tip, as we both know its freezing in Scotland just now so i'll wait for the weather to get warmer before I go for a roadtest.