Motorcycle Repair: 1980 yamaha 400 s, 2 stroke engines, vacuum conditions

Question
Hello sir,
 I have the 80 yamaha, got bike with 8975 miles on it woudn't run.  cleaned carbs the jets, now the bike will start and idle like new.  When under a load above 6000 rpms the boggs down and loses power and black smoke out the pipes.  I have adjusted the floats and changed main jets. should I readjust the floats of find smaller jets?
thanks
chris

Answer
Chris,

if you have a clean air box and air filter and
the choke is not stuck or needing adjustments,
it might need smaller main jets.

Always do a short fast run after jet changes
and check the spark plug color for tuning
clues.
http://verrill.com/moto/sellingguide/sparkplugs/plugcolorchart.htm

The black smoke means the fuel mixture
has either too much fuel or not enough
air flow. Plugged pipes can be a problem
but more so on 2 stroke engines.

A faulty vacuum operated fuel petcock can also
cause excess fuel to be sucked in but usually
at lower speeds or high vacuum conditions.

If you put bigger jets in already they may
be too large. The engine can only
handle so much fuel without other
performance enhancing changes like
pipes, compression, porting, cams etc.