Motorcycle Repair: motorcycle suddenly has a top speed of less than 40mph!!, yamaha xs 400, clutch discs

Question
My 1979 yamaha xs 400 was running okay (not great, but passable) for a while.  I got a flat one day and changed out the tire (with a mechanic). After a day or two, I discovered my motorcycle wasn't accelerating properly, it'd move very slowly after a stop, as though it were in fourth gear.  And then it would only reach top speeds of around 40mph, with full throttle and the engine roaring like mad.  I put in some oil and rode it again today, and it drove okay for the first five minutes and then would only go something like 20mph no matter what gear I was in or how much gas I was giving it.  What's happening and what can I do?  Is it the transmission (the shift lever seems to be shifting okay) or the carbs?  Please help asap.

Answer
Thesoorae,

have you checked if the wheel is turning
okay after you changed the tire? Are the
brakes dragging? If not it sounds a bit
like the clutch may be slipping which
would require replacing the clutch discs.
Does the engine speed up but the bike
doesn't move?

-Do both sides of the engine warm up the same?
Is it running on both cylinders?
Feel the pipes before they get too hot
and see if they both warm up the same.
Don't burn yourself.

-make sure the air filter is not plugged or blocked.
-is your choke off?

-If the cylinders are both firing, check your
fuel tank and petcock valve, see if fuel
is getting to the carbs. Try draining
the float bowls.

-Check your spark plugs, see if they look
wet or dry. Try some new plugs.

-Check the engine compression,
should have at least 130 psi on each
cylinder to run good.