How do you know when your power valve is shot?  Is there a physical change to look for in the valve itself or only the way the car runs?  I was given a used 850 to try on my car and was told it had suffered some backfires through the carb so I'm suspicious of possible damage.  

sounds like it is on the bench now....

you can take the power valve out and test it with your lips.. on the  diaphragm side.???

on the engine... if it will idle with the idle mix screws turned all the way in...  the power valve is blown...

you might want to check while you have it apart that the throttle blades close tight in their bores..  this had to be done with the base plate held up to the light...

nitrous backfires can also bend the throttle blades...

backfires that start a carb fire can melt things...     cranking the motor WITHOUT pumping the gas pedal will put out an internal carb fire..  in most cases...

and for everybody who reads this...  most carbs have the tips of the screws deformed so they don't unscrew and get into the motor...  you can loosen them in most cases to re center the blade in the bore...  as sometimes the carb gets set down with the throttle butterflys open. this moves them off center..  

Thanks waynep712 for that piece of info on sucking on the diapham. I saw in Jegs after I posted this question a Moroso tester that you have to hook up to a vac pump but this sounds so much easier and I always wanted to kiss my carb as my wife wants everyone to believe my car is the other woman.  

http://www.carcraft.com/techarticles/ccrp_0306_carb_questions_answered/carb_jetting.html