Is your engine sluggish after installing a new air filter, or fuel jet? Here are a few reasons why:

#1 – there is an air leak somewhere in your system, possibly between a mating surface like the carb to the manifold.

#2 – the fuel jet or emulsion jet may be fitted incorrectly. Check to make sure the emulsion jet orientation is correct, and the fuel jet is screwed all the way in. Often the emulsion jet falls out when installing the fuel jet and that can cause a lot of problems if it’s fitted back in upside down.
You can check the correct orientation of your fuel jet and emusion tube by clicking here: http://cutpriceracing.com.au/phpbb3/viewtopic.php?f=13&t=944

#3 – the jet may be too big for your engine – some engines do no flow as well as others, this is often because the head design is constricted and/or different. Try installing the original jet to see if it improves.