I am new to the motocross scene so can you help?
Answers:
with a free flow exhaust system, this is where it statrt getting expensive. new filter and exhaust will need a hotter plug and and bigger main jet in the carb.
A high performance expansion chamber (exhaust pipe). If the bike runs OK (power wise is acceptable to you), then it's not that necessary.
Air filter is and air filter on dirtbikes although i like the "Twin Air's", reeds like the "V-Force Delta Reed Block", and a good exhaust like the "Pro-Circuit" shorty. you might port and polish the exhaust valve for an extra punch!
More air in, more air out = more power in anything with a combustion engine.
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