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06 kx450f, where do i begin?

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hey guys, not really sure where to begin here... bought the bike used in spring of 09. the guy flipped it over backwards & broke the tailpipe, handlebars & rear fender. i replaced everything & installed an FMF factory 4.1 slip on. it has always had a bit of a decel. pop/backfire but i recently installed a powerbomb header & it seems to have gotten worse. the bike ran fine otherwise. once i installed the powerbomb header i was having problems with my boot melting to the pipe, so i wrapped it with that fiberglass exhaust wrap from head bolts to the midpipe/header joint. i also installed all new plastics/ seat/ & fuel tank. after this my bike has not ran propeerly. at first the bike would run out of fuel & not start for a while(or so it seemed) i cleanded out the fuel filter a few times & checked all the hoses. PROBLEM, with the added depth of the tank the hose was kinking where it left to meet the petcock, a quick turn of the fuel valve & it unkinked & seems t o be getting gas now. while it was apart i ckecked the gascap too, thinking it could be clogged. the bike starts fine, runs good but still has that popping & after about 5 minutes of riding today it began to die {as if starving} (on idle) in the apex of a turn with the clutch in. a quick dump of the clutch & it was running again but the problem persists. i just cleaned air filter(foam filter products unavailable do to location, used mineral spirits & chainsaw bar oil( , changed oil & filter, new antifreeze aswell. the guy who owned it before me must have played with the carb, i went as far as returning the fuel/air screw to stock position & it idled extremely high, backfired on idle! & spewed antifreeze from the vent ( this has been dripping now for a couple short rides). i returned it to where he had it (a whopping 3 3/4 turns out!!) & it idles normal again, but still has a bit of a pop. i dont know if any of this is relevant but i dont want to do any unnesscary dissambly/ "tinkering' if i can avoid it. also the bike seems to be leaking a SMALL amount of oil around the spark plug hole, where the cam cover meets. im ordering a new rad cap, gonna clean & return the carb to factory spec,  put a new petcock & fuel lines on just for piece of mind, & check for exhaust leaks. i assume i should check the intake boots aswell. any advice is REALLY aprreciated. where should i look first, what the best way to check for leaks? should i send the motor out if i cant fix it or save for a crate motor over the snowy season? i lost guys someone help me, i loved my baby girl till she started acting up... now i dont know what to do with her...

EDIT: im not sure if the rad cap is shot or if its over heating that bad but i cannot ride it now with losing that precious green liquid all over

Edited by KxFlinFl0n

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hey guys, not really sure where to begin here... bought the bike used in spring of 09. the guy flipped it over backwards & broke the tailpipe, handlebars & rear fender. i replaced everything & installed an FMF factory 4.1 slip on. it has always had a bit of a decel. pop/backfire but i recently installed a powerbomb header & it seems to have gotten worse. the bike ran fine otherwise. once i installed the powerbomb header i was having problems with my boot melting to the pipe, so i wrapped it with that fiberglass exhaust wrap from head bolts to the midpipe/header joint. i also installed all new plastics/ seat/ & fuel tank. after this my bike has not ran propeerly. at first the bike would run out of fuel & not start for a while(or so it seemed) i cleanded out the fuel filter a few times & checked all the hoses. PROBLEM, with the added depth of the tank the hose was kinking where it left to meet the petcock, a quick turn of the fuel valve & it unkinked & seems t o be getting gas now. while it was apart i ckecked the gascap too, thinking it could be clogged. the bike starts fine, runs good but still has that popping & after about 5 minutes of riding today it began to die {as if starving} (on idle) in the apex of a turn with the clutch in. a quick dump of the clutch & it was running again but the problem persists. i just cleaned air filter(foam filter products unavailable do to location, used mineral spirits & chainsaw bar oil( , changed oil & filter, new antifreeze aswell. the guy who owned it before me must have played with the carb, i went as far as returning the fuel/air screw to stock position & it idled extremely high, backfired on idle! & spewed antifreeze from the vent ( this has been dripping now for a couple short rides). i returned it to where he had it (a whopping 3 3/4 turns out!!) & it idles normal again, but still has a bit of a pop. i dont know if any of this is relevant but i dont want to do any unnesscary dissambly/ "tinkering' if i can avoid it. also the bike seems to be leaking a SMALL amount of oil around the spark plug hole, where the cam cover meets. im ordering a new rad cap, gonna clean & return the carb to factory spec,  put a new petcock & fuel lines on just for piece of mind, & check for exhaust leaks. i assume i should check the intake boots aswell. any advice is REALLY aprreciated. where should i look first, what the best way to check for leaks? should i send the motor out if i cant fix it or save for a crate motor over the snowy season? i lost guys someone help me, i loved my baby girl till she started acting up... now i dont know what to do with her...

EDIT: im not sure if the rad cap is shot or if its over heating that bad but i cannot ride it now with losing that precious green liquid all over

I would check the valve's and shims and make certain that they have proper clearance and are sealing the combustion chamber before doing anything else.

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New pilot jet.

Then check/replace head gasket.

That might do it.

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