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KX450f backfire problems

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My 08 KX450f has started backfiring and popping like crazy when I let off the throttle. It had run find before sitting up for a while. I cleaned the card really well but nothing else has changes. It runs fine and strong when on the gas but backfires and pops like crazy as soon as I let off. Does anyone have an ideas what the problem might be?

Thanks

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A few simple thing you can check are,

Make sure your hot start cable is closing all the way

Check your jets make sure they are clean

Confirm you pilot jet is the correct size as well as air screw set properly

Check for air leaks around your intake boot

These are just a few things off the top of my head, good luck I hope this helps.

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Well, deceleration popping is a sign of a lean condition.

- header crush washer leak

- header junction leak

- hot start stuck open or corroded

- loose/leaking carb boots

- missing o-ring from fuel screw

- fuel screw out too far

- too small pilot (or clogged pilot)

- vacuum plate seal on slide needs replacing

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I have the same problem on my 06, ill have to try to do some of those. I initially thought my valves were going bad or the top end needed done!

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I have an 06 as well and i have this problem also. i had a lot of problems with jets and timing in the past but iv got the timing sorted out and iv had 2 different people (who know what they are doing) look/adjust the jetting. iv also replaced my hot start plunger and spring and pvc thread but i cannot tell if the plunger is seating properly (or does someone have a trick up their sleeve for this). i have fantastic power and acceleration but as soon as i lay off the gas its popping like crazy. my next move was to take it to the shop and get the vavles done but ill go through the above checklist and go from there and hopefully save me some $.

Edited by jacobenn

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Would definitely check the exhaust header gasket and exhaust header assembly. Had same problem and the exhaust header was cracked right after gasket. Ran perfect otherwise and has since changing parts. Good luck and let us know what turned out to be your problem.:thumbsup:

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I have the same problem with my 06, cleaned the carb and jets but that didn't help so I put a carb kit with stock jets and that helped a ton but did'nt go away completely. I had put a FMF pc4 slip on on it and for some reason they didn't put a clamp on style for this one the instructions said to use a high temp silicone to seal the header muffler connection, this has never worked and I'm sure this is where the problem is so I'd check your exhaust connections as well.

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I have the same problem with my 06, cleaned the carb and jets but that didn't help so I put a carb kit with stock jets and that helped a ton but did'nt go away completely. I had put a FMF pc4 slip on on it and for some reason they didn't put a clamp on style for this one the instructions said to use a high temp silicone to seal the header muffler connection, this has never worked and I'm sure this is where the problem is so I'd check your exhaust connections as well.

i put a FMF power core 4 on my 06 and i didn't get the clamp either. i put a bead of high temp silicone on the header muffler connection then i used the original clamp from the original muffler. the original clamp is too big so i had to use the insert that sits inside to muffler side of the header muffler connection (sorry i dont know the actual name of this but its grey in colour and about 25 to 30mm long by memory). this fitted around the FMF, between and pipe and the clamp with a slit cut down one side to allow the insert to fit around the pipe.

if you give this a go then get back and let me know if this makes your bike better or worse. Cheers

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I'm gonna go try it right now, assuming I can find the old clamp.

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when I came home with my o7 new to dirtbikes the bike ran great my first tank of gas I put it was 93 with octane booster manual said don't run anything less that 91 so I assumed that the higher the octane the better. Clearly I was wrong bike ran like crap backfiring horribly. Since I new that was the only thing that was different It was a quick fix drained it threw the gas in the old pickup and put 93 octane with 10% or less ethanal bike ran good as it ever did After some reaserch and understanding jetteing I understood what I did wrong. So my advice would be try simply change your gas maybee you got a bad batch I now it sounds weird but how much can you trust gas stations I get mine now from the same one for the last few years and have never had a problem.

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I got my 06 running strong now!! I had to change my main to a 48. Problem solved, no more decel backfiring!! Thank god it was not anything else!! Thanks for the tips.

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