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05 crf450,backfire

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out of no where my 450 started running like shit and backfiring. last time this happened my hot start was stuck open,i took apart the hotstart and cleaned it and thats not the problem,it starts without using the hotstart when hot.

i was told to check the header bolts and intake boot to make sure there isnt any leaks.

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i was told to check the header bolts and intake boot to make sure there isnt any leaks.

did ya find any?.......if not check pilot jet for blockage.....check the valves........and back the fuel screw out 1/4 turn at a time till it goes away if it goes away

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didnt check yet i cant see why there would be i was riding,shut the bike off started it back up climbed a hill then once i got to the top i relized it was backfiring bad and its been like that since.the fuel screw should be fine. the pilot jet is possible ive had a clogged pilot on my xr before.

it runs normal at idle and acceleration but once you let off the throttle it makes a loud backfire.

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its not the fuel screw. ive been running my fuel screw where its at for awhile riding the same elevation without any problems so i know its not that.

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try ajusting mix screw anyway this new fuel we have is :):D it takes more fuel to make the same power plus it cost more..what a good idea!? :worthy:

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ok i took the hotstart apart cleaned it and whatnot its working fine,took out both jets cleaned them and im still having the backfire. i drained my gas and put in new gas as well. adjusted the fuel screw and nothing seems to help.

i guess i can check for exhaust leaks next,whats the easiest way,idk why id get an exhaust leak out of nowhere though,could the spark plug be bad?

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check for exhaust leaks, carb leaks, hotstart being open or the spark plug wire being frayed somewhere. Also check the stator output.

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try pulling sparkplug it could be debris on sparkplug

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OK, So this is the history.

My boy rides the bike all day Saturday without issue. Wake up Sunday and get ready to ride again. Bike backfires and runs extremely rough. Backfire is very loud and large flame at the exhaust.

Below are the steps we have gone through to date.

Change the plug - No difference

Look for exhaust leaks (Carb, manifold, and exhaust) - None found

Inspect and clean pilot and main jet - Good

Inspect hot start to ensure wasn't stuck open - functioning correctly

Inspect and clean carb - Found dirt in accelerator pump

Ensure Vacuum release plate is right side up - Check

Swap coil from known good 450 - No difference

Replace plug cap - No difference

Disconnect TPS - No difference

Disconnect kill switch - No difference

Check valve clearance - Good

Engine leak down test - Good

Swap CDI box from new 450 on showroom floor - No difference

Next step - Replace stator and pulse generator

Any other ideas?

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if you found dirt in acc.pump sounds pullcarb apart and clean clean tank& petcock too

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when i drained the gas it looked like there was some dirt at the bottom of the tank but none in the jets or bottom of the carb bowl.

ill try tightening the carb boot,exhaust and if that doesnt help ill try a new plug and cleaning out the tank and petcock.

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sounds like you will have to pull the carb apart and soak it

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Hey,

I had a similar issue with my '05 CRF 450R. It didn't take too long to figure it out because I could hear the leak at idle, my header was cracked around the inside and bottom just inches from the exhaust port. Occasionally when you fall down (don't we all ?) the header can bend even the slightest bit, when those bends develop into cracks you've got a problem.

Mine was backfiring on deceleration with the throttle closed like nobody's business, flames etc. etc. I put a new header on and it completely fixed the problem. Hope yours will be as easy...

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