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What causes backfiring??

This is a discussion on What causes backfiring?? within the Yamaha YFM660R Raptor (2001-2005) forums, part of the Performance /Technical category; My Raptor has been poping or sputtering when I let of the throttle. I have also heard it backfire a couple times when I hit ...


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Old 03-25-2002, 02:37 AM   #1
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My Raptor has been poping or sputtering when I let of the throttle. I have also heard it backfire a couple times when I hit jumps and let off the throttle. Is this caused by a lean or rich condition in the idle circuit? I am already running 27.5 pilots with my air screws 3 turns out. What do you think?
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Old 03-25-2002, 03:51 AM   #2
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Check the tightness of of your header pipe it may be loose or you snapped of a header bolt.
maybe.
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Old 03-25-2002, 10:59 AM   #3
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When you are on the gas hard and you let off completely and there is a backfire you are lean. I dont know from what, but did you do any mods lately? start from there. The cracked header is possible too. make sure your airbox lid is on tight, make sure your carbs are not partially unseated from a backfire.
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Old 03-25-2002, 11:15 AM   #4
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Like BlueRooster said, check any and everywhere for extra air getting in...this is if you haven't changed anything (mods) lately?! It is from being too lean. It'll be your main jet. But first, check all your carb boots to make sure they are seated good and no tears, check the airbox lid, make sure filter is seated, check for exhaust leaks, etc. You are too lean one way or another. Your pilots and air screw settings are okay.
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Old 03-25-2002, 03:07 PM   #5
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When I first got my Raptor it was doing the exact same thing when I let off the throttle quick. I took into the shop and they said it was running lean, as mentioned above.
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Old 03-25-2002, 05:13 PM   #6
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definatly lean condition, check carb boots make sure you dont have a leak, also possible on the exhaust side
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Old 03-25-2002, 11:56 PM   #7
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Mine done that For awhile an i was freaking out come to find out it was the Parking
break rev limter Mine came out of adjustment
i tighten it back up an now all seems find
and Yes it could be lean do a plug test
that is the only way to know for sure if
you are runing lean and then go from there
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Old 03-26-2002, 01:20 AM   #8
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I can't imagine it is running lean. I am running with no airbox lid, 27.5 pilots, 190/195 main jets and my air screws turned 3 out. If I turn the air screws in a 1/2 or 3/4 will it richen up my idle circuit (less air to mix with fuel from the pilot jet)? It only happens when I let off the throttle so I think it is a problem with the idle circuit. Also, I don't see any tears in my boots and my header pipes seem to be tight. Its crazy that I am running such huge main jets though!
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Old 03-26-2002, 02:43 PM   #9
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1fast, i had a similar experience as breadman did. i tightened down the parking break and the clutch cables first. then the only other thing i had to do was to adjust the throttle screw a little to get it perfect. mine is stock though. my next move is the old break blockoff.
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Old 03-26-2002, 02:48 PM   #10
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you could be lean due to an air leak not because of your jetting
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Old 03-26-2002, 05:55 PM   #11
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Any hints for finding an air leak?
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Old 03-26-2002, 07:34 PM   #12
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all i can suggest is checking your carb boots carefully and making sure your header pipes are tight. if you have the gear you can do a vacuum test at the carbs or get a good dyno on it
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