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engine backfiring into carb!

This is a discussion on engine backfiring into carb! within the Yamaha YFM350 Raptor/Warrior ('87-Present) forums, part of the Performance /Technical category; hey can someone help me out? my quad starts.... but like it bakcfires into the carb. u here a knocking in the carburator and u ...


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Old 07-26-2005, 09:19 PM   #1
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Default engine backfiring into carb!

hey can someone help me out? my quad starts.... but like it bakcfires into the carb. u here a knocking in the carburator and u see it like jerk kinda. can it be the valves? is the timing off? someone help me out here!! lol thanks.
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Old 07-26-2005, 09:51 PM   #2
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I had this same problem. I had a guy check it out. He done a leak down test on it and said my intake valve was leaking(not seating completely). He said that had to be my problem. I am not a mechanic, but it makes sense to me. I am currently having a three angle grind done to my head and putting me a hotcam in and replacing everything in the head. I hopefully will have it back together by this weekend to try it out.
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Old 07-26-2005, 10:09 PM   #3
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was there an air prressure spec for the leak down test? thanks
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Old 07-26-2005, 10:54 PM   #4
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[quote="spikek1"]was there an air prressure spec for the leak down test? thanks[/quote

Not that I know of, but thats not to say that there is'nt.
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Old 07-26-2005, 11:08 PM   #5
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The 4 stroke leak down testers I am familiar with are based off of percentage................100psi pumped in and the gauge reads 80 psi (tdc compression stroke) = 20% loss.

Before taking it in to have a leak down done, I would first double check the timing.
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Old 07-27-2005, 08:21 AM   #6
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Does it only do this when you start it up and its cold and go away, or is it completely unrideable? Definitely make sure the timing is set correctly and make sure the valve clearance is within spec.

A bent intake valve can cause poor sealing and can ruin the valve guide. This usually happens from severe over-revving which causes the valves to float (stay open when they should be closed) and then come in contact with the piston. The intake valve is usually bent since it is bigger, this puts it closer to the face of the piston when it is open due to the angle of the valve in the head.

If you don't have a leaking valve, then it could be a carburetor problem (either jetting, a blockage, ripped manifold causing a lean condition).
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Old 07-27-2005, 09:22 AM   #7
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sounds to me like the intake valve is either bent or partially burnt.
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Old 07-27-2005, 09:50 AM   #8
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Mine does that when its cold very seldom. My valve seals are going bad (thats what happens when you have a junky stock carb that doesnt adjust right). I would go with the valves seals to start. Or just rip it apart and take a look. Also does it or has it ever smoked on start-up?
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Old 07-27-2005, 12:56 PM   #9
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sounds like a good way to blow ur self and ur quad up....I mean its just me, but back firing into your carb....could cause a spark right? spark in a grab that is conected to a tank full of gas....Is that bad? Not sure on that one

I also say bent intake vavle or just woren out seals
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Old 07-27-2005, 08:15 PM   #10
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i rode my warrior today and she seemed to ride pretty fine... not to bad. i didn't rele notice any noise coming from the carb... ill take a look at hte intake valve and the seal. and ill start her up tommarow to c if i hear anything. thanks again
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Old 07-27-2005, 10:31 PM   #11
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Quote: Originally Posted by spikek1
I rode my warrior today and she seemed to ride pretty fine... not to bad. I didn't rele notice any noise coming from the carb... ill take a look at hte intake valve and the seal. and ill start her up tommarow to c if I hear anything. thanks again
Mine was pretty bad at start up. So bad that when it would backfire through the carb it would kill the engine. But after it warmed up it would hardly do it. It was like this for a while until I noticed it getting worse. Mine even smoked alot at start up. I replaced my valve seals, but that did not help the smoke. So thats why I decided to completely rebuild my head. Now I am waiting to get my head back so I can put it back together.
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Old 07-31-2005, 05:41 PM   #12
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if the parking brake sensor is on then it will let the bike idle but if you give it gas it will backfire and spit gas into the airbox and will not let it run more than an idle.
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Old 07-31-2005, 10:26 PM   #13
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i took the parking brake off awile ago... but i went riding for awile the other day and my engine started to slowly blow holes in my manifold that connects the engine to the carb... it blew like 3 slits and air started blowing out onto my leg. anyone know wat it might be? im gonna do a leak test tommarow. thanks
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Old 08-06-2005, 11:52 AM   #14
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There is your prob...excess air. Your bike will run lean , if at all with holes in the intake manifold....replace asap.
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