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Fuel Mixture Screw Setting

This is a discussion on Fuel Mixture Screw Setting within the Yamaha YFZ450/450R (2004-Present) forums, part of the Performance /Technical category; We just put a new pipe on and it pops pretty good on letoff. If I adjust the fuel screw out to 2.5 turns it ...


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Old 09-26-2004, 07:55 PM   #1
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Default Fuel Mixture Screw Setting

We just put a new pipe on and it pops pretty good on letoff. If I adjust the fuel screw out to 2.5 turns it goes away. The problem is the bike seem to idle and run better at 1.2 turns out. Thoughts?

Maybe a small exhaust leak? Anyone else have this issue?
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Old 09-26-2004, 09:49 PM   #2
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just to clarify, are you talking about the pilot screw?(under the base of carb) or mixture screw? (on side of carb) it sound to me like the bike is lean, popping on decelaration is a good indication of this (if you have ruled out intake and exhaust leaks first) I would dropping your needle clip one position and retest.
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Old 09-27-2004, 06:10 AM   #3
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I should clarify that I already had a 48 pilot and it was hard to start and had the same popping problem. Dropped to a 45 and it starts awesome now and idles better than the 48.

I have the ZipTy fuel mixture screw. That is the screw I'm asking about.

Thanks!
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Old 09-27-2004, 07:26 AM   #4
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Quote: Originally Posted by YFZ450_RDR
just to clarify, are you talking about the pilot screw?(under the base of carb) or mixture screw? (on side of carb) it sound to me like the bike is lean, popping on decelaration is a good indication of this (if you have ruled out intake and exhaust leaks first) I would dropping your needle clip one position and retest.
The yfz only has 1 mixture adjustment screw. It's controls less/more fuel thru the pilot circuit. Location on bottom side of carb. Pop on decel is normal and I would by no means try to jet according to popping. He says that it idles better at 1.2 turns, that sounds about right with having the larger 45 pilot in it. If the popping annoys or worries you, put the 42 back in, install a #70 or smaller slow air jet, and drill out the acv diaphram cover a bit to allow the air cut valve to open sooner.
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Old 09-27-2004, 09:33 AM   #5
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Quote: Originally Posted by FlyinYFZ
Quote: Originally Posted by YFZ450_RDR
just to clarify, are you talking about the pilot screw?(under the base of carb) or mixture screw? (on side of carb) it sound to me like the bike is lean, popping on decelaration is a good indication of this (if you have ruled out intake and exhaust leaks first) I would dropping your needle clip one position and retest.
The yfz only has 1 mixture adjustment screw. It's controls less/more fuel thru the pilot circuit. Location on bottom side of carb. Pop on decel is normal and I would by no means try to jet according to popping. He says that it idles better at 1.2 turns, that sounds about right with having the larger 45 pilot in it. If the popping annoys or worries you, put the 42 back in, install a #70 or smaller slow air jet, and drill out the acv diaphram cover a bit to allow the air cut valve to open sooner.
Guess there is a difference in mixture screw between my WR and my YFZ. I have to disagree with popping being normal is like some people saying that when the header glows red it is normal which is not. Popping is and indiction of a lean setting, however if it only pops every so often I would not worry about it. Popping would happen because of a lean setting in the higher speed cuircut on the bike not the low speed cuircut as suggested, 1-1/2 to 1-3/4 turns on the Zip-ty is about right so adjusting the clip position of the needle is what I would do.
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05 YZF450 White/Red no mods yet

03 WR450F, grey wire mod,James Dean blue needle,4th clip position,170 main 48Pilot,70PAJ (stock) ,1-3/4 turns on Pilot screw, Throttle stopcut, Powernow, FMF powercore4, 1500ft alt.
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Old 09-27-2004, 09:46 AM   #6
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Quote: Originally Posted by YFZ450_RDR
Guess there is a difference in mixture screw between my WR and my YFZ. I have to disagree with popping being normal is like some people saying that when the header glows red it is normal which is not. Popping is and indiction of a lean setting, however if it only pops every so often I would not worry about it. Popping would happen because of a lean setting in the higher speed cuircut on the bike not the low speed cuircut as suggested, 1-1/2 to 1-3/4 turns on the Zip-ty is about right so adjusting the clip position of the needle is what I would do.
The header glowing red is normal, sure you can richin the mixture to cool it down. The popping is caused by incomplete combustion igniting in the exhaust itself. Yes, it is a lean condition but only at higher rpm and throttle chop. Hence is why Yamaha has put an acv on the carb. It's only purpose is to create a richer condition at high rpm throttle chop.
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Old 09-27-2004, 10:42 AM   #7
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You can jet it so that the motor runs as best as it can which is the way I like it, or you can jet it so the header doesnt glow and the popping goes away and you never have to touch the choke knob.
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Old 09-27-2004, 05:04 PM   #8
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good way to put it Bluenova, I have said my 2cents on the subject.
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05 YZF450 White/Red no mods yet

03 WR450F, grey wire mod,James Dean blue needle,4th clip position,170 main 48Pilot,70PAJ (stock) ,1-3/4 turns on Pilot screw, Throttle stopcut, Powernow, FMF powercore4, 1500ft alt.
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