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Hello everyone, i have a 07 yfz450 and it has a 13:1 cp piston, cam, fci, full exhaust, and my jetting are p52 m185 needle poistion 3rd clip and my air fuel screw is 3 turns out. I am still backfiring but I notice that it mostly backfires when i do a WOT pull and when i have it idleing it doesn't really backfire. And my elevation is 52'.

 

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Also i don't really know what else to do besides putting a 55 pilot jet in it but i readed that if you have to use that big of a pilot jet u probarly have a air leak but have check everything and everything looks fine.

 

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Anyone??

 

[Nolifesigns]

What made you decide that jetting? That seems a bit big on both ends. I know with high comp you often go down some due to more vacuum.

 

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I just put those jettings for a base point and it runs fine and doesn't sputter or anything but the only thing is that it backfires after letting off the throttle when the motor has load on it/

 

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jets*

 

[Nolifesigns]

it's hard to say what you should do but I couldn't see needing that big of a main myself. Unless you had a big-bore kit. If it was mine I would have started somewhere near 172-180. Your pilot seems off to me as well because at 3 turns out you maybe be making up for your needle position or jet size. I also assume you are running at least 100 octane. Wish I had more input but you can always do a plug chop or a dyno to know for sure.

 

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I changed the jets to m175 and p50 and i am running 93 because its a 07 and the comp drops a bit so im basically running a 12.5:1 comp piston

 

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Should i mess with the needle position? And the needle is a dynojet needle.

 

[Nolifesigns]

it's 06+ that has the higher compression. Which CP piston did you get? M1001 or M1002 both would put you over 13.

 

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M1001

 

[Nolifesigns]

Should i mess with the needle position? And the needle is a dynojet needle.

Never messed with any of the dynojet jets or needles. I was always told to not use them because they were harder to get right.

 

[Nolifesigns]

M1001

that would make you at 13.25:1

 

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Never messed with any of the dynojet jets or needles. I was always told to not use them because they were harder to get right.

So should i change my needle?

 

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that would make you at 13.25:1

But i thought the 07s had lower comp

 

[Nolifesigns]

pre 2006 439cc after 2006 449cc

 

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High Comp Piston On Pump Gas

These guys are saying that its 12.5:1

 

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And I have a CP piston

 

[Nolifesigns]

So should i change my needle?

I would try to use it since you already have it. maybe on the 4th clip to fatten it up. More than anything I would put ether ethanol-free gas or some 100 octane.

 

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Do you think the octane is the problem?