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Discussion Starter #1
I have a 2004 ltz400. Yoshi slip on, new carb, raptor petcock, prodesign intake with 4in filter, stock main jet now, with 25 pilot. It runs good until you go wot. At wot itll cutout and want to die. I cant figure it out. Had to big of a jet before so I went to stock to atleast get it running right and still does the same thing. Any advice?
 

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If you went to a smaller jet and it didn’t change anything, it’s probably not jetting. Does it even reach full throttle or get there, then start dying?
 

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Discussion Starter #3
Itll get there if you just rev it. But will pop and crack pretty good. But if you hold it wide open itll just get up to high rpms then start dying and cutting out. It wont die all the way but itll get pretty close.
 

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Is the new carb OEM? Brand new? Vacuum port is plugged?
 

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Discussion Starter #5
Yes it's a brand new OEM. Vacuum port is plugged with one of those little vacuum plugs from the parts store
 

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Well, the factory carb is generally jetted kinda lean on a stock bike, meaning airbox lid is on, factory air filter, intake, exhaust, etc. It will definitely need a bigger main, possibly a bigger pilot. What brand of jets are you using? DJ, Mikuni, Keihin?
Still using the stock needle? You’ll need an adjustable needle. If you have one, what clip is it on?
How many turns out is the mixture screw?
What’s your elevation?
 

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I should also ask if the raptor petcok is an actual Yamaha part. Some of the Chinese knockoffs found on EBay don’t have a large enough orifice to supply fuel and cause problems like this.
 

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Discussion Starter #8
I was using mikuni jets. The one I had was too big actually measured out at about 165 DJ. So I went back to stock for some testing to see if that helped. The petcock I ordered was supposed to be 100% genuine yamaha. I'm roughly 2 to 2 1/2 turns out on screw dont remember 100%. Its got the airbox lid removed, pro design intake with 4in filter and yoshi slip on. Also it's got a stock needle with no shimming. Planned on doing that. At this point might be cheaper to just pick up a DJ kit.
 

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Probably need a DJ155 main and 25 pilot, 2.5 turns out on mixture, adjustable needle on 3rd clip. You can just buy the jets and needle separate, much cheaper. Most shops will trade for free.
If that doesn’t get it close, I’d look at reasons it is starving for fuel. Petcok, tank not venting, filter clogged, etc. Unless you’re living on the rev limiter, one of these should be your answer.
 

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It is also a good idea to check electrical wiring. Check sparkplug and sparkplug wiring all the way back to battery. My Ltz 400 was losing throtle while driving. Electrical wire was a bit loose and damaged. I would also check for compression if there is no problem with electricity.


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Discussion Starter #12
I'll look into the petcock. The one I got said genuine yamaha part. But you know how that usually turns out. Also for wiring i stripped it all the way down and checked and redid everything. Brand new plug and coil etc. I was also thinking maybe stator or cdi box. But I think a authentic petcock would work first to see. Anyway to test without ordering one to see if that changes anything first?
 

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Not that I know of.
 
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