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I may have found why the carb bogs when you stab the throttle wide open. Take the side cover off your carb and find where the throttle cable connects. You should see a little black lever connected to the main lever (the one the cable connects to) via a spring. Stab the throttle and notice the black lever lags back but catches up to the main lever. Roll the throttle and notice how the black lever stays with the main lever. My guess would be when you stab the throttle it takes a second or so for the spring tension to tighten up enough to pull the slide open. When this happens the pump is squirting fuel into the intake but you have no air, so the motor bogs. Simple, to much fuel and no air. When you roll the throttle on smoothly. The pump doesn't squirt that high volume of fuel into the intake. If your jettings close it shouldn't be that bad. I don't think you ever get rid of the bog with this carb though. Who rides around stabbing the throttle wide open all the time anyway? You don't do it as fast while you're racing, as when you're sitting there tweaking it wide open. I've never had mine bog on a holeshot! That's the only time I really pin the throttle wide open, real quick. Try it and see what you guys think.

:rock :rock :rock :rock :rock :rock

Fred J.
 

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Or....

It's possible, depending on your set up, that it could be the other way around. Too much air not enough fuel. I think that is more likely on mine with the k&n and pro flow adapter cause when I do that you can see the pre filter suck up to the filter as you open the throttle, but if the pump isn't giving it enough juice, you would have a similar situation. go to BLuetraxx.com and look in the YFZ forum there. One guy was sayin that he has increased the pump action on a couple of YZ dirt bikes and pretty much cured the problem.

Maxx
 

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Discussion Starter #3
Yeah, I saw that too. But there's also a guy there that says it's getting to much fuel. The next time you have the carb off, hold it in your hand, don't look directly into it but kinda' from the side. Stab the throttle and you'll see it's getting plenty of fuel. There will be a big squirt coming from inside the carb. If you just roll on the throttle, fuel comes out slowly. It's definitly to rich or to lean at that point, who knows. One thing though, if you haven't changed the spark plug since you've done all those jetting changes, you need to!

Fred J.
 

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The slide opens immediately (not vacuum operated).
The accelerator pump is on a spring so the fuel can squirt through the small opening without rupturing the diaphragm. (thus the delay) The bog is from the bike being lean to begin with. (have you rejetted?)
Mine bogged bad until I rejetted it. When I had it too rich (over-jetted it) the bog did not return. It only crapped out when the intake restriction caused it to pick up too much fuel (vacuum builds inside the carb) so I lowered the needle clip and jet size.
Power now has a plenum divider to keep the bike from going lean at part throttle, but for off idle to full throttle punch you should rejet.
 

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Hey Maxx, I'm that guy on Bluetraxx. LOL!

On the Bikes, it was a RICH condition, because the pump was shooting too much fuel into the carb.

The pump was squirting fuel for as much as 5-6 seconds, when the book said 1-2 seconds.

They may have redisigned that part of the Carb since the 426 bikes, so I don't know how that relates to the YFZ carb, but it is a good starting point.

And it sounds like you guys are farther into it than I have made it yet.
 

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:O the way i see it is when you stab it your sucking so much air the carb cant meter the air fuel mixture on any four stroke and it may seem like its running lean or rich its just a four stroke no fix.
 
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YA i noticed it boggs then stalls. but if u rev it about quarter throttle them stab it it fine. :D
 

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Hey guys I used to race cars before I got into this 4-wheeler thing. I know a holley
carb with mech. pumpers will do the same thing if you don't set the pumper setting
to squirt fuel at the same time the butterfly or slide starts to move off idle. My bike
does it some times if drag racing really hard. I'm going to take the carb. off & see If
I can make some kind of setting on the carb. PS: If anyone has found the trick to this one please let us know! :sup
 

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I have not played with mine yet, but there is an accelerator pump jet in the bottom of the float bowl. The stock size is a 50. A larger jet will let you get more fuel faster, and a smaller jet less fuel slower (not the best description, but br549 will recognize it as the same as the accelerator pump nozzle squirter size on a Holley). As mine doesn't bog I am not going to play with it yet!

Joel
 

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Mine is really bothering me. I don't think it did it when I first got it, but I rejetted immediatly to 168/45/5thclip. If you give it gas slowly until a few RPM's build its fine. From an idle to fast blip, farts,stutters and pops. Sooooo anoying. Getting really frusturated. Any ideas? Tried moving the clip up and down, Air box on/off, baffle on/off. nothing helps. Some say its a four stroke thing. Had plenty, this is the first problem. YZ450f is smooth as butta. Help! :yfz
 

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I talked to a great tech and the problem can not be fixed, and is normal. It is not designed to stab the throttle without a load on it. It is an accerated pump carb the accerator pump can not respond that fast. On my raptor they have cv carbs (constant velocity). It can handle the throttle stabs, but who sits around and stabs the throttle without load on it. these carbs are one of the best on the planet it is roll on throttle acivated with higher rpms. :yfz
 

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After adding a whites brother pipe(no spark aresster),white's foam air filter, k&n power jett kit. My yfz done the same thing. After a few ajustment's to the acc. pump, i solved my problem. you wouldn't belive how much better it run's.
 

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you can adjust the fuel screw on the bottom of the carb to help eliminat eht ebottom end sputter too. if you acel. pump isnt working correctly... that can effect things too... but it is more likely that your fuel screw is not set in the correct position.
 

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Hi guys!! I am new to this website, but I have found a way to fix the dead spot in the throttle. On my fiance's bike, I added a full pipe, filter, cam mod, and rejetted it!! I had a dead spot whenever I would hit the throttle hard exactly like everyone is explaining. I then switched the jet for the accelerator pump from a #50 to a #45. The bike now runs like a dream, and has strong power all the way through!! Here is the bad news!! The stupid jet is $12.00 at the local Yamaha dealer, and it is tiny!!!!!!!!!!!
 

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Sandhappy....
Are you talking about the pilot jet? Or is there another jet for the accelarator pump thats different from the main and pilot jets??
 

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This jet I am referring to is actually located in the bowl itself. You remove the bowl off the carb, and place it on the work bench. In the bottom of the bowl you will see a TINY jet. This thing is barely bigger than a grain of rice!!! Remove it, and go get a jet just like it from Yamaha in a size 45. It actually has a Yamaha part #. I would get the part #, but my fiance threw the package away!!
 
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