yz450f bog

2003 yz450f has a bog when throttle whacked open just wondering if any suggestions I've played a little with the fuel screw right now it at 2 turns out stock jetting and needle set at fourth clip. The only mods are slip on muffler white bros r4. The way the bike is is the way I got it.

You need to upgrade your accelerator pump system

Remember, on the stand throttle whacking will usualy give you a bog, no matter what, but riding, it never should. 


You can get all the parts here  



...and order a Toko mods apump linkage spring here, and install it.





Verify your squirt by lifting up the subframe and watching it squirt (dead engine)

You can save a few bucks at RMATV. Do a search for accelerator pump spring. It will show you KTM spring for like 5.00. It's the same as a Merge, it has one less coil then the stock to make it stiffer.

First, you placed your post in the WR forum rather than the YZ450 forum.  It's been moved.


Secondly, if you let the bike idle, especially in neutral, then "whack" the throttle, it should bog. 


The correct approach is to "roll" the throttle.  This technique involves doing the initial bit of the throttle opening just a little bit more slowly, as if you were only going to break it open a quarter instead of fully.  The engine will respond, and as it does, you finish rolling the throttle open as fast as it can take it.  If the bike is tuned correctly, and you master this technique, it will sound as if it went from idle to a heavier throttle opening instantly. 


Having said that, you may still have conditions that keep the engine from being as sharp and responsive as it should be.  These include a pilot jet that is either partly clogged or too small, a poorly adjusted fuel screw, even with a good pilot jet, an accelerator pump that is mistimed, or some actual fault in the pump system such as a torn diaphragm, or leaky checks.


The timing procedure is in the manual.  If you are still dissatisfied with the response, then, and only then, start with the beefed up linkage spring recommended. 

I've seen a few videos that people have wired the accelerator pump just under the throttle cable cover just wondering if this works as well 

Wiring the linkage spring out of function is a caveman approach.  You would also need to modify the AP diaphragm to make ti work without stopping the throttle short of full throttle, and you would end up with an accelerator pump that put out way too much fuel at the wrong times.  Don't.


If you just absolutely have to take a shortcut, at least avoid the wiring thing and use the "O-ring" mod in place of the stiffer spring.  But only after you get the bike running as well as it should be first.

Ok so don't do the caveman thing how about the 80 dollar quick shot will that fix my bog? 

Ok so don't do the caveman thing how about the 80 dollar quick shot will that fix my bog? 


Not very likely, no.

The previous owner of my bike took the caveman approach, and with a few other basic tuning tweaks, I have the most bog-free and responsive engine I could honestly ask for. You can kill it with an instant WOT rev from completely closed with no load, but use the short roll-on technique described no load or any throttle movement, including instant WOT while under load, and it responds sharply. I don't mean to contradict, and I know the WR and YZ carbs are slightly different, but the safety wire mod is viable even without modifying the diaphragm, and the slide does open all the way.

 Well, let's look at that for a minute.  On any YZ450F or any other Keihin carb I've worked on, the accelerator pump consists of a diaphragm and a return spring in a small housing at the bottom of the carb.  The diaphragm is driven by a cam on the throttle shaft as the throttle is opened.  But the diaphragm in all cases has a stop button on the bottom of it that limits the amount of fuel delivered by the pump in any one shot, so the pump pushrod comes to a stop before the throttle is entirely open, somewhere around 2/3 throttle in most cases.  To allow for this, there is a separation of the accelerator pump linkage into an upper and lower section such that the upper section is directly driven by the cam, and then pushes on the lower section through a coiled spring.  Thus, the pump bottoms out, but the throttle can continue to open because the throttle cam can "compress" the spring instead of being stopped by the pump shaft.


Proponets of the wiring mod contend that the connecting spring is so weak that it compresses almost immediately against the resistance of pumping the fuel when the throttle is opened, which is true to an extent, and part of the design.  The wiring mod takes the spring out of the picture and connects the upper and lower sections together with a loop of wire.  Without further tinkering, the throttle travel is limited to the amount of movement available in the pump diaphragm.  To correct, there are only two options; either time the pump late to give it enough room to move that far (most likely making it too late to be effective) or remove the diaphragm stop button, grossly increasing the amount of fuel delivered.  So, given only the information you offered, what you are saying is not possible unless the WR has a very significantly lower lift accelerator pump cam, which I suppose is possible.  Unless the linkage differs, there's some missing info in that which you posted.


In the case that the stop button is removed, one way of getting around the massive fuel dump that should occur is to install a much bigger leak jet, so as to reduce the shot a little and keep the engine from slobbering fuel down its own throat with every little twitch of the grip.  The trouble with that strategy is that the '03 being discussed here has no leak jet to change (nor does an '04), so that's out.  In any case, the stiffer linkage spring will do everything the wire mod will do without causing any of the attendant problems the latter creates. 


I should point out here that my '06 YZ450 runs very much as you describe the operation of your WR, but with no modification to the AP at all.  It was all done with the right selection of needle and pilot jet.

gray racer is right like usual... not sure if its possible on the 03's but when my 06 carb got tuned the guy said he worked the slide so its one smooth opening motion vs the stock 2 stage. 

gray - thanks again.  I definitely can't dispute anything here, based on my knowledge level, and am not trying to argue.  I've visually confirmed my slide opening and pump timing, but I'll take a closer look tonight or tomorrow as I re-jet my main to see if the stop button has been removed - I might even track down the previous owner's number to ask him if he changed out the pump cam or if he remember anything else about it.  I have verified everything else in the carb, and it's a 45 leak jet. 


I will not further hijack this thread...  Good information here, though.

Will moving the clip maybe help?

Clip position isn't really an element of the problem, no.

i had the fuel screw turned out more than i thought and reset it at 2 turns and then slowly worked it in just a hair and the bog is almost no exsistant. still looking into the accel pump stuff though. thanks all.

I have an 07 yz450 and just bought a boysen quick shot 2 and it will virtually eliminate any bog once its warm. I had a 48 pilot in and put back to 45 after and 158 main and needle in 3rd. It may seem like when it bogs its getting too much fuel but go the other way with the fuel screw it will help.

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