2008 yz450f bogs when throttle opens

I had put on an FMF powerbomb exhaust system and changed the main jet from a 160 to a 170, and changed the needle from groove 3 to groove 4. Could this be the problem why when i give my bike a quick twist of the throttle it bogs? Im thinking its too rich. Any ideas?

Neither of these elements have anything to do with opening the throttle quickly, so, no.


First, ask yourself whether your expectations are reasonable.  Even with the most correctly tuned carb you can come up with, it will still be possible (or, it should be) to at least almost kill the engine if you snap it open as hard as possible from an idle.  But you don't ride like that, and even when you need the throttle quickly, you should be "rolling" it open.  This only means that the first 1/4 throttle opening is slower than the rest by a small amount, and the practice becomes reflexive after a very short time.


That said, if it stumbles unreasonably while it's under way, then you should check the accelerator pump timing and duration per the manual, then examine the size of the pilot and the upper section of the needle.  IMO, your 170 is too fat.  I recommend the settings Todd used here:



The difference in the needle is the smaller "fat part".  This area of the needle is effective from off-idle to about 1/4 throttle, whereas the needle position has no effect on anything until the tapered section begins to lift out of the needle jet/nozzle, which happens at around 1/4.


Some people who ride MX exclusively do like either the heavier accelerator pump linkage springs made by Merge and others, or the O-ring mod, but neither of these are truly necessary.  Browse the fuel system section in Common Threads

Is it recommended to install a richer needle when I have the stock one in position 6 or 7?

Yes, but you need to note what part you're making richer.  The recommended needle here is richer only between off idle and 1/4 throttle because the upper diameter is smaller, but the taper is the same and is at the same height on the needle.  So an NFPP set at 3 is exactly the same at half or 3/4 throttle as an NFPR set at 3. 


6 or 7 strikes me as way too rich.

OK thanks for the info, I am going to rejet my bike again tomorrow and I just need to know what needle will work best and at what clip. 09 YZ with DRD full exhaust, I ride at So Cal deserts and tracks/sea level-1500ft. I still have the stock needle at clip 5, and I can tell its too lean.

Is NFPP the best one to go with?

That's what I would look to based on watching Todd's bike run and talking to him, yes. 

I installed the NFPP needle, so current jetting on my 09 is 45 pilot, 165 main, needle at 4th clip, fuel screw 1.5 turns out. It runs better than before but it sputters at 1/4-3/4 throttle, especially when I hold the throttle in one position. It did the same thing before this rejet but not as bad (had 48 pilot before and I think that covered up the problem a bit..that was too rich)

Should I raise the needle to clip 5? (I assume sputtering/cutting out = too lean) This is a much richer needle than stock acording to the manual so I'm surprised it still sputters

Edited by rhinoracin

Try this: disconnect the TPS and take the bike for a ride.  If it clears up the part throttle stutter, you're onto something.

As to the NFPP being "much" richer, not so, really.


Looking at the charts in the manual's tuning section, you can see that the needle is graded rich/lean on two different measurements that affect two different throttle position ranges.  The tapered section that you adjust up or down with the clip position starts to have an effect on things at just below a quarter throttle (where the taper first starts too lift out of the needle jet nozzle) out to about three quarters, changes having the most effect at about a third. 


The upper diameter of the needle does the bulk of the metering control at from just off idle to right around a third throttle, so it makes a lot of difference in the way the engine runs in that range, and how sharply it responds to the throttle being opened. 


What is omitted from the '09 manual that was included in earlier ones is the meaning, or at least, some of it, of the letter coding.  The first two letters have to do with the length and the minimum diameter at the bottom end.  These don't change within a given carb series for similar engine types except in extreme cases. The third character tells you how far up or down the taper is on the needle in half steps.  An NFPR is a half clip position richer with the clip in any position as is an NFLR clipped the same way.  That's shown in the column at the left of the chart attached. 


The columns across the chart show the effect of changing the major diameter of the needle, again in half step increments.  So an NFPP with the clip in 3 is only one half step richer than an NFLR-3 in the 1/4-3/4 range, but one full step richer in the idle-1/4 range.


So, in the range where you're having the most trouble, that needle is only a half clip spot richer.  It will let you run a smaller pilot and still get good throttle response, though.


A new spark plug fixed the problem

Edited by rhinoracin

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