so what is it with the 06?

The air cut valve on the WR has nothing to do with the AIS. The WR has always had the ACV and the AIS was put on the 2005 and newer WR to dilute the exhaust to meet C.A.R.B requirements. The ACV on the WR is a valve that cuts the air to the pilot circuit in half under hard decel to richen the circuit and eliminate popping.

My YZ pops some on decel ocasionaly and I think that just means your real close to being right on with your pilot circuit. If you can tune it out with a reasonable amount of adjustment of the fuel screw then your real close.


go 2 steps up on the pilot and the popping will go away. It is very common for a lean pilot to cause decel popping. a richer pilot will not effect the high speed jetting at all. while you are at it, make sure the spark plug is snug.

What do you mean by sealed the exhaust? I have an aftermarket one that has some play, and I was wondering if that affects performance, for instance popping. I thought about sealing it, but I was thinking that the pipe got too hot for even high temp rtv or anything like that? I have a pro circuit ti-4 header and silencer. What did you use, and how did you do it so as to not mess with the flow through the tubes?

ultra copper rtv silicone sealer. a leaky exhaust will make any bike pop on decel.

go 2 steps up on the pilot and the popping will go away. It is very common for a lean pilot to cause decel popping. a richer pilot will not effect the high speed jetting at all. while you are at it, make sure the spark plug is snug.
Better go to more than 2 steps. The recommended jet size at sea level is in the range of 48-50.
GrayRacer when I look at the parts diagram for the WR I see the whole AIS tubing and such, but on the side of the carb is the Air Cut Off Valve, diaphragm cover, etc. So these are connected? if so, how? And for all this discussion the main reason the person new to YZ's like me is the fact that the KTM and CRF didn't do it! I'm just super curious and want to see if I can correct it, if in the end I can't, then I can make the decision then to live with it or sell it.
OK, it appears that I incorrectly assumed that the Air Cut-off Valve was added with the AIS, and it was actually there already. But it serves the same need as the anti-backfire valves on cars does, but does it in a reversed sort of way. By cutting off the air to the pilot circuit, fuel is prevented from entering the engine under decel conditions, and without fuel in the exhaust, there won't be any popping. But Yamaha still won't put it on the YZF, because it still isn't that important.

The mechanics of decel backfires are pretty simple. There are three things needed for gasoline to go bang, and one of those is heat, which available in plenty inside the exhaust system. In an engine that has a well sealed exhaust, and is idling at the right mixture, most of the idle fuel burns within the combustion chamber. That which passes unburned into the exhaust won't burn because there isn't enough air for it, most of the available air in the chamber having been used up in partially burning the idle fuel as it passed through. Obviously, an air leak in the exhaust supplies the missing air, and that can cause popping.

If an engine is too lean, it will not burn in the high vacuum of the chamber under deceleration. When that happens the fuel enters the exhaust WITH the air it was supposed to have reacted with, and burns there instead. Similarly, in a very rich condition, it doesn't burn in the chamber under decel conditions, either, and the fuel and air charge enter the exhaust together with the same results.

Assuming a sealed exhaust, what you have in an engine that pops on decel is one in which the idle fuel didn't burn during deceleration, which is hardly anything that should concern anyone, except for the fact that it CAN indicate that the idle mixture isn't right. But if that's the case, I still don't recommend using the decel pop or lack of it as a measure to gauge the correct mixture by.

Now, in the case of the '06 YZ450F, they are well known for being way too lean as delivered.

Read: (first post)

I'm sure you've noticed that the bike not only backfires, but has a really bad off-idle stumble, and stutters quite a bit under conditions of very light throttle at moderately high revs. That's what should bother you, and that's what you should correct. Once that's done, you may well find that the popping is gone, as David did. But the YZF is more prone to this than other engines (thus, perhaps the motivation for dealing with it on the WR...sparks and all, you know), and it will usually be present to some minor extent under certain conditions. If you feel driven to sell it for that reason, PM me.

No expert on the 06 but have you tried dropping the needle, one clip can make the world of difference to deceleration popping, you said you are running on the 5th clip, on shutting off acceleration a rich neddle position will still suppy to much fuel to an correctly adjusted pilot circuit, thus this fuel is burnt in the exhaust causeing popping.

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