The valve on the side of the carb?

I don't remember what its called but if you disable it, do you really notice it? I know it leans everything when you get on the gas and richens it when you let off. The last few days, sometimes when I really lay on it, it revs out fine but doesn't have all the power. I guess this thing has something to do with that. I'm trying to figure out what I want to do next to my bike. I have a feeling that when I get the new back tire, its going to feel a little underpowered again when it doesn't spin as easy. I figure I'll either get a YZ cam or go with a 13.5-1 piston since it won't run right on pump gas anyway but this thing on the carb is bugging me. Can you really tell any difference when its disabled?


The only "thing" on the side of the carb is the choke and the hot start.

Choke is black, it's used to start your bike when cold - usually just when cold out, on warm days most of these bikes fire cold without it.

The hot start is (on older versions) a red choke looking thing that essentially does the opposite of a choke - and allows you to start the bike easier when the engine is hot. on an 05 your hot start is remote controlled by your finger.

The bike should never be ridden with either one of these "ON" and should never have an impact on riding performance unless either one is broken and malfunctioning.....

I believe he is referring to the TPS (throttle position sensor). Sorry, I have not messed with mine....nothing else to add.

Is it on the left side? I know the 02 has a cover held in place with two allen screws and under the cover is a diaphragm that does what you are describing. It's called the air cut valve. There is a mod to disable it because it can make it harder to jet the bike.

Here I thought he was talking about something physically touchable/changeable - that would be why I said "Huh?"

ACV - most people don't notice much of a difference with or without it...

here's the mod:

Here I thought he was talking about something physically touchable/changeable - that would be why I said "Huh?"

ACV - most people don't notice much of a difference with or without it...

here's the mod:

I read that and thats why I asked if you could really tell a difference. If the valve is operating up to half throttle, that could have a lot to do with why the motor acts funny riding fast through the woods because its half-nothing-1/4-nothing-1/4-half-wide open-nothing....and so on with the throttle. I CAN'T get it running consistantly through the pilot circuit. I'm playing with the fuel screw constantly and I might have 30 minutes and I might not get 20ft before its acting off again. Once the bike is HOT, I can leave it but the motor has to stay around a certain temp or it changes.

As mentioned, it's the ACV (air cut valve), aka, coast enricher. It only works on decel (or coasting) by "cutting" the "air" to the pilot circuit in half, effectively "enrichening" the circuit to prevent decel popping. When you let off the gas, the vacuum signal increases, which is what closes the valve. It does not lean anything when you get on the gas.

There are many methods for disabling the ACV, some of which block the valve open (both air passages to the pilot remain open), ala WR's, which let you keep the pilot and pilot air jets the same, and other methods block the ACV closed (only one air passage remains open), ala YZ's, which require you change the PJ and PAJ to YZ spec.

The reported benefits of this mod are slightly crisper throttle response and easier tuning of the pilot circuit. The 2nd advantage makes sense because the circuit won't be continuously changing depending on the vacuum signal.

I left mine alone.:thumbsup:

I just turned mine over so the pin is now facing out and can't open the valve. Seems to make little to no difference either way.

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