Which ACV mod works best?

In reference to the following link, for those that have modified the ACV valve, have you had good results? I am currently using method #1 but can't tell any difference. I have an 06 wr450.


I too wondered this, I have all yz type mods done on my bike, maybe i should do as you done also..... Who here has done this one?

I played around with it, and it did nothing to enhance performance. I put it back to stock and left it alone. There are a few supposed "mods" out there that are really nothing but the result of unnecessary tinkering. In my opinion, the ACV mod is one of them...SC

I was hoping I could then set the fuel screw easier (it says thats a reason for doing this mod) but I guess I am not proficient at that either. I think I'll just put the spring back in.

I did #3. Plugged the port and turned the diaphram around.

I had to go to a way smaller pilot jet (40 from 48), and went to 100 PAJ as per YZ450F.

Throttle just seems crisper with this mod than with out.

As with Clarky said, it is a lot of tinkering for little gain (gains are still there), but what else are you going to do once you already have done everything to your bike?

I also think going to a factory pro jetting kit would have the most benefit of everything to do with the carby. I got the JD kit, that's only because I learned about factory pro after my purchase. D'OH! One will be going on my next bike, I might still get a kit for this bike and sell the JD kit.

Zip-ty sell it with everything you need for your carby:


I did the ACV mod early in the life of my 06 - the reason was so I could sort the jetting out.

With the ACV done, the fuel screw seems a lot more effective at dialing in the on/off throttle fuel setting, that is about all it is good for.

I took the spring out and "blocked" the diaphram with a spacer so it remained shut. This method allows use of all OEM jets.

Once I got the jetting sorted (with ACV blocked you will get some more "popping") I reinstalled as per OEM and accept that it will be a little rich on the overrun on "shut" throttle.:worthy:

Nice zip ty link, good stuff forsure.

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