Low idle, adjusted all the way high

I'm having an issue with my idle adjustment on my 426, I currently have the idle adjusted all the way high (screwed all the way in) but it still won't idle. The jetting is as follows, 160 MJ, 40 PJ, JD (red) 4th position. The fuel screw is ~1.5 turns out. I did some modifications over the weekend that included swapping out the ZipTy fuel screw with the stock one, replaced the accelerator pump diaphragm. The bike runs well (above idle) everywhere else, the plug looks good and everything (slide etc.) else on the carb looks good.

Could it be a vacuum leak?

Should I start with the fuel screw by adjusting it in and out?

Any input is appreciated.

I would start with the fuel screw. I have an 06 WR450 and just completed the AIS removal. Once installed I could not get the bike to idle without the choke being pulled out. The instructions that came with the AIS kit stipulated two turns out on the fuel screw. At 1.5 turns I got my idle back. Hence my comment to play with it first. Have a look through the posts around jetting and they will telll you how to adjust it correctly. At that point roughly the idle seems to return.

Hope this helps.


I believe your are talking about the black plastic knurled knob, correct? If we are talking the same thing then don't mess with anything until you check the other end where the knob wire goes into the carb (on the opposite side of the carb). If that gets dirty and yucked up you are turning the knob and winding it and the springing like sheath over the actual wire but you are not actually turning the screw that goes into the carb. I am not explaining it well but before you mess with a bunch of other stuff clean the area, spray a little WD-40 and see if the knob now frees itself up and turns the screw. Had this happen to me. Good luck

If you suspect vacuum leak use carb spray with nozzle tube to locate the leak. Start the engine hold a constant throttle and spray various areas like the intake manifold and the hot start system. when you find a leak you will experiance a change in engine rpm then repeat procedure to pin point leak. you may want to search for hot start system plastic nut which holds hot start into carb these nuts crack break or seize, there may be some threads on this.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now