Jetting for a 400-426 conversion

I'm converting my '98 400 to a 426 and using my 400 carb. I'm running a Hot cams exhaust cam and a white bros E series. Foam filter in a stock YZ box.

I usually run at about 1500-3000 feet elevation.

I was thinking of starting out with stock 426 jetting, but the carbs must be different because the 426 has smaller stock jets.

Does anyone know of a good baseline to start with?

Thanks! Some good info. I believe i am a bit lean already with the 400 at mid range after reading that post. It would miss at low throttle cruising.

hi, im new to thumpertalk and i;m wondering if your 426 motor bolted right up in your 400 frame. how about the electrical? my 400f blew and i cant find a new motor and the damage to my 400 is way too $$$ to fix. thanks!

I 426 uses all the same externals as the 400. The swingarm bushing might be the only issue.

How much larger should I go on the pilot and main as a good starting point for the 426 with 400 carb?

You should discard the 400 carb, or at least make a paperweight or mobile of it. The carb from a 426 is much more sophisticated, and the carbs from an '05+ YZ450 even more so. They bolt right in place, too, with only minimal mods. You won't have a hot start with the 400 carb, either.

Either way, start with the jets already in the carb and let the motor tell you which way to go with it.

Thanks gray. I'm going to see how the 400 carb does before I discard it. The hotstart port is built into the yz400 head that I'm using with the 426 cylinder so it will still be functional.

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