Is your idle speed high enough? It MUST be north of 1400rpm (ideally 1450) otherwise it will cut out on a sudden closed throttle (usually when going down through the gears).

Remember that any changes you make to intake or exhaust will almost always affect the idle speed. Idle speed should be checked and adjusted after any change made. Even a fresh spark plug or new stock air filter can alter the idle by 50-100rpm, which is enough to trigger the typical XT cutting out behaviour.

You might also want to consider a fuelling device with the mods you have made. Not essential, but it will help a lot. As would an O2 controller.

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