The popping and backfiring is perfectly normal. It is (as Jaques says) unburnt fuel igniting in the exhaust when the throttle is snapped shut.

On older bikes (with carbs) like your KLX the fuel wouldn't ignite (unless you had a leaky head/downpipe gasket) as it is starved of oxygen and would just pass out into the atmosphere unnoticed. Current emission regs mean that unburnt hydrocarbons in exhaust gases are a big no no, so air is deliberately introduced to the exhaust port downstream of the exhaust valves to make sure any excess fuel is ignited and burnt off.

On the XT this is known as AIS (Air Induction System) and an ECU controlled valve opens to let air into the exhaust from the airbox to burn off excess fuel when the throttle is suddenly closed. It also opens on a cold start to clean things up as the engine runs rich, and also to speed up the warm-up process as the head, exhaust ports and system will get hotter quicker because of the fuel igniting in them.

AIS is important if you have cats fitted (stock exhaust system) as unburnt fuel quickly damages the catalyst.

If you don't like the popping effect of the AIS (and you don't have cats) then you can simply block or clamp the pipe to the AIS valve that runs from the airbox (or remove the whole system altogether). You cannot just unplug the AIS valve under the tank as its default position is 'open' and the bike will pop and bang all the time!

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