My 07 DRZ 400SM backfires when I let off the throttle.

Dennis, Maybe try changing your plugs, unburnt fuel may be getting in exhaust because of weak plug.. Read the following links, very informative in telling causes of and some suggested fixes on backfiring. Check the attached links,instruction and guides, Good luck
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Exhaust backfire popping when letting off throttle Motorcycle Forum

What causes bikes engine to backfire Intake Jetting Fuel Injection

Preventing Backfire on Motorcycle Exhausts ' DoItYourself ...

www.doityourself.com/.../5-tips-to-preventing-backfire-on-motorcycle-e...

When the engine on your motorcycle has an emission system malfunction, such as anexhaust leak or a moment of running rich or running lean, backfire can ...

Exhaust backfire, popping when letting off throttle - Motorcycle Forum

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Apr 26, 2009 - 28 posts - ‎17 authors
Exhaust backfire, popping when letting off throttle General Motorcycle Discussion. ...You can damage the engine by running too lean or rich. Backfires on deceleration
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clean exhust pipe.
is it 2 strok or 4 stroke engine?

After-market exhaust system or stock?

A lot of after-market mufflers don't cover up a backfire pop on over-run in the way that the stock system does. Look at The Marble Mod on GIXXER forums for ways to doctor the bike's emissions system to stop the popping.

Another cause of an over-run backfire may be a slight leak where the header pipes emerge from the cylinder head. Check the bolts are tight. Consider new copper washers.

Get the fueling set up on a rolling road. Bike makers set the fueling up to match the exhaust they fit. Your new exhaust will have less backpressure and this will cause popping when you close the throttle to slow down. It will also cause hesitation when you accelerate.

I would check the jetting to be safe but it could also be dirt in the carb. The main jet has very little to do with lower engine speeds and especially idle. That is controlled more by the pilot jet and circuit. Your best clue is how it responds when you add more choke. If it gets worse then there is a rich condition but if it gets better then you have a lean condition. From what you have described it sounds like a lean condition.

You are running really lean. gas is not getting to the idle ports. try removing the idle mixture screw and shooting some carb cleaner in there. make sure you get all of the mixture screw parts out first. Reinstall screw and set to previous adjustment. you may have to back out the idle mixture screw a bit to allow it to idle properly. There is a very small idle mixture jet that may be clogged as well.

I had the same prob with my 97, It was not the head gasket but the O-ring on top of the head. Replaced the O-ring and has been running good for a year now with no backfire. Hope this helps.

They should be around 33hp at the rear wheel. They should wheelie ok, though you will need to try a bit for decent ones.

Your bike is most likely running a little lean on the idle/slow-speed circuit. The jets might need a cleaning, or you've got a slight air leak somewhere. Other possibilities ... - Idle mix screw out of adjustment (Probably not as it's sealed and 'supposed to be' tamper free) - Carbs slightly out of synch