exhaust leak = backfire?

So According to the mechanic, my bike is jetted correctly for my elevation, yet it still backfires a LOT. he says he thinks there is an exhaust leak where the pipe meets the head. would this cause a lot of backfiring? what else could be the culprit?

Is the bike new to you? If no, has it always been this way?

What bike do you have?

When does it backfire?

Where is the fuel screw set?

I've read that these bikes are typically a little lean on the pilot, you might try backing out the fuel screw 1-2 turns and see how that affects the backfiring.

I'd say it's more likely that there are jetting issues than a leak at the header, but I see no harm in pulling it off and visually inspecting the sealing surfaces.

The bike is new to me, 99 yz400f, and its been this way since i bought it. When my mechanic put on the new carb, he said he jetted/adjusted it correctly for this altitude, but i dont know exactly what the fuel screw is set at. It backfires on deceleration, almost every time i let off the throttle. I'm 90% sure i can hear the exhaust leak at the header.

Some popping on decel is normal. If it sounds like a machine gun, then it needs attention. Look for carbon build up (black soot) around the exhaust joints. If you don't see any find or make a stubby screw driver and adjust the fuel screw (recessed under front of carb). Check and write down where it is at first!

... he thinks there is an exhaust leak where the pipe meets the head. would this cause a lot of backfiring? what else could be the culprit?

Yes. The two main causes are lean idle jetting and exhaust leaks. Leaks are easily located by having someone hold a rag over the end of the muffler to increase the pressure in the system while you look for the leak. The most common point is at the joint between the mid and header pipes.

As Frankie pointed out, a correctly jetted YZF will normally backfire out the pipe on decel to some extent at least some of the time. DO NOT chase the problem with jetting until after you have discounted the leak issue.

OK So! for those that give a crap - After i received both new gaskets from rmatv, I dismantled the exhaust system to find that there was NO gasket on the head. Nothing. so i plugged the new one in, and replaced the middle one as well, and not one single backfire yet. two rides later. It seems to run better as well (but i also cleaned and oiled the air filter).So hopefully my jetting is fine after all. Thanks for the help!

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