puff sound


Just wondering if anyone can tell me why I keep hearing this muffled puff sound coming from my bike. It usually happens when I’m at a light, and I rev the engine just before takeoff. It sounds like its coming from the air box.


K&N Air Filter
Small Box mod
Yosh RS-3 Race Stainless Steel Full System Titanium Sleeve (USED)
Try checking the breather hoses to the airbox. Especially the big one that goes down to the PAIR valve. It may be sucking air for the PAIR injection. Normally it is connected to the clean side of the air filter but it may have become disconnected up under the tank at the airbox and is extra noisy? That connection is at the rear of the box on the right side bottom. Lift the tank and you will see it clearly as a black hose about 1/2" in diameter. While you are there, you can adjust your idle too. It is the "star" knob below the connection in question next to the throttle bodies and cable pulleys.

If not that, maybe your exhaust header is leaking up near the top under the tank and behind the radiator?
Thank for your response SierraFlyer. I just checked what you suggested, and everything appears to be ok. One thing I did notice is that I started having the problem only after I did the small box mod. I might have to do the pair valves, and get a custom map and see what happens.

You didn't mention the small box mod............... That is probably the reason. With the flapper removed, the throttle body sucking sound is free to resonate in the airbox and come up the intakes like a megaphone.

I did a similar mod to my Ducati Monster air box by cutting off the top entirely, leaving just the rim that holds down the K&N air filter. The first time I fired it up it scared me! The carb noise was loud and it sounded like it was sucking me into the engine. Then it backfired! ouch. The upside is that I could no longer hear the clutch over the sucking sound!