Rough running.

[Shafty]

My dear old NTV650 appears to be struggling a bit lately, it seems to splutter and fart a lot now when I close down the throttle and there's some back firing too, it never ever did this before. I replaced the plugs hoping that that would do the trick but sadly not. Does the back firing mean unburnt fuel igniting in the exhaust suggesting that it's not burning as it should in the cylinder? I'd be grateful for your thoughts.

[ntvandy]

Backfiring may be caused by air leaks into the exhaust either at the cylinder heads or where the pipes join the silencer.

[richierichl]

Could also be air getting past the carb rubbers if they have cracked or hardened over time.