Exhaust popping on a closed throttle at high revs, coming down the gears. Why? I presume one or both jets are too small but which?
You could have a leaky jet which is still allowing the fuel to dribble in when there is not enough Oxygen in the chamber for combustion. This unburned fuel can then ignite in the exhaust system or on the opening of the throttle. This is more common the older a carb gets.
You could have the mixture too lean.
You may have a fouled spark plug which is causing too much fuel to go unburned.(usually more apparent on acceleration)
You may have a failing HT lead (spark plug lead) which fails to provide enough power to the spark plug.(usually more apparent on acceleration)
Improper ignition timing (unlikely on a computer timed bike such are the majority these days).
Improper valve timing (unlikely).
Also check your air filter and box to make sure you are not choking the air supply too much with a dirty filter.
Too lean?
It’s not like the sound of an exhaust backfire, it’s a muffled pop-pop-pop.
My bike has done it from new but I know they changed the carb from 26 to 28mm in the shop the day I collected it and I wouldn’t trust them to have done it properly.
Is there likely to be a mixture adjustment screw on my carburettor?
The other thing that can cause popping on the overrun is a combination of too rich mixture and an exhaust leak. You pull too much fuel and the air that gets sucked into the exhaust on overrun allows it to ignite inside the exhaust.
Try turning the idle mixture screw in an eighth of a turn to make the mixture richer. If the popping gets worse you have a bad exhaust gasket or clamp. Probably the gasket at the cylinder head. If it goes away you were just lean on the idle. The perfect setting changes with the season so you should be retuning about twice a year.
How would I know I’m turning the right screw? Are there any other adjustment screws on a carburettor beside this one?
Which way will be richer, in or out?
[quote=“jaame”]How would I know I’m turning the right screw? Are there any other adjustment screws on a carburettor beside this one?
Which way will be richer, in or out?[/quote]
QFT
Go identify your carb there, grab a diagram and probably download a tuning manual while you’re at it.
A muffled pop pop or as if a machine gun goes of, that just depends on the speed you are making and from what rpm you are coming down.
Mine does it too, especially when going down hill and I love it! 
People hear you coming way before they even see you.
All my Harley’s had it, my Suzuki Intruder and my current CPI does it too.
It can be altered a bit but why bother…
I always thought this can happen if the mixture is slightly to rich?
Whasa… :lick: Look at this!
sudco.com/Exhaust01.html
I would love to shoot potatoes with that pipe haha
Thanks Redwagon