For circuits that are relevant to MSUK (and many kart circuits are) the limit is supposed to be 95dBa measured at 2/3 throttle with the meter 1 metre away from the exhaust, directly behind it with no sound reflecting or absorbing surface nearby other than the road (and supposedly hard surface). I only learned this on Tuesday when I asked a scrutineer.
Some places get away with noise at the boundary of the property, I wasn't able to get a number for that, it's individual councils.