i think you got it. just tested comp on the 79 xt500 friday, 120#.
its running much better now since changing the k&n clamp-on to the stock airbox & running stock tt500 exhaust. i'm using tt500 carb, vm34ss with 340mainjet. the 310 flattened out too quick & the plug looked pretty clean.
so i had a 340 & thats what i'm using for a while. now it runs like a normal machine.