Engine run on can be a too rich or too lean LSN. If its too rich the engine will load up with excess fuel that will burn off when it sits at idle. Too lean and it may machine gun as it starves for fuel. There is a fine line between your idle gap and your LSN tune. Check idle first and then tune your LSN till the engine idles low and steady without that up and down effect. Run a few laps first to get everything heat soaked before tuning.