After the install of a cline system on the OS 1.60 all the needles will now be much richer than before due to the excellent fuel delivery. The problem should disappear when the engine is adjusted properly. 8100 rpm is very low using the 18x6W APC prop. I tach around 9000 to 9200 rich for flight according to the humidity that day. We have alot of humidity here on the Gulf Coast. Switching to an OS F plug should help also.