Truzo

Hi people i'm new to the forum. I was trying to find help for my Rd and signed up but I couldn’t find anything using the search function so I thought I may as well post ! OK, so I have a UK MK3 Rd with working YPVS. However the bike is spluttering and backfiring at about 8.5k rpm. The power valve seems to work OK and I think the rest of the bike is standard. My first thought was the carb but I only have a workshop manual for the mk3 without working ypvs. So does anyone know the jet settings for this ? Also any more ideas why I am having this problem will be very grateful Thanks Truzo.