High RPM Backfire when fully warmed

i know many prefer it but i don't trust/like the air pressure method to hold the valves up when swapping springs/guide rubbers myself. i have a length of cotton rope (washing line pulley rope, lol) that i work into the bore through the spark plug hole at bdc. then turn as close to tdc as i can at which point the rope jams the valves shut. no possible chance of losing air or having the compressor noise in the background while you're working so for me it's better. the cotton rope is chosen as any tiny 'threads' that come off in the bore will burn away cleanly.
neil.