okay ive spent another day under the bonnet to no avail. that bronze looking rocker does appear idea did not work it was the same as the rest. all valve stems seem to be the same in length etc. i did notice that inlet valves have longer stems than exhaust ones is the correct?

piston probs slap or gudgeon pin rattle wouldnt cause this prob would it because i am thinking the backfiring im getting when going downhill in gear would be valve probs / timing but timing is all done automatically

when i pull off 3rd ht lead it goes away but obviously runs rough