So the tuned 04 YZ450F survived my valve shimming efforts but was popping and banging badly on closing the throttle sharply. It always has a bit but this was worse than ever. Could be just the fairly cold weather I thought, but we ran it at Hartland Quay in March last year at 4 degrees centigrade and it wasn't too bad. So I decided to tweak the pilot fuel screw. Then I decided I might as well get a fancy RFS screw in blue with a numbered knurled knob. On dropping out the standard screw there seemed to be no spring, washer or o-ring ! Put the new one in properly equipped and set it to a default 2 turns out as I was far from convinced the previous position was optimal.
What a difference! Started first kick and warmed up easily settling down to a steady idle - in fact it needed reducing a tad. Very responsive to a snapped open throttle with no popping or banging on closing. A tentative tweak either way suggested it was at about the right place but I'm looking forward to riding it in March on a setup day at Pembrey circuit. Not a championship round and plenty of runs so I can fiddle with the fine tuning.
Just got to replace the rather ageing race wet tyres now and take it to John at Revs for a suspension setup to suit my lad's weight. Might need re-springing, but he is still growing so perhaps not.
Oh, and I started the orange bike up for it's spring awakening after valve check (all OK) fitting a new plug, air filter and used titanium FMF Megabomb system. It's been a good day