Iain, doesn't higher compression cause a leaner condition and pinging. An example is Diesel engines.. knock..knock..knock... is that the lean condition you are getting?
Taken from thumpertalk "Several things cause pinging...
1. not enought fuel (lean)... amount needed varies with temp and altitude
2. too much air, air leak = lean
3. ignition too advanced
4. squish too large so hot end gasses pre-ignite fresh charge
5. compression too high/head poorly designed... during combustion pressure increases too rapidy causing spontaneous combustion of charge = detonation.
1-4 are all 'pre ignition' caused by chamber temps which are too high
5 is 'detonation'
Both detonation and pre-ignition sound the same while you're riding."
http://www.thumpertalk.com/topic/689696-what-causes-pinging/I think a fix for this is running the best high octane fuel you can get. An example is my Jeep V8. I have to run Premium 98 or my knock sensors go crazy. This is because i have the more powerful HO 4.7 which runs higher compression.
This is also interesting.. you have just rebuilt your bike... are you running more oil? This can lead to a lean state and i have read this in many places...
"When talking about being rich or lean, we're actually referring to air/fuel ratio.. not oil/fuel ratio. When you add more oil to your fuel (eg. going from 40:1 to 30:1) your actually leaning out your air/fuel mixture because the extra oil takes up the space that the fuel would have otherwise taken up. When going from 30:1 to 40:1 you're richening up the air/fuel ratio because now the fuel is taking up the space that the oil used to take up."
http://www.dirtbikeworld.net/forum/archive/index.php/t-2463.htmlSo, Iain, due to the higher compression head, your bike is running leaner..... all the readings suggest that to fix this problem, you can try retarding your timing slightly (going closer to TDC), use higher octane fuel, run less oil in your mix, jet your bike richer, or find another standard head.
You could eliminate one problem at a time. Go through the timing, fuel, oil changes above... You have changed ignitions so maybe the new ignition runs a little more advanced than the old one. Swap just the ignition first.
If no changes, put the ignition back and swap out the cylinder head.
Looking forward to finding out the answer..
Cheers,
Alex