Detonation probs.

[micks]

could it be a crap load of fuel?

[pancho]

Hey Mick Ive noticed a pinging noise at Nepean when the outfits come 'round the speedway bend, I always assumed that it was the passengers head-butting the witches hats! cheers wally.

[micks]

it`s the metal plates in our heads

[evo550]

OK.
It appears KTM offer 4 different thickness base gaskets.
When the cylinders are cast then gasket surfaces turned down there are slight variations in height between head and base surface of each cylinder, before the cylinder is bolted to the bike it's measured and matched to a base gasket. This is done to ensure each bike that comes out the factory is running the same (correct)compression ratio.
This process is also done for pistons.
Their advice was to stick with the original gasket thickness.
Seems that's what I'll do.
Their advice was also that you maybe able to jet to prevent deto with thinner base gasket, but would probaly blubber elsewhere (YZ490 comes to mind)
Lozza,
How long does it take for deto to cause the type of damage you demo'd ?

[Lozza]

Sometimes 1 WOT run sometimes a whole race but it doesn't take long, the delay is between when the deto occurs and you notice some drop off in performance.
Jetting to prevent deto sounds like pretty dumb advice to me, retarding the timing a bit or using a lighter power valve spring would be better.

[evo550]

Jetting to prevent deto sounds like pretty dumb advice to me, retarding the timing a bit or using a lighter power valve spring would be better.

Yeah, KTM didn't recommend it either, I asked if it was possible, they said maybe, but........
It's far less hassle to just spend $4 and put the right size gasket in.
Race day this Sunday, will see how it goes.
Thanks for everyone's help.

P.S. already running the "Angry" spring.

[vmx42]

already running the "Angry" spring.

Always good to know!