killing spark plugs

my Dax ST70 has run 80km since a complete engine rebuilt.
its runs well engough, when it does. every few days however the plug dies..i replace it with a new unit and it works fine for a few days..then the same story again.
this has happened thrice!

the 'dead' plug when attached to the cable and ground shows a perfect spark..but the bike wont start unless i put in a new plug.

the used plug is usally wet with gasoline and sooty.
I will reajust the air mixture to be a little leaner, however, is there somthing else i need to look at to stop this from happening?

The deposits on the plug will be causing your spark to leak away (short out) over the porcelain insulator.
This will happen when under 100/150psi compression but not perhaps when checked outside the engine.
The fouling will be either oil or fuel.
Excess fuel(when burnt) will leave a' sooty' deposit but wet if its very excessive and un-burnt ,but then evaporate.
Oil fouling is wet and of course 'oily.'
If you have a look on 'OO racings website they have some good plug condition info which should help determine your problem.
Cheers.

Just another quick note,
Make sure the coil is earthed properly on the frame,(this is easier said than done)

Alot of problems can be caused by bad earth returns through poor or corroded connections. Wire brush and or emery back to bare metal then use copper paste before clamping down.

All the best Si

...and check if the sprak plug threads in the cylinder head are still good! + set the spark plug gap correctly.
After you checked the earthing, you'll have to dissamble the cylinder head and cylinder.
Valve leakage, worn cylinder or piston rings, bad head gasket,...

@ Waynus
The the engine is just rebuilt..
i thought correcting the jetting/valves/mixture would help, no?

FAS wrote:@ Waynus
The the engine is just rebuilt.

Oh yes, didn't read that part. Did you do a good running-in?
Are you 100% sure the rings are in the correct position?
Also, recheck if all rubber seals (oil channel) are in place + the positioning bushes.
Only new gaskets were used?

To be affected by a rich mixture only, the setting should have to be waaaaaaaay off to get this result.
But indeed, if you're 100% it's only fuel building up on the spark plag (set the gap correctly and use the correct warmth) you could try to make the mixture leaner.
Easiest way to test this is to remove the air filter and go for a (clean) test run.
Also check in which rev range the problem occurs the most.
(for example: let the bike idle 2-3 minutes with a clean spark plug and check the colour)
Make sure the choke is off!!!

Good luck (and have fun!)

PS: you didn't paint the cylinder head?

Waynus
Thanks for your input. will rever.

no the head wasnt painted, just buffed.
the cylinder was painted