Plug keeps coming loose-426EX

[Knowpig]

I built a 426EX for a good friend. He's in Glamis even as we speak.
He called me yesterday saying the 426 is running great, his 14yr old son just beat a Banshee up Olds on it (Just go with it, I was nice, you can be too.) but after the 'race' it was making a puffing sound from the engine. He asked what it could be. I tried to stay calm and went through everything in my head real quick. Then I started thinking, what did i NOT do on the motor. He installed the exhaust, intake, plug, etc.........."hey, is the spark plug loose?". And yes, it was. The spark plug had backed out a bit and was indeed loose enough to let by an audible puff at idle. I would like to include that this build did get a brand new head. He has installed another new plug, but it also keeps coming loose while riding. I have not yet laid eyes on the quad, but he said the plug went in nice and the threads in the head and on the old plug seem fine. It's not blowing the plug out, just backing off a bit and getting loose.
Has anyone had this happen?????
What advice can I offer him? (or you to me!)
:confused:

[Rootar]

try a very minimal amount of lite thread lock on the plug threads

[GPracer2500]

What piston and what fuel? No place I've ridden will bring out deto like making runs up Olds. Detonation has been known to knock plugs loose from their threads.

Just a thought if you're grasping for explinations.

[Knowpig]

I am grasping!!!!!!!!!!!
It's the 11:1 JE piston 426 you helped me with!:D
I am going to look further in this direction.
Keep the suggestions coming!!!!!!

*and it's running pump 91 mixed w/vp110.

[GPracer2500]

Originally posted by Knowpig
I am grasping!!!!!!!!!!!
It's the 11:1....and it's running pump 91 mixed w/vp110.

That would make the deto explination unlikely. Hmmmm......:confused:

[bansheemorphine]

i had that problem with my 440 before it slowly loosened and then blew right out is all i did was tightened it back up and it was good to go never happened again i'd suggest not tighten it too much tho because that wouldn't be good

[Knowpig]

Originally posted by GPracer2500
That would make the deto explination unlikely. Hmmmm......:confused:
Unlikely, but not impossible.
Do you have a friend that you dearly love, but they just don't listen?
It's 11:1, HCstg1 cam, stock pipe, slip on, no porting, open lid, k&n filter. I told him 91 premium from Shell or Chevron. For extra safety, mix in some VP. So what did he do???? He went to the local Vons grocery store and filled up with "vons 91". And I am unsure how much VP was in the mix.
This is why I am forced to drink.:eek:
j/k
But that drives me up a wall. I think that's what it was. I will get eyes on it later next week.
Thanks!