Well that sucks.

What octane gas were you running?

Mods?

Jetting?

I don't think that small of a leak was the problem I have sucked a cylinder gasket 2x on a Pilot http://www.pilotodyssey.com/cylindergasket.htm and http://www.pilotodyssey.com/cylindergasket1.htm

octane--only run 91 octane that's the highest we have up here. mixed with motul oil 40:1

mods- 39mm pwk, power pros pipe, uni filter open air box, hoserized cylinder 79mm bore. i did notice that compression was lower last time i check 110-115. was planning on putting new rings on.

jetting- 180 main jet and 45 slow jet, needle on middle clip.

that leak was the only visible leak i could see and hear. i haven't pulled the balanceer cover or mag cover off yet to check the seals yet.

you have any ideas of what might have happened

How was your jetting?
Have you ever kept installing bigger main jets until you can get it to blubber at WOT (Wide Open Throttle) then work back down then keep raising the needle until its over rich in the mid range?

What plug?

8 psi to 4 psi in 5 minutes is not a big leak I would be looking at other things.

Really? I could have sworn I read on here that it should hold at 10 psi for at least 20 minutes,im actualy shocked your saying 4 to 8 psi in 5 minutes is not a big leak.Im not saying you don't know what your talking about,I"m just surprised that you don't think that's a big leak

Its not a big leak when you look at the big picture, and leaking out 4 to 8 psi in 5 minutes is not a acceptable leak either, the goal is to have a leak free Engine, I just don't want him to rebuild the Engine assuming the sole cause of the poofkaboom was the air leak then have a poofkaboom happen again.

I don't think that small of a leak was the problem unless he had some really poor fuel, I have sucked a cylinder gasket 2x on a Pilot Visit these two links and have a look both those leaks would not hold 10 psi for only a few seconds both were driven until a problem was discovered, http://www.pilotodyssey.com/cylindergasket.htm and http://www.pilotodyssey.com/cylindergasket1.htm even after discovering my Pilot was leaking I drove it from one end of the dunes at Little Sahara to the other to get back to camp, drove it on the trailer then drove it off the trailer into my shop when I got home, look at the crowns of the pistons and the heads.

Fatcats 350 was leaking worse than 4 to 8 psi in 5 minutes look at its piston and head viewtopic.php?f=1&t=9826" ."..

I would be talking to others in his area that use 91 octane in ATV's see what others think and are saying, sales on high octane fuels world wide are way down maybe the 91 octane he bought has been in the tanks so long its no longer 91 octane? Time to seek out another gas station that has documented proof they are a high volume 91 octane seller?

Guys running high compression 4 stroke ATV's and bikes and are normally running 91 octane should be able to notice and detonation problems right away from the excess heat between their legs if they have a after market exhaust they will never hear it, not much use in talking to guys that race most them burn so little fuel they all run race gas then you get the lecture on how you should only run race gas bla bla bla total waste of your time talking to most racers trying to sell the same snake oils they use...

Ran across this the other day while researching http://www.rotaxservice.com/rotax_tips/rotax_feed2.htm" .".. I always thought adding 2 cycle oil to gas make it harder to ignite (same affect as higher octane) not easier to ignite (same affect as lower octane) , trying to get confirmation on this.

Always knew about losing octane to storage and age, recall reading some place the parts of the fuel that give it higher octane evaporate at a faster rate than the rest of the crapola that makes up "gasoline"