My kid was out riding the atv and it died. Hauled it into the shop and noticed the plug wire came out of the boot. No problem, right?! Reconnected, started it and fired right away and redlined without touching the throttle. Shut it down right away, waited a few minutes and tried to start it again, no dice. I could hear chirping in the jug as it was turning over. Check spark, all good. Tested compression, about 60-70 now. The topend and bottom end was rebuilt 200 miles ago.

Do you think it's a top end issue or bottom end or both. Why would it redline from just starting without touching the throttle?

 

well either the throttle was stuck or it got really lean. Maybe an air leak or a plugged jet. Better take it apart and look. What kind of piston was installed?

hmm early failure. maybe seized, caught ring, dislodged clip.
Is this still oil injected?

 

agreed w/ above and maybe rings broke? need to pull it apart for sure to see what happened. how did the plug look when you checked it? white(lean) or tan/black(good/rich)

 

I had just replaced the plug when I resolved the plug boot issue, so it was nice an shiny.

 

oil injected. Odd thing with the carb is I had recently tore it down, clean in solvent and reassembled. It was running like a champ until this issue.
It had a wiesco piston in it.

 

by any chance did yu put the slide in wrong and it hung up wide open?just a thought.

 

I don't think so but easy to check. It had at least 20-30 hours on the carb clean and ran fine.

 

Don't know if this makes sence or not but, when the spark plug wire pulled out, it most likely got flooded bad. When you started it after fixing the lose of spark, it most likely took off because of the flooded condition.

When you again shut it off, it most likely was still flooded and fouled the plug. A badly flooded engine will wash the cylinder wall down and kill your compression.

I'd try putting a table spoon of oil down the spark plug hole to re-oil the cylinder and turn it over for a bit with the plug out. Then throw a new plug in her and give it a try.