Crank seals?

Well I guess I have a blown crank seal.Engine's been behaving strangely,reving up at idle and
running a little jerky at cruising speed.After ruling out most of the usual culprits I began looking for air leaks.While it was idling without clutch cover I directed an unlit
propane torch at the small gear.Wow! I just about blew my engine.It went straight to red line and the kill switch wouldn't stop it for some reason.Had to drop it off the kick stand to stall it.
Strange thing is,aside from a few quirks the engine runs strong and there is no visual evidence
of a leak at the seal,although I haven't removed the gear for a close look.I do have an extra set
of seals and if I can ask a stupid question,is it possible to replace the seal without taking the
case apart? Thanks,Ron
Sorry,I should have stated the engine is a Dax 70cc.

Thanks for the replies.
Finally pulled the small gear and the seal was trashed.When this engine was new it had a flaw.The hole for the large head screw that holds the small gear was not tapped deep enough for the screw to tighten against the gear.The gear would drift out enough to allow the key to slip out.
I fixed that problem and it ran great for many miles.I can only guess that when the key fell out it became stuck between the gear and the seal.Probably didn't help that I was revving the engine trying to figure out why clutch wasn't engaging,lol.Took awhile for the seal to finally fail.
Anyway it was easy to remove as it was pretty much in pieces.A spark plug socket was a good fit for pressing in the new one.(which I just happened to have,That's a first)
Couple things I learned.If your engine want to rev up it idle everyone will tell you your carb slide is sticking.It just might be a air leak.
If you check for an air leak with propane make sure you have means to quickly kill your engine.Mine was close to melt down,although the piston and cylinder checked out fine. Thanks again all,Ron