when you say missfire there are so many meanings and i dont think any of them are applying here
a pop and a bang through your exhaust every so often is a missfire (missplaced firing)
but below 3000 rpm no bang, above 3000rpm a bang things are a little different
this is where things get interesting and somewhat difficult to identify through here (but have before)
starting with easiest to get access etc
can you smell petrol in your oil (take filler cap off and get your nose in there)
(if you can then blown gasket or piston rings kaput)
are you able to perform a compression test ??? check all 4 and write them down you will see if something is wrong
(below 3k rpm pressure could just leak into crank through a couple of routes gasket/piss rings or even badly seated valves
[size=78%])[/size](above 3k rpm its still leaking but not fast enough so still ignites)
these are only the two i would start with here because this has popped up a few times but described differently and has turned out as 2 blown gaskets (including my own) and one badly seated exhaust valve - thankfully both of these are relatively easy to fix if you have the time, patience and a little know how (or this forum to be pointed in right direction)
and in tommorows lesson we shall be looking at fuelling issues
THE FIXES
Blown gasket - REPLACE GASKET (bet you couldnt have guessed that
[/size][size=78%])[/size][/size]Non seating valve - Valve lapping then check clearances - its easy its basic its like being in the scouts before we had lighters (some might get that joke)[size=78%][/size]piston rings - pray that it isnt them (it is the least likely to be a problem)[size=78%]