still got missfire, someone HELP!!!

hi guys, my landy sIII 2.25 petrol, has still got a missfire, so far Ive done, timeing, new distributor inc points, condenser, rotor arm, ht leads, the whole lot, set the gap on the points to 12, no pitting or burning, new spark plugs, checked the carb no air leaks, setup the carb with a sniffer, and it idles good, soon as you rev it or drive it, she missfires, not everytime but its there, so where do I go now, im a total novice and all out of Ideas....

coil or plugs.

is the vacume advance ok .. ?

yep put a new coil on it and plugs, no differnce, I havent done the vacume advance tho, how do I alter that and what should it be set at, bear with me im on a massive learning curve here and am in well over my head.... thanks for the replys guys

do a cylinder compression test, could have a burnt valve/valve seat, not necessarily noticeable at idle but as soon as you load engine may show up as misfire,

The vacuum advance pipe goes from the carb to the disc shaped thingy on the side of the dizzy - it's not uncommon for the pipe to split or the rubber joins to perish. You can often hear this as a hissing noise.

Check valve clearances,a tight tappet could cause a missfire if it was holding a valve open.