Constant misfire ??????

[Bob Kneale]

Hi

At risk of adding yet another voice.  You describe good starting and running OK for several miles before getting a misfire under load.  This suggests something changing when it gets thoroughly warm, and presumably coming good again when it cools............

John's sound advice reminded me of a similar malady on my wedding day! Very similar symptoms which turned out (2 weeks later when I investigated in more detail) to be a cracked tappet block which expanded when hot to seize in its guide, causing the problem! Replacement of the item restored performance instantly. Moral- don't over tighten your locknuts!
As you say, systematically check one element at a time and don't tackle this one till you have eliminated the simpler  ones, especially as this will involve removing the head!
Good luck with your investigations .
Bob