2 guys built modded 681's on another chainsaw forum...both worksaws...both had modded ignitions, i think from a husky 272...I believe there is some stuff here about that...search solo 681. I wouldnt bother with the big bore...quality just isnt there.....porting the OEM is an option for sure....I think these require a slightly different approach to your husky/stihl's....they have very tight base volume and pretty good port timing stock...if you do buy one, #1 let me know where you got it and how much (im chasing one) and #2 run it stock first, run it in and see what its got before you open it up.
problems ive heard are the odd rod failure....av,s a bit weak, dawgs to small adn average filter....its filter is better than the stock dolmar 7900 but not as good as the HD dolmar....I dont think the HD dolmar can be adapted???
you have posted this in the right place, but unfortunately 90% if the guys in the chainsaw section dont read here....so may be better over there?
good luck
