Yes very odd, I had the exact same noise on mine after complete rebuild. Fired it up first time for the heat cycles and it was fine for a couple of minutes then start the scraping noise, sounded like the fly wheel scraping on the cover, but no. Fired it up again, still doing it so I listened closer and it sounded like it was coming from the depths of the box, bollocks, so turned it off. Like you pissed off 😥 Fire it up again in denial reved it, guess what? Gone. Fcuk knows what it was. Since done around 500miles and seems ok, neutral can be a bitch to find even though I've done all the shift pro and star mods so may be linked. Had the clutch and basket off and all looks fine. Will probably pull the lump out in the winter and pull it apart to get to the bottom. Sorry no help but I feel for you,hope you sort it, the only thing I could think of was maybe the box oil was taking its time to get into the close tolerance areas of the box and clutch?
It idles great, pulls off then machine guns at just over 3 grand then at 4 grand it settles down then feels flat to 5 grand which is where it hits a wall No spluttering at all, just sounds like the engine is trying to "draw" I have feathered the throttle here and wound it right round to no effect, it only gets in the band if you hold the throttle steady(maybe a third open) and wait !! It has hit the power band a few times at 7 grand and really comes in with a wallop and you can see the addiction here LOL I think it's a lean spot as the plugs are quite light I have raised the needles , so ran out of adjustment there , that is why I was going to try bigger pilots
It may well be the exhausts and Athena barrels mismatch and be impossible to cure A bit of fine tuning on the ignition and carbs may make it less severe
Been told to try putting the chokes on to see if it will rev through the flat spot
Will also mark the throttle position too just to see what openings it is playing up at
Looked at a few dyno charts with Lomas and they are as flat as a pancake in the lower revs , making less power than standard !