If its carboning up, the previous posts are all good recommendations.
The worry I have with engines that potter is that apart from not getting hot enough so that carbon builds up, they can also glaze rings, which can encourage the carbon build up.
So I'd be looking to give it a deglaze and fit fresh rings.
Rings need to do hard work early in their life. They don't appreciate gentle running. plenty of revs, not laboured.
Fully understand wanting a cruising top gear, but it does mean the engine is further encouraged to labour and any hills or passing require down changes, so can be counterproductive - lower oil pressures, more load on the bearings...
As a counterpoint; not that I'm recommending it, Eddie Dow noted a safe continuous max rpm of 6800 for a sound engine and my race engine routinely pulls into the sevens in top gear. Just watch you don't over gear is all I'm saying.
While the box is out, it might be a good opportunity to review the gearbox bushes and layshaft for wear.