Eegore wrote on 07/27/17 at 20:55:19:It may be the tire. Once I run through two more 150 tires, if the bike doesn't catastrophically fail, I will try a stock 140 and see if there's a change.
The stock 140/80-15 tire is only available from IRC, and it is a mediocre tire.
It really doesn't sound like you engine is making the power it should, as 4th gear should overcome any issue with a tire that is making the gearing too tall and you should be able to reach 70 - 80 mph in "one" of the gears.
If you want to keep the big/bold/bobber look - get a 140/90-15, if you want to get the most performance and a tire almost identical to the stock size - get a 130/90-15. If your bike can't go 80-85 with either of those sizes.....it is not the tire.