My VN will do a little better than 80 in third gear... your problem is either in fuel delivery or in spark.
Have you checked to make sure that there are no kinks in the vacuum hose going to the petcock and that the fuel delivery hoses aren't kinked?
Ya might even try a "speed run" with the petcock set to prime (all the way back - PRI ).
Curtis - Albany Oregon.
Currently receiving therapy from "Doc" - My 1985 VN700.
Daily rider in NorthWET Oregon.
Ammo Can Saddlebags
Shaved and Rejetted. Coastered. Degoated w/ Hardley pipes.
Bought Feb 2007 with 12K miles.
Hit the 24k mile July 2008