Hey I had an exhaust valve go in my 1950 BMW R 51/2,while travelling across Canada from Vancouver in 1970.At that time I was in Ontario,and I first noticed loss of compression in one cylinder,and a "Phut-phut-phut" sound on that side. I adjusted the exhaust tappet every evening,but it just got worse. Also,just as you describe,it ran OK up to highway speed-barely- 50 MPH only. Any more throttle caused it to slow a little. At Ottawa I found a BMW dealer and was able to purchase a replacement valve and get back to speed.The old one had probably contracted a case of a carbon piece burning into the edge until a big chunk was burned away from the edge and the whole shape of the valve was compromised.Hence the need to adjust the tappet,to no avail. Bill