Originally Posted by jasonh
Tighten all bolts on motor? (head studs, intake and exhaust manifolds...)
Thanks, Jason. I was just getting on here to write...
I fixed it!!!!!!!!!!! I routinely tighten all of the bolts before a ride, but apparently I hadn't tightened the motor mounts enough -- doh! I re-seated the motor to pitch it forward a tiny bit more, then snugged the mounts good and solid. The problem is not only gone, but now I'm easily cruising at 31mph!! It seems so strange that loose motor mounts would cause this problem, but I guess the vibrations were bad enough to make it an issue.
God it's such a relief to solve a problem after you've been tinkering with it for four days!