GL1500 Bogging down at high speeds

On older bikes a plugged fuel cap vent would cause this. Not sure on 1500, but on older bikes soak the cap in vinegar for a while and blow it out.

Probably running out of fuel. Check your pump output and replace as necessary. This is not an uncommon occurrence on the 1500. (That's what Ken told me, but he doesn't have a 1500 either)

Yup, running out of fuel. At those speeds manifold vacuum is low and first thought is auto petcock leaking vacuum causing a partial closure cutting off full fuel flow. Test petcock with external vacuum pump and check valve flow of fuel.

Next thing to check is fuel pump output and pressure. Or first thing however you want to go.

I agree with all of the post.
Check gas cap vent AND hose first. have seen those stop up a lot.
Fuel filter last changed when?
Check fuel pump flow

This would be the order I would check.

what should the fuel pressure be? I'm about to pull my wing apart to replace a bad shift fork and want to go thru everything possible so I don't have to pull it apart for a looong time

Mine is 2psi (+/-) 1/2psi

Also some fuel filters are only designed to flow one direction, if it was put in wrong way accidentally that might be the issue also since u just had that done...something to check out anyway..