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Jessmansimon wrote: After all I did, the bike runs well, UNTIL I get up to about 60mph. It just loses power. Doesn't stutter or backfire, but just loses power, til I have to back off to about 55mph, and it stays running fine. What is going on?

I had the exact same problem on my BS38 carbs and it turned out the float valves that came with the carb kit were too small. The kit came with 2.0 I switched to 2.5 float valve and the problem went away. Mikuni 29mm smoothbores use the same float valve www.z1enterprises.com/mikuni-needle-valve-assembly-25.html

are you syncing the carbs with a vacuum gauge?

have you checked voltage to the coils? this is a common cause of dirty plugs and weak spark

Check your timing with a timing light, and adust it dynamically if need be.

I tossed the stock ignition right away on my bike. Im running a C5 optical ignition. they ain't cheap but they work great and you never have to adjust them. quote from their website is "this ignition really wakes this engine up"

I had the replacement valve seat and pin when the problem occurred. Switched back to the original, and BAM! Problem fixed!!
Yes with vacuum syncing, she's riding beautifully!
Although, even though the vacuum is right where it's supposed to be (17-23cm Hg) the gauge dial says that range (about 9-10inches Hg) is where there is a vacuum leak or late valve timing. Should I just ignore that on the gauge? In the gauge manual it says the markings and warnings only apply to automotive use and to disregard for other applications. Automotive is a term for any self propelled vehicle though, right?
Either way, she is running great! Thanks for the help everyone! This forum is the bees knees!

In the U.S., the term automotive refers to cars and light trucks.
Motorcycles, boats, gopeds, and such are not included in spite of the fact that they are self-powered (auto-motive). Yes, it is confusing. Welcome to the English language.

i wouldn't worry about how much vacuum its pulling as long as the cylinders are balanced.