I'm at altitude (Denver) and my bike is still running very lean above 10k.

What can cause this aside from jetting?

 

Air intake, exhaust, carbs out of tune.

 

Carbs are stock, and I'm altitude so I figure it would run rich if anything. I've got a K&N filter and a Yoshi exhaust.

 

There you go. Whenever you change your exhaust or your intake you have to re-tune your carbs.

 

As hamlin6 said.
Typically with performance pipes, they come with a jetting guide but in your case the k&n filter chucks a spanner into those recommendations.
Lean is obviously not good but your " very lean " needs sorting sooner rather than later.
Lean means more fuel required so bigger main jets would be the starting point for me. Confirm floats are good and carbs clean whilst your in there as well.

Best of luck

 

So there's nothing mechanical or electrical that would only affect the top end performance?

And for what it's worth, when I say very lean I mean it will *not* go above 10k. In neutral it will, but under load it's like it's hitting the redline early.

 

Generally speaking, no. If your bike wasn't tuned after changing out your intake and exhaust, you can pretty much bet that is where you need to look.
Also, instead of looking at your tach, where are you in throttle position when you have experience this issue? Are you at idle, 1/8, 1/4, 1/2, 3/4 or full throttle? Knowing this gives you a good idea where in the carbs you should start.
+1 on what Wes17 said also.

 

It's happening around 3/4 to WOT so I went and ordered 2 138s, 2 140s, and 2 142s.

Now just to be sure, I'm trying to do a vacuum test. When I hook up to any cylinder the needle jumps around like CRAZY, almost all the way around the dial. When I increase the idle to 2k it smooths out to ~5in, and when I blip the throttle it jumps to ~11in then drops to 0 and back up to 5in.

I've looked and looked but I can't find any info on those readings. I'm hoping it's just user error. Should I start a new thread or do you guys have any ideas? Thanks for all your help 😇

 

The bouncing needle is likely because the valve on the guage is open too far. You barely want to crack it open. If that doesn't fix it, you might have an air leak.

The actual values on the guage dosent matter. What matters is all four cylinders are as exactly the same as much possible.

 

My vacuum gauge doesn't have a valve of any sort (harbor freight vacuum gauge and fuel pressure tester). Am I missing something or do I need an entirely different gauge?
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