1977 KZ650c, When gunning throttle, engine will rev up but won't increase speed.

Hey guys, I've got a 1977 KZ650C. Its got stock exhaust, stock airbox. Carbs were just cleaned and re-jetted. All electrical re-done. Engine was just statically and dynamically timed. New carb boots. When i am driving it in 1,2, or 3rd gear, when I gun the throttle, it has power and will go. But when I'm driving in 4th or 5th and gun it, it will just rev up but won't increase speed or won't increase it very fast. I have to slowly apply throttle to get it faster. I know i have a vacuum leak on the carb holder side between the engine and the carbs. I have purchased new carb holders and am waiting for them to show up. But the bike has about 50,000km and I haven't checked the valve clearance yet. 

So what I am wondering is, would this issue be caused by a vacuum leak, or valve clearance? Or maybe it is my jetting? Or clutch not disengaging well? I also only did a bench sync, I am purchasing a vacuum sync this weekend.

I have attached pics of the plugs as well. #1 looks good, #2 looks rich, #3 looks lean, #4 looks good.

Thanks

Sounds like you have the exact problem I have. A slipping clutch.
Check your clutch freeplay first. Pretty easy.

Okay,  thats something to start with. How exactly do I do that? Or would it be in my clymer manual?

Your clutch is slipping.  What motor oil are you using?  Ed

Rotella T4 15w-40

#1 very lean
#2 good
#3 good, maybe on the hot side
#4 slightly lean

To get accurate readings you need to perform plug chop test at Idle 1/4,1/2,and WOT. 

www.dragstuff.com/techarticles/how-to-read-plugs.html
 

Yeah, the Clymer will have the procedure. It may be in the first chapter as it's a basic maintenance item. 
Your clutch cable needs to have a certain amount of slack. The manual will tell you how much slack and where to measure it.

Oh that's what you mean, clutch lever adjustment. Yea was gonna do that as well. Already did it when I put a new clutch lever on. But I'll do it again. 

If that doesn't fix it, there is a possibility you will need to replace the clutch.

That video is just about lever play adjustment.  There is a procedure for actually adjusting the clutch in the repair manual.

This is for a 78 650 via the owner's manual.
Check your owner's manual for the correct procedure for your specific model.

Manual excerpt via file base.. OKC_Kent

Be sure to follow the procedure exactly, as the order of adjustment matters.

If adjustment does not solve the problem, you will need to inspect the clutch. You will need a new cover gasket.
The most common problem is worn out springs. They are fairly inexpensive and easy to replace. The factory service manual lists specifications for clutch plate and  tab to basket wear. Inspect everything carefully and ask many questions.