Topic: 5k rpm cutting out

[antleo]

Bike is new to me. I purchased it less than 2 days ago. i knew it would need work, but this stumps me. The bike starts and runs great, when its on the stand in neutral it revs straight through the range. but when under load the bike pulls hard out of the hole but will not rev past 5k. It sputters and practically dies. I have the carbs off as this was my first guess. It has 120 main jets. I have a set of 97.5s I think I'm gonna throw in it as i did a plug chop and the plugs (which where brand new) are black. meaning rich. stock jetting i believe is 90? so i if that goes the right direction and helps get me up in the revs ill check that out.   I also found that one of the coil packs(if sitting on the bike looking forward, the one on the left) is cracked straight down the center.

I purchased the Bike from a local used bike dealer who has a plethora of used parts as well. It did not have a side cover or airbox cover, but if i  can find one in the plethora of parts(i found a side cover already, need the airbox cover) it'll be tossed in. 

So what else should I be looking at if these don't solve the issue?

[Scott S]

 What bike? You didn't state the year or model. Any mods to the exhaust? What part of the air box are you missing, exactly?
 Also, 120 mains are pretty huge for any of the SOHC4's.

[mrbreeze]

Spark advance?

[PeWe]

It can be the ignition coil that get a spike to ground. I have had similar with bad wires/caps in wet condition.
Try other coils with wires and caps. The seller has loads of parts, right?

[calj737]

+1 that 120's are probably too large for main jets. But, dropping to 97.5 jets from there is a HUGE change. Before you commence re-configuring your carbs, you need to perform the full 3,000 mile service. This site has a link to all the manuals for each model, download yours and start going through the list.

If your bike's motor is basically stock, set your carbs to the stock configuration, but DON'T use aftermarket jets. Only use Keihin brand jets and needles. Clean yours thoroughly and properly, noting each numerical stamped value and position. Then when you commence tuning, you can relate that info and proper guidance can be offered.

You should use a multimeter to test whether the coil, plug wires, and plug caps are within spec. If not, replace them.

And tell us the year and model of your bike in the topic or first sentence of your posts 

[antleo]

that is my bad, the bike is a 1980 cb650. The airbox lid is the part that is missing. it is a stock bike. exhaust and all. stock jetting as i was told was a 90 main. the rest are pressed in and i do not think have been changed. the 97.5 i have are manufacturer jets i have from another bike.   valves are in spec, oil was changed as soon as i got it, chain lubed and adjusted, plugs done, i plan to do wires and apparently a coil soon here.

[calj737]

It's a CDI ignition, so there's ample issues to investigate before you go replacing parts.