76 kz400 breaking up/missing above 6k

[whereamigoing]

New to me bike..... stock air box with k and n
Filter. Runs good all around but above 6k
It pops and misses. I'm leaning toward ign.
Problems, any thoughts

 

[VonYinzer]

Set EVERYTHING (ignition, carbs, etc) to spec. Make sure it's getting fuel correctly. Clean the carbs. Do a plug chop.

 

[whereamigoing]

Plug chop...?

 

[VonYinzer]

It's a home mechanics way of checking for proper air/fuel mix and ignition issues. On my phone at the moment so searching for good how to's is a pain. Use the search function here on DTT. There are several good write ups floating around.

 

[coyote13]

What Yinz said, but also check for air leaks around your carb boots. Spray a little WD-40 around the rubbers, if your issue goes away you've probably got a leak.

Google plug chops, no need to hash that out here

 

[Sonreir]

Air leak is not likely an issue. At 6000+ RPM, you're probably closer to WOT than not. With the throttle open more, there is less vacuum.

Air leak definitely a problem at idle, less so the higher you go in the RPM range.

whereamigoing, do you notice the problem more if you're under hard acceleration?

 

[whereamigoing]

Yea it's more when i Crack into it.... if i slow roll the throttle it is not as bad

 

[Sonreir]

Dollars to donuts your ignition coil is on its way out.

Replacements are about $35 bucks for a Chinese replica.

 

[whereamigoing]

Thanks if it's the coil....dounts on me

 

[whereamigoing]

I was hoping to buy you donuts but the new coil
Did not cure the 6k plus jitters. However
The idle seems a little stronger with the new coil so thanks

 

[Sonreir]

OK. Other option would be the fueling. If it's popping, then it's leaning out.

I'd pull the carbs and do a general inspection and cleaning.

If they're CV carbs, give the diaphragms a good look over, too.

 

[DohcBikes]

Stop throwing parts at it.

How's the pressure at the tailpipe at 6000 rpm? Plugged exhaust can cause this and is rarely mentioned as a possibility for a top end tuning issue on this site.

When did it start acting up?

Was it all at once or did it happen over time?

 

[whereamigoing]

Exhaust is hallowed out-- previous owner And it's been like this since I bought it..Got a bunch of rain comming so I guess I'll tear into the carbs next. Thanks for the continuing support.