1982 XJ 550 MAXIM 6k rpm in 6th gear only doing 120kmph

I have a 1982 xj550 maxim I've had for a few months and it preforms poorly at highway speed, it rips through 1st to 4th gear no problem but when it comes to 5th and 6th i give it throttle and wont speed up nor does it rev any higher, i can get it to 140kmph in 5th gear at 7-8k rpm but 6th wont hold that speed

 

Yeah, well I have the same issue but with an 82 xj650 maxim, only 5500 miles, finished all maintenance, and took it on the highway, 1st to 4th I can get to about 7k, 8k rpm max downhill and about 60mph, but as soon as I pass the 5th gear, drops to 4.5/5k rpm and I just loose speed.... Not revving higher, not accelerating or very poorly. I'm stuck at max 68mph with a 650 so I envy your 140kmph (86mph)...

I was searching the forum and it seems like the 550 needs to really push the rpm to at least 7k from 3rd before you start shifting, otherwise if you shift around 4.5/5k you're not gonna go much faster apparently. Tried that for myself but couldn't get higher rpms. I guess tomorrow is compression test day

 

I don't know if you fixed your issue, but for those who have similar issue in future I'll list what I've done and fix the issue.
I checked my plugs and adjusted my mixture screw back to 2.5 turns out, cleaned my air filter area, vacuumed soap etc. Checked my gas line and fuel filter, fuel filter was bad and when I noticed there was some rust in the gas line. (Recently phosphoric acid cleaned, finished with oil coating wash to prevent flash rust) well oil and rust sticked at the bottom of tank and because of bad filter it was getting inside my carbs.

Decided to take the carbs out for a clean, and I'm glad I did, my #4 carb main jet was completly unscrewed, along with the copper washer hanging loose for their life under the float.

Did a nice clean without braking the racks since I've done that very recently. Clean the gas tank, gas lines, changed fuel filter.

Just took it for a quick spin to check if that did it. Well HOLLY SHIT. This time it was me hanging for my life, I wasn't even trying and got to 95mph in 3rd at 8krpm... (might have been in 4th, but I really think it was 3rd, will update on that) got home cause highway was busy at this time, and I'm not trying to get a speed world record, just wanted to have the bike run right.


So to summarize, check and clean airbox and air filter, ignition coils and make sure all cylinders are working, check gas line and carbs, it could also be a jet issue, if you got cheap ebay jets, maybe get some from Chacal to make sure you get the real size you need.

Now I'm going back to work on my Seca! Good luck