No power below 3000rpm, black spark plugs (XJ750 Seca)

Hello everybody!

My bike was a trash, a wreck, took me 3 years to rebuild (suspension, wheel bearings, brakes, speedometer, lights, wiring, paintjob, but that's off topic).
I've read almost every topic related to carburetors, but I haven't found the solution to my problem.

I rebuilt all four carbs (new jets, bowl gaskets, butterfly-shaft seals, idle screws + o-ring + washer, springs, etc. Everything's new or cleaned in ultrasonic washer), every little holes/routes are clean. Fuel levels adjusted as described on this forum.
New spark plugs with proper gap, adjusted valve clearances properly, new valve cover+screw gaskets, new oil+filter.
In short, everything that was needed.
Stock air filter box, new filter, new stock exhaust system with original collector box.
Carbs are synchronized, engine sounds good, idle mixture screws adjusted like described.

I rode for the first time after I rebuilt it last summer. The first few miles was good, the bike had power. But then it weakens, lost the idle and wants to stall. But if I somehow manage to get over 3000rpm, it gonna be a rocket. Like a wild animal. But no power under 3000rpm.
It was necessary to raise the idle speed, otherwise it would dead. I just got home barely. Took out spark plugs, all black, more, pitch-black!

I bought a Gunson colortune, adjust the mixture screws as stated in the descriptions.
New test: rode a few miles, it got better but a little later the same thing happened. I heard the exhaust crackles and pops because of too much fuel.
I checked again everything, and cannot find the problem.
Please if anyone has any ideas, help me with this.

(ps. I'm not too good in English, please excuse any errors)

 

Black spark plugs means you are running VERY rich, aka way too much fuel. The bike is running so it is probably not anything to do with spark, which leaves your carbs as the culprit. I wouldn't think that with a purely stock setup with synchronized carbs and idle mixture screws adjusted, as you claim, would do that, which makes me think something is not stock. I would bet that either your idle screws are NOT adjusted correctly as you think, which i'd bet is not the case, OR, your jetting inside the carb is the wrong size. We never know what the P.O. did, could be he switched the jetting away from stock. The only way to tell would be to tear them apart, unscrew your jets, read the numbers on the side, and compare to what should actually be in there. Did your bike have pods when you got it, and you switched back to stock airbox? Do you have a stock paper filter?

 

A PO could also have drilled out the stock jets instead of trying other sizes. The markings would not be accurate. You never know.

 

I believe that overly rich could also be caused by the pilot and main AIR jets being installed incorrectly (swapped).

This is a common problem since it is shown incorrectly in at least one of the service manuals.

 

@Greg Thunderpants welcome to the forums and your English is fine, better than some of the native speakers! As @Dan Gardner wrote it could be the air jets in the top of the carburetor bodies that are switched. The good news is that you can check them and fix if they are switched without taking the carburetors off the bike again. Here is a photo of what they should look like. Some carbs came with covers over them and some did not so no worries if not. Good luck.

 

Dan and Fuller had my same thought as this perplexed me when I rebuilt my first carbs

Also, you mention new jets.
Where did you get the jets?
Did you use emulsion tube or main needle from some cheap Chinese carb rebuild kit? (Don't, better off cleaning original)
What did you do for the float valve filter screens? Did you clean and reuse the existing ones or put new ones in? If new, where did you get them and any pictures of the kit you purchased?
Did you inspect the diaphragms? Maybe open again and verify you did not crease/crimp/fold them when closing it up.

If its not the flipped jets as described above, One thing, you mentioned it was fine, then weakened. If you let the bike sit for a while, then start it up and run it, is it instantly weak or does it run good for a while then weaken?
if its instantly weak, I'd suspect a clogged pilot jet somewhere. Since you say rich, I"d suspect Pilot air jet. BTW, pilot air jet can be different #'s depending on the year but 195 should work anyway.

Is your fuel enrichment circuit working correctly? does it increase as you move it left and then drop back down when you move it right? A stuck fuel enrichment can cause havoc on the low end.

 

Where did you get the jets?
Main fuel (120) and pilot fuel (43) jets are form a rebuild kit, the pilot air (195) and main air (80) jets were manufactured in a machine shop (because the thread of the screws in the kit are different). I think there could be a problem here, but more on that later.

Did you use emulsion tube or main needle from some cheap Chinese carb rebuild kit? (Don't, better off cleaning original)
From a repair kit. I will replace the original in worst case, but I dont want to

What did you do for the float valve filter screens? Did you clean and reuse the existing ones or put new ones in? If new, where did you get them and any pictures of the kit you purchased?
New filters, new floats.
Kits bought form Ebay, probably chinese.

Code:

https://ibb.co/C9LZ70W

Did you inspect the diaphragms?
They are fine, no pinchings, no holes.

I wish I had swapped the air jets but sadly I didn't. The bike is running good for a while, it weakens only after a 4-5km. There is no difference between 5 min or 15 min idle running before ride.

Is your fuel enrichment circuit working correctly?
Yes, it was the first thing I checked.


About the new jets from the machine shop... It's possible that it was manufactured incorrectly, so I put back the old 80 and 195 jets.
The holes (new-old 195) are quiletly different. Either the new jets size incorrect or the originals are drilled bigger.
I haven't had time to re-synchronize and try it yet. Need a few days to have some spare time.

 

We don't know what bike model you have. the old 80 and 195 jets might be stock, but they also might not be. What is your bike?

 

'82 XJ750RH

 

According to Chacal:
Main FUEL Jet Size: #120
Pilot FUEL Jet Size: #43
Main AIR Jet Size: #80
Pilot AIR Jet Size: #195
These numbers look like they line up with what you said you had, apart from the whole machine shop situation. Like the other guys said, check to see if you accidentally swapped the jets internally. Some how you are getting WAY too much fuel at lower rpms, there has got to be a reason!