XJ900 31A Misfires, sputters, backfires at higher revs.

Having issues on my Yamaha XJ 900 -31A (1983) misfires, sputters and backfires at higher revs, a carburetor has a fuel leak when engine is off and the peacock is on prime.
Bought it as is.... and didn't now what was working (well did some adjustments and tried to get it more stable)

Its stock airbox, air filter, carburetors but the exhaust system is some unknown made, i did put some inner exhaust wadding to compensate as the original exhaust with simulated backpressure.
But with no luck to sort the problems.
Also I changed the gas tank and don't have the vacuum peacock, its ON-OFF with prime.

What could i do to at least sort some problems? Also what can I do when I don't really understand with vacuum clocks on how to adjust. There is some where told in the manual that a special tool for the adjustment of carburetor synchronization must be used. I only have 4 gauges china made tool, can i synchronize them with that?

What I think could be some issues but I need to take the short cut as we all want.
Have done:

1. Adjusted the air screws on the carburetors, 3,5 turns without the 4 gauges tool and did not synchronized them.
2. Cleaned the damn carburetors several times and reassembled them at least 3-4 times.
3. Fuel is sufficient, peacock isn't with vacuum and its on prime.
4. Float levels has been measured (i think correct and nothing is stucked)
5. Did a carburetor repair kit installation on all 4 cabs.
6. Checked for Intake leaks but it seems ok, for the moment. Some visual cracks on the intake boots, unsure if they leak... doesn't seem to.
7. New spark plugs.
8. Ignition Coils and Wires, ok

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What I want to do but I am unsure how to.......

1. Adjust the air-fuel mixture and sync the carburetors. (Can I do this with 4 gauge tool and any guidance how?)
2. Fix the leaking carburetor. (Dismantle the diabolic carburetors again and test the floating bowl thing)
3. Check the spark plugs and ignition system. (Maybe adjust the gap on them)
4. Inspect for vacuum leaks in the intake system. (change the Intake boots)
5. Consider rejetting the carburetors if the new exhaust affects the engine. (DONT WANT TO DO THIS)
6. I don't have any original exhaust system but can I alter the engine's backpressure, affecting its breathing with putting more wadding?

My biggest concern is the peacock that doesn't have any vacuum as the original, does it affect with the carbs (i have plugged the vacuum carb) ?
And also the exhaust is my other concern, don't want to rejet the carbs.
The reason I am writing all this is simple what to begin with instead of starting at the wrong way.
Also can i synchronize carbs with 4-gauges tool, Yamahas service book states a very complicated.
Any guidance are welcome and also how to do it the right way, cause this has taken a lot of time dismantle- mantel the carbs that is a hell doing it.
Thanks in advance.

 

Hi and welcome. Did your carb rebuild kit include jets? Hopefully not, and you still have the original jets in.
A misfire at high rpm is nothing to do with carb synch, although you should perform this - a bench synch using the first progression hole is sufficient. fine tune with the 4 gauge tool.
Look at the resistor plug tops for breakdown - did you get the correct plugs? ends of the ht leads for corrosion.
Are the air jets in the top of the carbs in the correct positions? small one for main jet, large one for primary?
Has someone swapped the needles out?
All the diaphragms good?

 

Backfire is interesting.
To me this means it explodes the gasses and then they go out the inlet valve into the air filter instead of exhaust.
Common issue with the old mechanical points being out of whack.
Not tci stuff though, well unless it's faulty of course.

If it really is backfiring at high rpm then you have an issue with the cam chain as the tci is instructed to fire by the crank and the valves open and close by way of the cams which are connected to the crank via the cam chain ie. The weak link.

But if your talking about popping from the exhaust well that is too much fuel and fuel escaping into the exhaust unburnt. needles too high or main jets too big generally (very generally).