Fuel gushing out of number 5 cylinder, what solution do you have for this problem?

If you have compression and spark, that leaves fuel. A fuel injector balance test would be next step. Unfortunately not a simple task for that model year.

possibility that the float needle is not shutting off and the fuel is leaking past the valves into the cylinder past the rings into the sump
drain the sump, refill to the correct level and turn off the fuel tap under the tank
if the level doesn't change after a period of time then overhaul the carby and turn the tap off after each use

My first guess would be the power brake booster is leaking vac.

you need to run new injectors and injector leads.

Cylinders 1,2 and 3 are the cylinders on the passenger side and cylinders 4,5 and 6 are the cylinders on the driver's side. They are numbered front to rear. The coil pack will have the cylinder numbers marked on the top of it. The actual firing order is 1-4-2-5-3-6
The top of your coil should look like this:
1 2 3
5 6 4

Your engine looks like this:
1 2 3
FRONT OF ENGINE
4 5 6

I'm assuming the code that was pulled is P0305. Best case would be a bad spark plug on the number 5 cylinder. Keep in mind, the spark plugs are torque sensitive when installing. These are the possible causes that are stated in a Jeep service manual for your vehicle. Check the basics first......Camshaft,crankshaft,valve timing or vacuum problem,ignition coil output is low,engine internal mechanical problem,secondary ignition pattern is irregular, fuel contamination, Fuel pressure leak down is out of spec, fuel pump ampere out of spec, fuel pump capacity out of spec, fuel pressure is out of spec., engine compression, PVC system restriction, Fuel injector #5 is defective or the connection on the injector itself is loose or corroded, Intake gasket defective. It could also be freeze frame data in the PCM, but this is a very very rare cause.

All these codes indicate you have a misfire on cylinder number 5.
P1399/P0300 - Random/Multiple Cylinder Misfire Detected
P0305 - Cylinder #5 Misfire Detected

Causes:
- Improper spark plug at cylinder #5
- Insufficient compression at cylinder #5
- Incorrect fuel pressure
- The injector circuit is open or shorted at cylinder #5
- Fuel injector #5 may be faulty
- Intake air leak at cylinder head

Solutions:
- Perform Injector Service
- Replace Spark Plug #5
- Replace Injector #5
- Replace Ignition Coil #5

CYLINDER NUMBER 5 IS STANDING IN FRONT OF VECHICLE IT WILL BE THE 5 CYLINDER COUNTING FRONT TO THE BACK CYLINDERS IS IN ORDER 1 2 3 4 5 6 THAT YOUR CYLINDER ORDER. YOU HAVE A INLINE 6 CYLINDER ENGINE.YOUR MISS FIRE COULD BE THE COIL - SPARK PLUG - CLOGGED OR FAULTY FUEL INJECTOR. ALSO INTAKE MANIFOLD VACUUM LEAK OR SLIGHT HEAD GASKET LEAK WILL CAUSE MISFIRE.

For the 2000 Buick Centrury 3.1L V6:

The # 5 Fuel Injector is located above the #5 Cylinder.

For the 3.1L V6:
Cylinders:
.......(back)
1........3.........5 <-----
2........4.........6
.......(front)

Firing Order:
1-2-3-4-5-6

Ignition Coil Connections:
..........(back)
|.5..2.|..|.3..6.|..|.4..1.|
..........(front)

Let me know if this helped, or if you have additional information or questions. Feel Free to contact me at FixYa.com!

What size Engine?
The Number 4 fuel injector goes to Cylinder #4.
On 3.3L and 3.8L it should be the Front Middle Cylinder.
If you are not sure where Cylinder #4 is. Look on your Electronic Ingition Coil = Distributor. It has small numbers on them #1 thru #6. The numbers are small and black, but they are there. Follow that #4 wire to Cylinder #4. Fuel Injector #4 should be right over the top of that Cylinder. Usually the Cylinders themselves have numbers on them on the engine block, but it is easier for use to use the Ignition Coil as a reference.
Cylinders Numbers should be:
1 - 3 - 5 Rear
2 - 4 - 6 Front
Ignition Coil should be:
6 - 3 Rear
4 - 1 Middle
2 - 5
Let me know if that helped, or if you have any additional information, problems or questions. Feel free to contact me at FixYa.com!