cylinder missing or not firing? Possibly?

Hello,
My bike is an '80 maxim 650. I think I have a problem with firing in my 4th cylinder. Heres my symptoms:
- exhaust has strong fuel smell
- small leak of fuel smelling liquid out of where exhaust meets 4th cylinder
- exhaust pipe for 4th cylinder is different colour than the rest
- fuel mileage is only about 11km/litre, im told it should be about 20
- bike sometimes backfires and also lurches if i give it hard throttle, especially above 100km/hr.

The carbs were cleaned about 4000km ago, when i got the bike.

The cylinder has a perfectly fine spark, the bike has 29000km on it. Also a previous owner installed a 4 into 1 exhaust header on it. Any ideas on whether it is a misfire? And if so what would be my solution?

 

What do your plugs look like?

 

Heres the plugs. Left to right, 1 to 4 in oder.

 

Kind of hard to see the ends of the plugs in the pic. # 4 does look a little too black. Post some close ups of the tips.
Have the valve adjustments been checked?
A compression test might give more information about the condition of that cylinder.

 

Check your valve clearances immediately. Put them in spec.

Then get a compression test.

If everything checks out, pull the carbs, and wet-set the float levels.

Meanwhile, I would recommend that you pop the oil filler cap and stick your nose in. Have a whiff. If it smells of gasoline you've got float issues for sure.

I wouldn't keep running the motor until sorted or you may damage it.

 

Okay, thanks for the info! Ill check the valve clearance to start