Yes 8 km is less. It should be 10 or more as I said.
My father has a 86 FX and it gives I think a good 12-13 km/litre. Im not sure about this but I could check in a few days when Im back in Karachi.
Anyway that FX of ours has done just over 100k kms recently and has all fuel/service records from day one!! So it cant be compared to most FXs on the road.
The sad thing about Suzukis is that spares are of very cheap n substandard quality so even if you fit a brand new part you can be sure what you have.
Mechanics can tak eyou for a ride by fitting cheaper/seconhand spares and claimng its the best in the market. Its good to shop around and research yourself when u go for any major work.
I wont be surprised if the carburettor is a second hand or substandard third party unit. I have actually SEEN mechanics do this.
Sorry my post is not informative or helpful to you as I cant give u specific tips to follow.
All I can say is take your car to another mechanic.....someone known to you or your friends etc and just ask him to tune your car saying its giving you bad mileage. Keep an eye on him and see what he does and see what comes up. Whatever he says take with a pinch of salt and report here.