My sv650s, speedo and back lights die'd... where can I start to look? The rev and other lights are stil working. eg: N - light, Feul low - light & High beam is stil working...

You have low voltage in the battery,all the best.

Without the make, model, year and engine type and size can't help you

What's the color of the smoke? white, white with a blueish color, or black?

The most likely problem is a a short circuit in the ignition barrel. It would give all of the symptoms you are experiencing. Although the speedo may have an internal short as well. The ignition barrel is just a tube with switches in it, it controls the power distribution for your entire bike.

Probably a break inside wiring loom of one/more of the three wires that eventually connect to the speed sensor on top of gearbox. The break I fixed in my 2003 Leon 1.6SX was in the loom under battery tray. My symptoms were no speedo working, and erratic fuel guage (particulary when under 1/2 full). I repaired one broken wire.
Jim Nichol
Seat leon 2003 problem with speedo not working and fuel gauge faulty is...

sock in the feul tank may be stopped up.my just need a new feul pump. check feul pressure first when car will not crank.

I suggest you check the fuel pressures and delivery volume before you replace any more unnecessary parts...

Don't spend money until you know what is wrong and that shouldn't be guessed but discovered by careful and systematic checking.

In addition check the plugs and spark power and quality

would change the ECU with the chip from the key from a car in scrapyard ,but before you get carried away try another seperate earth on the back of the speedo printed circuit board ,i wouldnt mind betting this is your problem

speed sensor on the back of gearbox