My problem is that i have no spark in my savage. At first it started to loose ignition at full throttle, like the spark was weak, next day i lost it completely. I checked the coil, signal generator and the ignitor - they are ok. I only found that the voltage on coil's primary circuit is low, only 10 volts while on the baterry there is 12.5v. What could be the reason?

Check the voltage going into black box -- you know the voltage going out of it already.

I hope Justin is right and you have corroded connections -- them black boxes cost too durn much and to test the theory really requires a plug in a known good black box test which you can't do unless you happen to have a spare known good black box lying around.

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PS  Make sure your battery is fully charged before doing any electrical troubleshooting -- it can screw you up otherwise.

Then take your ohm meter & look at your wiring diagram, look for a wire that has a long run, find each end, isolate ( remove) one end & measure Ohms down the length of that wire. If its not REAL close to zero ohms, start studying down the length of it,
Find youself a good ground on the motor, use an alligator clip on a bolt or something, If you dont have alligator clips that slip on your test leads, you need them, YOu should also have a 3 or so foot long wire with a good clip on it, handy T/shootin stuff.. BUt start testing the wires/connections in the circuit. Easiest place to start is pull the tank, take connectors apart, look in them for signs of corrosion, put them back together,, maybe some kinda dialectric cream, IDK Ive never gone that far,.