No sparking on a 1979 YZ 125

This could go several ways #1 engine mechanical check cranking compression should have 125 psi or greater in all cylinders if low but even squirt oil (1/2 oz.) in 1 cylinder and retest goes higher then rings sealing has been lost. if uneven check timing belt for jumped or misalignment. #2 electrical does it have electrical spark to plugs crank engine with a spark plug connected to spark plug wire laying plug on engine can you see spark? No test ignition coil wires with a logic probe both terminals will be red with key on one will flash red/green when engine cranking does not the primary ignition needs repair Ignition module? or pick-up coil/ crank sensor. #3 fuel system pressure and injection pulse. injection pulse can be tested with logic probe DONOT USE A TEST LIGHT YOU CAN RUIN THE ECU .

Electrics have had two days to plot...lookout!
I've just spent an entire summer re-beauting an IT250J. From all the dealers, parts seller's and on-line info I've gathered, ignition coil is usually first "component" to go. (assuming all wires/ connections 100%) Black boxes (cdi) and stator coils are fairly robust. I can tell you testing them is very expensive and WILL lead to more $$$ if you are any way ignorant of Ohms and resistance parameters. For $40 odd, try a new coil first.
Bear in mind, loose connections will have an effect on all electrical components in the system if not fixed quick enough.
Cheers.

if you are sure plugs are good, work your way back to the ignition coil, other end of spark plug cable, make sure all connections are clean and tight. if that is good next step is to test your coil, it may be toast

G'day i am a . YZ 125 j ...lets see now! You could perform these tests yourself.Check your plug CAP,if in doubt replace it with a new one. Take the below ohm figures and see if they match up. Pulse coil: white/blue and white/red= 310 ohms. low speed coil: Green/white and Black/red= 900 ohms. high speed coil: Green/blue and black= 55 ohms. hope this has helped in some way to helping you solve your problem.If it has please leave feed back.Ride hard , see ya,Tabbycat

Not much info Smitty but if that weak spark is constant (ie:pulsing ) i would guess that the trigger coil is ok.I'd make a check on your ignition coil earths then do a couple of continuity tests on your coil primary and secondary , orange terminal to ground,orange terminal to ht lead ,if they check out ok according to service manual specs then i'd run continuity test on the source coil.....Hope this is helpful in someway ...Smittycat.......from Flabbycat