Hello VMX experts!
Short version - on a 1978 YZ125, where would I expect to see the bike firing at idle with a timing light as compared to the static timing marks on the rotor/stator? I have the bike timed spot-on to the factory spec of 1.2mm BTDC using a dial gauge. Adjustment screws are pretty much exactly in the middle of the slots. Bike starts and runs nice and crisp with good power on the pipe but seems a little soft on the bottom (I know, they don't have much on the bottom). I shot a timing light at it just out of curiosity and at idle, it is firing closer to 3.0mm BTDC or so (see attached pic for approx where I see it firing using the timing light) and with revs it retards towards the timing marks on the stator but never reaches the middle mark where I actually timed it to. Is this expected? If not, what could be causing this behavior? I guess I thought the timing marks would line up at idle, then maybe advance some with revs?
Longer version - I'm working on a nice 1978 YZ125 for a friend. At some point it had been converted to a YZ100 for the Marty Tripes series and we want to put it back to a 125. We rebuilt the crank and installed a '78 YZ125 cylinder and head. At some point, somebody had installed an outer rotor/flywheel style ignition on the bike, I assume from an IT model. The bike started and ran but seemed super sluggish and down on power. We suspected the massively heavy flywheel and wanted to get back to a more original ignition system.
I bought what was listed as a 1977 YZ125 stator and rotor. The keyway on the rotor was pretty beat up so got a different one also listed as 1977 YZ125. From what I can tell, the CDI appears to be original to the bike as a 1978 YZ125 CDI box.
With the original style ignition, the bike runs WAY better, but seems slightly soft on the bottom. I'm confused by what I'm seeing with the timing light but have honestly never used a timing light on one so not sure what I should expect. Have always just set timing using dial indicator and left it alone.
One thing I did notice - from the 1978 YZ125 manual, you can see the rotor is stamped M100-22. Would I need this specific rotor to work with the 1978 CDI box? Or, I see a '77 YZ125 CDI box on ebay - I wonder if I should try that? Or maybe I just throw it all out and put a PVL on it 
Here are some details on the components on the bike if it's helpful!
Rotor: M100-20, stamped 602
Stator: M100-20, coils stamped 712A, plate stamped 803
CDI: TIA01-16 711
Coil: Mitsubishi F6T411 (pretty sure this would not have been original)
Using a timing light, it appears to be firing at this point at idle and lower rpms and will retard to the first shorter mark with revs:
"soft on the bottom" compared to what benchmark? seat of the pants testing? What pipe is on there '78? What carb? Reed valve condition? Stock airbox? What plug? Stock Gearing? I've owned several 1977s & 1978s & 79s raced as 100cc in Marty Tripes. I'm not an expert on timing those and never needed a timing light on mine- just went static like manual shows. Timing stock (make sure you do use the exact Rotor & stator plated called out in manual and there's multiple versions of that TAI01- CDI box with different curves too. I read some old MXA & Dirt bike articles claiming the 1978 YZ125E was the best of the 3 125s ('77-79) with best power- but peakier due to modified pipe, different head and porting IIRC. A couple VMX racers I knew swore the 1978 best racer. I thought the 78 was better than the 77 I raced in VMX too.
There's a lot of variables here that could be issues, so In your situation I would make everything stock like the manual. Check the resistance on the primary coil and the individual stator coils too. Plug and fuel run is important too. Could be as simple as changing sprockets to get more low end, (but those coils on stators do fail sometimes), and a brand new correct plug and fresh gas. Good luck!
Thanks for the reply!
I had a 1977 YZ125D and feel like it had a bit more pull down low.
My main confusion is what I'm seeing with the timing light (wish I'd never used it, ha!).
The pipe, as far as I can tell, is from the '78 YZ125. Running a B10EG spark plug which should be correct for '78 YZ125. No airbox, just a UNI foam pod filter. Stock gearing. Carb is a new Mikuni jetted 100% to stock specs. Reeds looked fine. But I do have some new ones from Vintco on the way to install.
Since I believe the rotor and stator I have are '77, I found a '77 CDI box on ebay and have it coming to try. I also bought a NOS primary coil since I don't think what's on there is original (but close). Going to be interesting to try on these new parts.
Thanks!
Kelly
ur welcome. 1 of my 1977 D models ended up a bitsa Franken bike after I got it going from non-running. Had a 80/81 IT125 lower end laying around and put the YZ top end on it. Had a 34mm carb when I got it and was surprised it didn't need re-jetting. Stock ignition, pipe/silencer, airbox (with UNI) and it really pulled down low. Test rode it before selling and front end would come up a little in 1st & 2nd gear. On my 78 100 and 125 I ran B9EV plugs and jetted for my altitude (sea level) and always ran 91+ pump gas unless racing- Used VP.
Anytime I don't know history of a bike, I change L&R crank seals too. Curious what size rear tire is on there too. Mine went better with a 4.10 rear than 4.50 that made it feel sluggish.
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