Digging out an old thread here. I'm working on a 75 DT400, hasn't run in a while. Compression seems fine. I cleaned the carb, put in a new (stock Yamaha) air filter and attempted to start it over the weekend - key word here being "attempted". It has a nice spark when I pull the plug and ground it while kicking. It wanted to kick back a few times and it just plain stopped the kickstart lever forcefully several times. It barely fired a few times but nowhere near close to running. I want to check timing but none of my magnetic bases/arms will hold the dial gauge in the right spot so I ordered the mount that screws into the spark plug hole and it will be here in a few days.

I've set the timing on the older points motors but this is my first time working on a CDI model. I see in this thread how the mark on the stator plate should align with the mark on the case, and it sounds like there's also marks on the stator. With the dial gauge I'll know where 2.9mm btdc is... But to adjust the timing you move the stator plate, so that mark will no longer line up with the mark on the case...

My question: How do I check the timing if the bike isn't running? I have an automotive-type timing light I can use once its running, but until then, how do I set the timing to get it close enough to start? And once its running, what marks am I looking at to check that its firing at 2.9mm btdc? Thanks for your help!

You have a pre-77 bike so technically you're looking in the wrong place if you have the stock ignition. 

The 75 400 has a slotted trigger coil (aka pulser) that is moved to adjust the timing.  Flywheel is rotated until piston is correct distance BTDC and then mark on pulser is aligned with mark on flywheel. 

Those ignitions sometimes act very strangely when they get old so sometimes you can't go by the marks.  If it's kicking back, then you can move the pulser counter-clockwise until it stops kicking back.  Then once running you can check and time with a light. 

P.S.  I set timing on the 400's at 0.100" BTDC (2.5mm)

Thanks Mark. I haven't taken off the stator cover yet, I just assumed it would be the same as the 78 in this thread (oops). Yes I believe its the stock ignition so I'll pull the stator and move the pulsar/pickup a little bit counter clockwise and see if it'll start.

On this bit: Then once running you can check and time with a light. What will I be using as the reference points to check the timing with the light? I guess when I measure out the 2.5mm btdc I can scribe a line in the case (or stator plate) to align with a line on the flywheel and adjust the pickup to get me firing when the marks align? Or is there a better way?

Marks on case and on flywheel that align at correct timing is the best IMHO.

P.S.  You do not need to remove flywheel to adjust pulser position on that model if I remember correctly.

Without reeading the story fully just yet I'll guarantee its broken off the flywheel locating key. Check that first & if that, replace it & forget the timing unless you've altered it too far away from original timing marks. That's my guess for the day.

Well I tried pulling the rotor last night and its really stuck on there. The threads on the rotor, where the puller screws in (left hand threads right?) seem boogered up so I couldn't get more than a couple turns threaded, and then the puller just pulls out of the threads. I tried some heat and that didn't help. Anyways I looked in the groove where the woodruff key sits and I'm pretty sure I'm seeing the key in there, down a bit inside the groove, looking like it should. I also see how the pickup coil can be adjusted through the slots in the rotor so maybe I won't need to pull the rotor off anyways. It all looks fairly decent, not a ton of corrosion/etc. Anyways I'll retard the ignition a bit by moving that pickup coil as far counterclockwise as it will go, try and get it running and then check timing based on marks I'll scribe at 2.5mm btdc. It'll be a few days as I'm waiting on the dial gauge fittings.

Oh damn, problem 'is' if you look in at the key & see it, enemy if it's broken you still see the top half & looks ok. Alright let's guess it defo is ok so how about set the piston at 2.9 & observe is the timing Mark is aligning & that should probe key is good. I'm thinking on those the timing Mark is a notch in the window aligning at s tab on the pulser, not an external stator mark around the perimeter. If they align then that's good but follow MarkT & move pulser forward anticlockwise to 2.5 if you want. Good if you can do that through the window. I had a problem on my dt360a 35 years ago where the start retard part of the pulser failed so wired it to start on advance. Worked good, just had to kick it like ya meant it. But talking about pulser I don't suppose someone has cross wired the pulser on yours. Theoretically not possible with the double rubber plug but if it had been replaced with individual connectors & green/white had crossed to red/white then--I'm guessing at this point haha & may not work at all.

I'll start a new thread in the correct sub-forum for my '75. See some of you there!