Stator not CDI? Wassup w/that?

[Colorado]

Hi all,

I'm in trouble with the wife and need help :uh: . I went and bought another old bike yesterday --- a hybrid '75 MX400 frame/suspension and DT400 engine with a new top and bottom rebuild (professionally done -- new old parts). I saw the ad and called, and it turned out the guy was almost on his way to buy a used CDI for it. I've got a '77 IT400 parts bike (rebuilding a '78 IT400) and said, "Hey! I've got a CDI!" :laugh:

Bought the bike for about what he had in the motor, but it turned out my '77 CDI didn't match. OUCH! :bang: Bad points with the wife --- another bike that I can't ride (yet ... but she doesn't get it). Anyway, he claims that he tested the CDI by putting in a known good one, and got spark with a spark tester, but never tried to fire the bike.

I know that most CDI replacements are uneccessary. 95% of the time the real problem is with the stator. I've read it here, and been told the same thing at the scrap yard I use. What I don't know is exactly what that means!

The stator assembly seems to be prone to failure only in the charge and pulse coil windings as far as I can tell. Does that mean that checking resistance (I got thte factory Yamaha book with the impedance #'s with the bike) in the magneto coils will diagnose the problem? Or can something with the stator be bad even if the coils test good?

I can buy a used CDI for the bike for $75. The existing one is a hard epoxy body, so I'm not sure I want to attempt a CDI rebuild like I saw described searching here. I definately would rather test the stator before springing for another CDI. I just need to know if checking resistance of the two magneto coils amounts to testing the stator. If not, what's the test? Throwing parts at the bike isn't what I had in mind.

Thanks!
Steve

 

[MadArrow]

Checking resistance worked for me on my old beater 1985 KDX 200. The source coil was about 20-30% to low. It still had a spark and would start up great when the engine was cold. After it got hot and I let the bike sit for a few minutes it would never start back up. You might also disconnect the kill switch when troubleshooting.

 

[Colorado]

Thanks MadArrow. My multimeter doesn't have the correct ohm settings for the test, so I need to get another ohm meter. If both coils test good can I be certain the stator isn't the problem?

 

[MadArrow]

My understanding is no it is not a complete test. It is possible that it will malfunction when it actually tries to produce power. You could try seeing if there is any voltage coming out of the source coil. I got between 20-30 VAC with a good kick with no spark plug installed. It is deffinately not a true test, but might give you an indication. Do you have a weak spark or no spark at all?

 

[Colorado]

I've got no spark. I bought a new ohm meter today that I can set at multiples of one/ten, so will take it from there. I also have a '77 Yamaha 400 which may (or may not) have an interchangable stator plate. I can get a used CDI for $75 at a motorcycle scrap yard here, but can only test it, on the bike, there. My truck got rear-ended, and my bumper hitch is bent too low to pull my trailer, so I'm kind of screwed until the DrawTight hitch I ordered arrives, or until I get the insurance co. to fix my (work) truck, which I use daily. It's a pisser!

I understand how things make other things move mechanically much better than I understand electrical "black box" stuff. By 'source coil' do you mean what my Yamaha manual calls 'charge coil', which puts a charge into the CDI, as opposed to the pulse coil, which sends a jolt to the ignition coil?