1979 Yamaha YZ400 CDI

Brett Campbell
Edited Date/Time 5/30/2020 4:17am
Does anyone know If a 79 YZ250 CDI will work on a 79 YZ400 engine? The part numbers are different and one of the six wires is a different color. Any harm in giving it a try?
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project racer 5/8/2020 4:49pm
5/8/2020 4:49pm Edited Date/Time 5/8/2020 4:56pm
Stator and flywheel ? The tapers are different . 78 started the small case 250 era, and crank ends changed as well. I fit an 87 YZ80 system on a 79 YZ250F with minor work. The 400F uses the large taper crank ends, like the 73-75 MX250-360-400 , 76-77 YZ250,and 76 to 78 YZ400.

I used a 79 400F system on my old 360MX. Never tried one, but I would bet that the 465-490 shares that same large taper.
Brett Campbell 5/8/2020 5:09pm
5/8/2020 5:09pm
project racer wrote:
Stator and flywheel ? The tapers are different . 78 started the small case 250 era, and crank ends changed as well. I fit an 87...
Stator and flywheel ? The tapers are different . 78 started the small case 250 era, and crank ends changed as well. I fit an 87 YZ80 system on a 79 YZ250F with minor work. The 400F uses the large taper crank ends, like the 73-75 MX250-360-400 , 76-77 YZ250,and 76 to 78 YZ400.

I used a 79 400F system on my old 360MX. Never tried one, but I would bet that the 465-490 shares that same large taper.
I am talking about using just the 79 YZ250 CDI on a 79 YZ400 with the original flywheel and stator coils. Could it damage the 79 YZ250 CDI?
project racer 5/9/2020 7:50am
5/9/2020 7:50am
Brett Campbell wrote:
I am talking about using just the 79 YZ250 CDI on a 79 YZ400 with the original flywheel and stator coils. Could it damage the 79...
I am talking about using just the 79 YZ250 CDI on a 79 YZ400 with the original flywheel and stator coils. Could it damage the 79 YZ250 CDI?
250 is internal rotor. 400 external flywheel. Do the wire lead plugs match ? Plug it in and see what happens. Surely the advance curve is different between the two boxes.

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Brett Campbell 5/9/2020 4:33pm
5/9/2020 4:33pm
Five of the six wires are a color and plug (male and female) match. I thought the advance curve might be different. I guess I will give it a try.
project racer 5/9/2020 7:06pm
5/9/2020 7:06pm
Brett Campbell wrote:
Five of the six wires are a color and plug (male and female) match. I thought the advance curve might be different. I guess I will...
Five of the six wires are a color and plug (male and female) match. I thought the advance curve might be different. I guess I will give it a try.
With the YZ80 system on my old 250F engine, it seemed like it would twist a few more R's than the 250 internal rotor. I was trying to get some flywheel weight on it for short track. Moose added 14 ounces to a new stock flywheel. It would twist.
Brett Campbell 5/10/2020 3:48am
5/10/2020 3:48am
Project Racer....I found your old post where you told how you used the YZ80 ignition system. Very interesting!
What part of Florida do you live?
project racer 5/10/2020 6:17am
5/10/2020 6:17am
Brett Campbell wrote:
Project Racer....I found your old post where you told how you used the YZ80 ignition system. Very interesting!
What part of Florida do you live?
Tampa Bay area. Threw the stock 79 internal system on an old 88 YZ250 I came across with no fire. Went right on. Used to spend quite a bit of time at a local salvage yard with a few pullers. Learned that many Japanese bikes share the same 3-4 tapers on the crank.
Brett Campbell 5/28/2020 10:20am
5/28/2020 10:20am Edited Date/Time 5/28/2020 10:22am
Project Racer- I am working on a 79 YZ-IT400 and it has the IT stator plate with the lighting coil. I removed the lighting coil thinking it is not needed because I am not running a light. The microfiche shows the YZ400 having a bar in place of the lighting coil. Do you know what this bar is for? Is it needed? Please see attachment. The part in question is circled. Thank you for your help! BTW The CDI from the 79 YZ250 did not work.

project racer 5/28/2020 3:13pm
5/28/2020 3:13pm Edited Date/Time 5/28/2020 3:28pm
Don't think the bar is wired into the circuitry ,if I recall. The YZ80 system I used had one as well. Maybe it is more of a support system for the rotor / flywheel, in order to keep things in phase. Like a chain guide.

Edit: Never had much luck on mixing boxes / stators, even if the wire leads were correct. Certain models will swap it seems. I have an NOS YZ100 box that color/ plug matches ,and some NOS replacement stator coils for my old 125. Have not needed them so far. It is probably 50/50 they would all match and run.

Brett Campbell 5/28/2020 5:06pm
5/28/2020 5:06pm
I have the correct CDI on the way. Hopefully it will work. Thank you for your input!
Brett Campbell 5/29/2020 5:22am
5/29/2020 5:22am
Do you know if the spark plug cap is a resistance cap? I can't find any info in my manual.

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