Both CDI boxes show signs of extreme overheating

[CharlieB]

Acquired a super clean, low mileage 79 750F a few years back. It ran only sporadically ...and I parked it. Then I got the bug again ...and recently discovered that the CDI boxes were both cooked from overheating. Any ideas as to what the culprit might be? Thanks so much for any help.

 

[dirtdigger]

They pretty much all do it

 

[bluellew]

It seems you didn't get much of a response. Perhaps the facebook page will throw up more assistance. I've just come here from there. I have a similar problem and still trying to work out some test for things. Best wishes.

 

[Rampton]

Acquired a super clean, low mileage 79 750F a few years back. It ran only sporadically ...and I parked it. Then I got the bug again ...and recently discovered that the CDI boxes were both cooked from overheating. Any ideas as to what the culprit might be? Thanks so much for any help.

The resin in the spark units(CDI) is prone to melting but this won't affect them from working, they put poor resin in them you can re-resin them if you feel the need. If you have issues with spark just replace them. Have fun but stay safe

 

[RobHolst]

The resin in the spark units(CDI) is prone to melting but this won't affect them from working, they put poor resin in them you can re-resin them if you feel the need. If you have issues with spark just replace them. Have fun but stay safe

Hey there, busy troubleshooting spark issues on a '80 cb750 K and and my spark units look beyond melted. Do you know of a way to test these units with a mutlimeter?

 

[kirky]

i found mine had melted and there was also a crack all the way along a coil , i thought these things were causing a bog out at 5000rpm . it wasn't either of these that were at fault, it was a 102 jet instead of a 75 which should have been in to correspond with the air flow on a JDM 750FZ

 

[RobHolst]

i found mine had melted and there was also a crack all the way along a coil , i thought these things were causing a bog out at 5000rpm . it wasn't either of these that were at fault, it was a 102 jet instead of a 75 which should have been in to correspond with the air flow on a JDM 750FZ

very interesting, it's hard to ignore something that looks this trashed as a possible fault but I understand that they can be totally fine despite their appearance. I am slowly replacing all the components to future proof this bike so would still like to know how to test them.

 

[kirky]

there is some information on the web about testing with a multi meter, in fact quite a bit of stuff. google testing a cdi

 

[RobHolst]

there is some information on the web about testing with a multi meter, in fact quite a bit of stuff. google testing a cdi

Thanks @kirky from what I can tell the spark units are quite different to modern cdi units. I managed to find the original manual for the bike that shows the correct way to test.