Electrical Problem

[Flightboss]

I have restored a 1980 CB 750 F and when it runs it is beautiful, unfortunately it has an annoying habit.  When it gets hot cylinders 3 and 4 do not fire, (carbs have been boiled out and rebuilt) let it cool for 5 minutes and it is fine until it gets hot again.  It is weird because # 3 and 4 are on different coils.  I am about to start checking the various values on the pick up coil, ignition coil etc. but I am hoping someone might have an idea of what is wrong.

Thanks for the help

Don

[CB750Winger]

   This is kind of strange. I'm fighting an issue right now with my 75. It runs great until about 15 minutes and starts missing. #1 plug is sooty but seems to be missing on more than 1. I'm thinking my coils are failing as the heat goes up. When it cools off it runs good again. I've done everything else. Have you checked all your spark plug caps and connections and checked spark with the plugs out?       Paul

[Flightboss]

Hi Paul

I have just finished testing the pick up coil and the ignition coils and all the values are within the normal limits, however the ignition coil values are on the low side of normal, and I am wondering if they fail as the heat goes up.  I have checked the spark at the plugs when the bike is cold but not when number 3 and 4 cylinders have quit and everything is hot.  That will be another test, right now I am looking on line for the normal values for the spark unit and the proper technique to test them.  

I'll let you know if I find anything wrong.  Thank for the post.

Don    

[CB750Winger]

 Don,  I'm not sure on testing these components correctly (I'm not great on electronics) but sure you will find out. Almost every part of my 75 is new except the coils and I'm sure they are the issue as heat builds up. Nothing smells hot but it gradually runs like crap after 15 - 20 minutes. I ordered a new set of coils and will post results when I get them on. My post below "75 has me stumped" may give you some other ideas. What do your plugs look like when you take them out?       Paul

[woodsrider250-2]

Coil can very well start failing when hot.  Swap them and see if the problem switches to other 2 cyls...Same goes for the ignitors.