Topic: K4 CB750 No Spark

[BakedPotato65]

Hi new to the forum. About a month or so ago I was given a 1974 CB750 as a project for me to fix up. The bike has 4K miles and is all original except for it was missing the seat and headlight lens from a fairing from the first owner. The engine was also seized so over the past month I worked on getting the pistons unstuck then rebuilding the top end with new pistons and gaskets and the same with the carburetors.

I’m at the point where I’m ready to start the engine but I’m not getting any spark. I bought fresh plugs, I’ve tested the coils, 5ohms on the primary test and then 15ohms on the secondary test. The points look to be perfect but I’m a little confused on how I can test them. I’ve already tried running voltage with the multimeter through the blue and yellow points wires and I’m getting negative numbers so I don’t know if I’m testing it right with the multimeter. I’ve also cranked the engine and held the plug wire copper (I’ve done the same with the plugs in the boots also) close to the frame bolts and it’s not getting any spark. So I’m kind of stumped. All of the rest of the electrics seem to be working when I turn the key also. I have a brand new battery charged up and it reads around 11.9v so that should be good.

There is a huge mess of wires in the headlight I’m assuming it was for using the fairing or something but there’s only a few wires disconnected up there. There is also two wires unplugged from something next to the fuse box area, one red and one green, I cannot find where they plug in unless something is missing. Other than that the bike is ready to start up but I can’t figure out why there isn’t any spark.

Thanks a lot for the help

[robvangulik]

Voltage of 11/9 isn't very good, should be around 12.6 minimal...

[rotortiller]

Make sure the coils have power at the black/white stripe wire under the tank.

[TwoTired]

Simple test is to probe on each side of the points.There should be 12V across an open point set, and zero volts with a closed point set.  When closed, the coil charges, when opened the coil dumps energy into the spark gaps.

If you can't get both 12v open and zero volts points closed, you won't get spark.  The 12v gets to the points via the coils.

Cheers,