Topic: CB750K1 Sudden loss of spark on all four

[Prospect]

1971 cb750k1. I was riding around near my house and on two different occasions I had a sudden  loss of spark on all four.  I can crank it over but using my spare spark plug I can't see a spark. After waiting a bit and putting the bike on the side stand it would come back to life.  Sometimes, after a couple of days the bike would not start because there would be no spark.  Again leaning the bike to the left would fix the problem.  Obviously it's an electrical issue.  I went through most of the electrical wiring near the coils and cleaned it.  Also, inside the headlight and under the seat.  It seems to be ok now but you never know with electrical faults that are intermittent. 

Anyone care to guess what this could be. 

[fire113]

1971 cb750k1. I was riding around near my house and on two different occasions I had a sudden  loss of spark on all four.  I can crank it over but using my spare spark plug I can't see a spark. After waiting a bit and putting the bike on the side stand it would come back to life.  Sometimes, after a couple of days the bike would not start because there would be no spark.  Again leaning the bike to the left would fix the problem.  Obviously it's an electrical issue.  I went through most of the electrical wiring near the coils and cleaned it.  Also, inside the headlight and under the seat.  It seems to be ok now but you never know with electrical faults that are intermittent. 

Anyone care to guess what this could be. 

...maybe kill switch, fuse or bad contact.

[Vinhead1957]

+1 on the kill switch

[mrbreeze]

Just had the same problem only I was dropping 1-4 only. I'm running Accel coils and it turns out that the screws that mount the coils to the frame were loose (bad ground). IT was an intermitant problem but with the tank off and engine running.... I could wiggle the coils and the engine would die. Tightened everything up and we're golden!

[cheftuskey121]

+2 on the killswitch. thats what got me. I did not want to believe it because every single wire on my bike was replaced by me brand new. turned out it was my black/white stripe wire (killswitch) in the bucket. it was connected and seemed tight but it wasnt tight enough. I found out by starting the bike with the headlight out, and jiggling that wire. I could control the bike shutting on, make it stutter etc. so I soldered that connection and now it isnt going anywhere!

side note: dont get all excited after you fix it and try to drive away with the headlight just dangling alongside the tire....don't ask me how I know

so get that bike started and jiggle some wires and pinpoint the problem. probably a good time to go through and clean up connectors and add some di-electric grease too. good luck

[Prospect]

Just had the same problem only I was dropping 1-4 only. I'm running Accel coils and it turns out that the screws that mount the coils to the frame were loose (bad ground). IT was an intermitant problem but with the tank off and engine running.... I could wiggle the coils and the engine would die. Tightened everything up and we're golden!

I thought the coils were grounded to the engine via the spark plugs?

+2 on the killswitch. thats what got me. I did not want to believe it because every single wire on my bike was replaced by me brand new. turned out it was my black/white stripe wire (killswitch) in the bucket. it was connected and seemed tight but it wasnt tight enough. I found out by starting the bike with the headlight out, and jiggling that wire. I could control the bike shutting on, make it stutter etc. so I soldered that connection and now it isnt going anywhere!

side note: dont get all excited after you fix it and try to drive away with the headlight just dangling alongside the tire....don't ask me how I know

so get that bike started and jiggle some wires and pinpoint the problem. probably a good time to go through and clean up connectors and add some di-electric grease too. good luck

+1 on the kill switch

Good.  I have a spare kill switch that I'll try and install.  The wiring is a bit of a mess from the PO's and there really is no kill switch because it's hard wired in the run position.  I'll check the bucket again.  It makes sense that it would be the kill switch. 

[stickman]

Prospect,  If not the kill switch,  investigate the ignition switch.  At idle, just wiggle the wire harness, electrical plug and switch itself.  If it repeats the problem, look first at the back of the switch at the wires and solder joints.  Has your ignition sw been relocated by chance?

[Prospect]

Prospect,  If not the kill switch,  investigate the ignition switch.  At idle, just wiggle the wire harness, electrical plug and switch itself.  If it repeats the problem, look first at the back of the switch at the wires and solder joints.  Has your ignition sw been relocated by chance?

The ignition switch has not been relocated but again, the PO hard wired it to the harness. There is no round plug. I'll check your theory.

[mrbreeze]

Just had the same problem only I was dropping 1-4 only. I'm running Accel coils and it turns out that the screws that mount the coils to the frame were loose (bad ground). IT was an intermitant problem but with the tank off and engine running.... I could wiggle the coils and the engine would die. Tightened everything up and we're golden!

I thought the coils were grounded to the engine via the spark plugs?

Right you are Prospect. It was just a coincidence that my bike picked up 1 & 4 when I tightened the coil mount. I just dropped 1 & 4 again yesterday and tracked it down to a bad connection at the power wire to that coil. It is one wire that branches off to 2 wires ( bullet connectors). One goes to 1/4 coil & the other to 2/3. The female side was loose fitting with a small bit of corrosion. You just can't read enough on this forum about checking and cleaning ALL connections. Intermittents will drive a guy crazy!!
+2 on the killswitch. thats what got me. I did not want to believe it because every single wire on my bike was replaced by me brand new. turned out it was my black/white stripe wire (killswitch) in the bucket. it was connected and seemed tight but it wasnt tight enough. I found out by starting the bike with the headlight out, and jiggling that wire. I could control the bike shutting on, make it stutter etc. so I soldered that connection and now it isnt going anywhere!

side note: dont get all excited after you fix it and try to drive away with the headlight just dangling alongside the tire....don't ask me how I know

so get that bike started and jiggle some wires and pinpoint the problem. probably a good time to go through and clean up connectors and add some di-electric grease too. good luck

+1 on the kill switch

Good.  I have a spare kill switch that I'll try and install.  The wiring is a bit of a mess from the PO's and there really is no kill switch because it's hard wired in the run position.  I'll check the bucket again.  It makes sense that it would be the kill switch.

[mrbreeze]

I screwed up on that reply but it's in there and I'm not changing it due to the PITA it is to post from my phone!!!

[Prospect]

I changed the kill switch to a spare old one I had kicking around.  It looks to be working for now.  Cleaned more of the wiring as well.  Thanks for the replies!