1993 Nighthawk 750 ,Ignition problems,Help needed

[Rick the Ratchet]

Hi Guys .
I've had my CB 750 Nighthawk about six years now and love it .About a year ago it wouldn't get in gear and the shop decided it was the clutch so put a new clutch in .It wasn't ,it was transmission,and they said they thought wasn't worth doing because it would cost over £1000. But they kindly didn't charge me for the clutch .
Anyway after she sat in shed for a while I decided to rebuild her myself .I stripped the casings apart by turning whole engine upside down to get at the transmissio,so not touching the head,pistons at all .In end it was a pawl spring on the selector drum mechanism that was broken so shop didn't have a clue what they were talking about .
Well,got her back on road and she was good ,until other day .
Went to start her and kept blowing Ignition fuse,the 15 amp one .So stripped all harness and tidied wires looking for any shorts etc .Put everything back ,all works ,no blown fuses.Think was solinoid to starter motor .But tried jumping across terminals to see if had spark ,and no spark .
The coils have a green connector and a black one ,can anyone advise me where the black and white wires go on each coil ,and other wires to coils .Could it be the CDI unit .Do you think that until I put a new Starter Solinoid on it will not spark .Im really confused now,because it's not blowing fuse anymore ,but don't think I've got a spark .
Any comments and advice would be most helpful.
Rick The Ratchet
In the UK .

 

[Chris in Dorset England]

Hi Rick, nice to meet you. You should find all the help you need here. You must be quite close to me. I have a DOHC CB750 F2c 1982. I am in the process of rebuilding it, but it will be a while yet before it's back on the road, very expensive hobby!

 

[amc49]

The service manual calls out the ignition fuse as a 10. Sure in all the fun you haven't flipped the kill switch on? The starter solenoid is controlled under the 30 amp fuse right at it. Black/white wires to BOTH coils and 12 volts with ignition switch turned on. The grounds are yellow/blue for 1 and 4 and green/yellow for 2 and 3, both go to the ignition module. You can have key on to disconnect the two grounds and simply manually ground each one yourself with power on and a plug in the coil wire and grounded at plug and then each coil will spark the plug, that happening says ignition module may have fried somehow. All it does is to ground each coil at the proper time.

If you do show power at the coils through the black/whites with key on then the system is NOT CDI regardless of what anything says.

Service manual here............

https://www.manualslib.com/manual/804798/Honda-Nighthawk-Cb-750-1991.html

 

[Rick the Ratchet]

Cb750 Guru
Hi to you and thanks for all your interesting threads on repairing motorcycles .No,the kill,switch and all those safety switches are fine .Have tested them all .Also cleaned and rewired the starter switch as was dodgy.My electrical experience is ok but because the coils on the wiring diagram have no + or - identity for them ,I'm thinking I may have put wires wrong way round,not sure.Thats why I'm thinking may be cdi.Yes you are right ,it is a 10 amp,my mistake .At least I've cured the fuse blowing problem ,just spark to sort out .Many Thanks again ,and really nice chatting .
Rick The Ratchet
Somerset ,UK.

 

[Rick the Ratchet]

Hi Chris
Yes I'm near Ilminster .Yes these things(bikes) can be addictive and expensive ,have been messing with them for over forty years now and still learning .Just can't stand electrics .Fuel,Air ,and nuts and bolts fine with ,just electrics,on bike bikes with all those chi boxes,regulator/rectifiers etc .
Rick The Ratchet

 

[amc49]

You can still get spark with coils hooked up backwards but it will be too weak. The black/whites are the two + there.

 

[Rick the Ratchet]

Thanks
Think I thought black/white were negative .The coils look like the terminals have a green band on one terminal and a black band on the other .Would this mean green live and black earth .Confusing these coils.Why don't they have + and - marked on them like old British bikes .Thanks Guru .

 

[Rick the Ratchet]

Hi All
Checked coil continuity all ok .But no power getting through to wires that go to coils,with power on,kill switch on,side stand up,clutch in .Still have Old starter solinoid on as new one not arrived yet,but this shouldn't affect power to the coils surely,it's just for starting isn't it ?
Rick The Ratchet ( now loosing will to live after being three days trying to fix it now ,help)

 

[Rick the Ratchet]

Hi
Ok guys,thought this might help you out .I brought some cheap spade fuses of e bay,about 50 for couple of quid .Well when I checked my 15 amp ,it looked fine as though it hadn't broken the wire like you can usually see .But to my amazement when I checked the continuity the fuse had blown .So put another one in and hey presto,coil live and spark.Horrah,hurrah.So pleased ,so can put back together,new solinoid when arrives and back on the road ready for the summer .
Thanks everyone for all your advice ,will try and help others who need help ,anything I can help with .Good luck with all your projects ,.
Rick The Ratchet
Somerset
Uk
In the shed

 

[Chris in Dorset England]

Hi
Ok guys,thought this might help you out .I brought some cheap spade fuses of e bay,about 50 for couple of quid .Well when I checked my 15 amp ,it looked fine as though it hadn't broken the wire like you can usually see .But to my amazement when I checked the continuity the fuse had blown .So put another one in and hey presto,coil live and spark.Horrah,hurrah.So pleased ,so can put back together,new solinoid when arrives and back on the road ready for the summer .
Thanks everyone for all your advice ,will try and help others who need help ,anything I can help with .Good luck with all your projects ,.
Rick The Ratchet
Somerset
Uk
In the shed

Don't you just love electrics! Glad you found it.
Chris.

 

[Rick the Ratchet]

Cheers Chris
Any chance of any photos of bike .Glad to help in any way .Guy round corner rebuilding his dads old F2.Think it's about 78 .Has this massive round metal disc for brake disc on back wheel,no holes in it at all .Recon you could use it as a dining room table it's so big .
Best Wishes
Rick The Ratchet

 

[Chris in Dorset England]

Cheers Chris
Any chance of any photos of bike .Glad to help in any way .Guy round corner rebuilding his dads old F2.Think it's about 78 .Has this massive round metal disc for brake disc on back wheel,no holes in it at all .Recon you could use it as a dining room table it's so big .
Best Wishes
Rick The Ratchet

Photos? I have dozens! Showing the good, the bad and the downright awful. I'll get some together and post them, probably Monday. The other bike sounds interesting.
Chris.

 

[Rick the Ratchet]

Thanks Chris
Just found out that the wires on the alternator where it goes into the casing were bare ,so this was causing the fuse to blow .Have mended now ,but not put back on yet ,fingers crossed .Just re covered the seat using an electric stapler gun and some cheap leatherette ,looks ok .
Well will keep you in touch .
Rick The Ratchet .