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1983 CB650SC Nighthawk rescue

dougrun

dougrun

Veteran Member
Joined
Dec 1, 2021
Total Posts
700
Total likes
170
Location
slo, ca US
Got the bike. Bike fires up fine but stalls after a few seconds. I'm going to soak the carbs in seafoam spray and let it sit for a day. PO says he rebuilt the carbs but I need to check his mixture settings and he says he didnt synch them.
Other Issues:
1. Clutch is spongy and clearly does not engage. I have not tried anything yet. PO says he bled it 5 times and rebuilt the MC and its still like that. I will first try to warm up the oil and see if the plates are stuck. Would like a clear attack plan on this as clutches are new for me to work on.
2. Turn signals dont work. I think its the switch as the lights come on but the switch spring feels weak. The whole left side switch housing is faded a lot, I might replace the whole thing.
3. Brake light evidently doesnt work either, have not dived into yet,

20231110_164015.jpg
 
Reminds me a bit of the '84 VF1100S I had, similar styling in many areas. Cool bike in nice condition.
 
Issue #!. Brake light doesn't come on but taillight does. I have tested all fuses and all are ok. I followed test of taillight indicator light and that is fine. It comes on if the bulb is removed or either tail or brake wire are removed. I have tested 2 1157 bulbs, both work as taillight and both light up the brake filament if inserted backwards so bulbs are ok, fuses ok, something is not triggering the brake wire. Front brake switch has 12v to ground. If I short that with a paper clip, nothing. Rear brake switch has 12v to ground, I can yank on that spring as hard as i can and nothing. I'm thinking it has to be a bad brake light sensor box. What else can I try?
 
dougrun said:
Issue #!. Brake light doesn't come on but taillight does. I have tested all fuses and all are ok. I followed test of taillight indicator light and that is fine. It comes on if the bulb is removed or either tail or brake wire are removed. I have tested 2 1157 bulbs, both work as taillight and both light up the brake filament if inserted backwards so bulbs are ok, fuses ok, something is not triggering the brake wire. Front brake switch has 12v to ground. If I short that with a paper clip, nothing. Rear brake switch has 12v to ground, I can yank on that spring as hard as i can and nothing. I'm thinking it has to be a bad brake light sensor box. What else can I try?
Click to expand...
Follow the wiring harness starting at the light and work forward, possibly a disconnection.
 
did that, but i suspect a bad tailight sensor box, as the rear signals dont come on at all, front ones try but dont blink. have one coming from ebay
 
Doug do you have a multimeter or are you working with a test light?

I searched the Nighthawk forum for help but I think I only found your thread there. They've been hacked and lost everything a few times over the years, tons of good information was lost each time. Hopefully some members there will step up and share what they know.

Do you have the FSM got this bike?

I poached this from another forum:

wiring650.jpg
 
thanks, I have all the manuals (FSM and Clymer). Has to be either that tailight sensor or the the turn signal relay somehow. Both switches (rear brake and front) show a increase in voltage to the brake light wire, bt doesnt trigger it and neither rear turn signal light, which both are wired to that sensor so it makes sense.
 
ok, tests done today:
Brake light module is getting good power and ground but still no brake light, only taillight. Neither rear turn signal lights at all, which doesn't surprise me since the left control switch seems to be shot. No horn from the button and front turn signals only come on, no blinking. I ordered a new winker relay and will look for switch options. I should get the brake module replacement this week.

The clutch I think (hope) is the product of a bad MC rebuild. It looks like he replaced the hoses, but i found some brake fluid near the banjo bolt and when I tried to bleed it, it provided hardly any pressure, I didn't see any air bubbles coming out but then again, i could not get enough pressure to force the fluid out anyway so it has to be an upper (above the clutch) pressure issue. I'm not going to rebuild this one, the screws on the top are almost stripped so I ordered a new one.

I also removed a accessory power lead from the battery and rewired one of the grounds.
 
can anyone tell me what that spring is just rear of the rectifier? it freely moves. 20231114_142842.jpg
 
Looks like a release for a latch of some sort, maybe the part fiche might give a clue to what it is for.

EDIT: it's part of the inner fender. Toolbox door release?

honda-cb650sc-1983-d-f-26-rear-fender_big3IMG01163385_c66e.gif
 
Last edited:

Honda CB650SC 1983 (D) F-26 REAR FENDER - buy F-26 REAR FENDER spares online

www.cmsnl.com www.cmsnl.com

hmm, maybe a latch for the tool box lid??

Anyway, today I tried re-bleeding the clutch, still no pressure. Tried rocking the bike back and forth in 1st to see if the clutch plates were stuck but nothing helped. Will wait for the new MC. Fed the carbs straight seafoam spray and ran her a little, will let that sit a day or 2.
 
That's a pretty cool looking bike! I like the trailer too! Dual over head cam, 6 gears. And some interesting features for a 83. I was looking over the schematic Alan provided. The tail/brake light 'sensor' looks like a treat. What does it do? I was trying to figure out what the ignition used. Was thinking TCI, or maybe DC-CDI. Some researching showed it as an early TCI(transistor control, 12v). Here's some of the posts I read on a 82 on that: http://www.hondacb650.com/viewtopic.php?t=488
Good luck working out the gremlins!
And here's a review I thought was pretty good: https://ridermagazine.com/2017/05/26/retrospective-1983-1985-honda-cb650sc-nighthawk/
 
ok, i was sure i checked 4 fuses yesterday, but when i went to replace them today, there were only 2. I replaced all 4 fuses, brake light work (although front switch needs replacement as it goes out when fully engaged), Now I just need to find out why the rear turn signals dont come on at all. front work fine.
 
found the other 2 fuses, fell out. the fuse box is iffy, will need to replace it I think. ignition is also twitchy, not sure on a replcement, may have to clean that. rear right turn signal had water in it and can't get bulb out yet.

Also got the luggage rack, in great shape, don't think it will be hard to mount it.
 
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Once i clean the fuse box, I think it wil all work ok but I ordered new rear turn signals since the current ones clearly are not sealed.
 
got the fuse box out, one of the fuse lips broke off so the fuse wont stay in. ordered repl on ebay. fit the luggage rack loosly to see where things would go, It came with a uber-long flange bolt that helpled me figure I need one about 2" shorter and itll use the seat bolts. Then i just need to bend some steel supports off the turn signal posts.
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so looking at the photo of the NH i saw with that rack, i see that the seat bolt method would require cutting into the side panel, thats a no go. I looked and there is an empty thread socket just behind the helmet lock on both sides, unused. I think a chrome P-clamp with a rubber insert would work if I can find one with a U-shape maybe.
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Hi dougrun, ancientdad is correct, the spring lever releases the tool kit cover.The bike looks pretty good, clean. Good luck in your rescue. I have an 85' cb650sc. Though i don't have much spare time,I started a thread, as i didn't want to hijack yours, to share some of my triumphs and frustrations i had and have with this bike. garjoIMG_0184 (1).JPG
 
replaced the front brake light switch, which I cant test until i get the repl. fuse box installed next week. Ordered 2 Z brackets to support the rack on the rear turn signals but they were way too big, If anyone neds these, let me know, not worth returning. They would not work anyway as the signal bolt mounts have a guide pin.
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I already had some mounting brackets I never used to mount lights on that fit around the outside of the posts perfect so im using those. Now just waiting for the chrome pipe support I can bend into a U shape to support the front bar.
20231118_174322.jpg
 
Any advice on polishing the silver metal? im using 000 steel wool and elbow grease. working ok but killing my arms.
 
painting tip, if you paint plastic with gloss black, do not spray clear coat over it. ruined the paint. repaint..
 
The grab rail and the trim around rear are aluminum(non-ferrous, non magnetic) . I removed the rail and removed the rear section to wet-sand and polish(mothers). I was removing everything for cleaning anyways. A fine grit 1000/1200 works fine in removing the blotchy oxidation i had to contend with. I also have much finer, 2500/4000/5000 grit, but i usually save those for engine covers and such . Plenty of mothers, using a cheap hard cotton, like the red mechanic towels, work much better than microfiber for polishing in my experience. Save the microfibers for the final go over detail. A buffing wheel is also a great detailing tool for parts removed, use white compound for aluminum. A couple pics of mine before treatment, blotchy, . Wd40 or even better and more long lasting is the s100 engine black(not cheap) to make faded black paint black again. Chrome is very hard and can only be cleaned/ polished to my knowledge. No abrasives. hthIMG_3187.JPGIMG_3190.JPG.
 
not sure yet on the groove. I replaced the left hand controls just now (cant test yet, still waiting on fuse box). They only came with 1 set of horn connections, the other one had 2. was there a model that only had 1 horn?
 
dougrun said:
not sure yet on the groove. I replaced the left hand controls just now (cant test yet, still waiting on fuse box). They only came with 1 set of horn connections, the other one had 2. was there a model that only had 1 horn?
Click to expand...
Can't say personally, but you could look through the years of that model on CMSNL and see what years had either 1 or 2 horns.
 
I just checked and the description says CB550SC controls, my bad. I hope they work the same.
 
finished polishing aluminum. now waiting for some gel superglue to dry on a cracked end, then i'll touch up the black.
20231121_131213.jpg
 
ok, so next dilemma. this is the old fuse box, which isnt that bad inside. the issue is the fuses fall out (they are too loose). I need a thicker fuse or a tighter clip. The clips are old and do not bend well. Do they make thicker fuses or is there a way i can make them thicker??
20231121_140603.jpg
 
Ignition connection was iffy so i am cleaning it. Should I put something in there to prevent rust or lube it or nothing at all??
20231122_103651.jpg20231122_104743.jpg
 
Got everthing back in. now I cant find the fuse box cover. duct tape will have to do for now.
 
I approve and highly recommend duct tape. Leaves a good residue after a few years. Hopefully you find the cover before then.
 
found the cover.
Headlight issue: cannot get the screw holding it all in to properly start. The melted zip tie corner didnt last long when you have to fist-pound the light to get it in far enough.
I changed battery to my Llion from my cm400 so i can start more. ALL LIGHT WORK!! Yay. Horn does not work, Installed new rear turn signals and the rear rack.

20231122_140943.jpg

Engine:
It starts on choke. and I can get it warmed up on choke, but any real throttle and it dies. Cannot idle without choke, PO said he rebuilt carbs but didnt synch them.
 
also, cleaning the bottom of the ignition did not fix the key issue, it sill doesnt turn on until the key is far right in the on position. it stays there but im not riding and think it make go off if bumped.
 
The horn issue can have many causes, start by connecting a test light or meter to the two connectors from the horn, press the horn button and troubleshoot from there.

Horn buttons sometimes can be cleaned up by spraying some electrical contact cleaner in around the button and pressing it a hundred times.... Repeat. Sometimes horn button springs rust out and break, look for that too.

Horns don't always recover from years of disuse. You may be able to wake it up by knocking it with a screwdriver handle and connecting it directly to your battery.
If your horn has an adjustment screw on it, snap a photo of it with your phone before adjusting so you can set it back if messing with the screw position helps it to work again. The screw fine tunes the sound the horn produces.

Running only on choke is a classic symptom of dirty carb jets. The tiny passages get clogged easily when old gas is left to evaporate, bigger jets don't clog as easily. When you crack the throttle open no fuel is passing through the jets and the engine goes lean and stalls.
 
Horn is getting 12v so I guess both are shot. secondary issue atm.

Clutch: Installed new master cyl. Getting more pressure than the last one but its a smaller MC anyway. Still no clutch release. Bled like crazy, not getting any air bubbles. Took slave cyl cover off, slave piston does move when i pull lever but i cannot push the clutch shaft with my own strength, not sure if that says anything. I may reinstall the previous MC because the choke cable mount is part of it, had to manually hold the choke open to start it and try the clutch. its idling better but any throttle still kills it. What should I try next? Should I tap the shaft with a hammer to see if it frees it? Or do i need to dive all the way in and replace all the plates?
 
Maybe my bleeding isn't working enough, i pull the clutch lever, turn the bleed nibble, fluid comes out about 3 inches, stops, i close the nipple, release lever, wait a few seconds and repeat. It doesnt seem to get enough fluid to fill the tube but im not sure the tube is connected to the nipple well enough as some leaks down around it.
 
dougrun said:
Should I tap the shaft with a hammer to see if it frees it? Or do i need to dive all the way in and replace all the plates?
Click to expand...
Don't hit the clutch rod with a hammer, it won't noticeably move that way either and you don't want to do damage accidentally. At this point the clutch plates may well be stuck too, so even if you get the clutch hydraulics bled properly it still might not disengage. To be safe, the bike should be on the centerstand and when you feel like the clutch slave is fully bled, then with it running try to put it in gear while holding the rear brake. If it stalls the clutch is either stuck from sitting or the slave cylinder isn't moving the clutch rod.
 
I wonder if I should get a 1 piece clutch hose and try that, currently it has a metal portion that winds through the middle of the body with hoses attached to that at each end. PO said he replaced it with one off ebay.
 
I fixed my vacuum pump brake bleeder so i'll try that on it tomorrow, see if it can suck the air out that i bet is in there.
 
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