81 CB900C questions

skyish

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Joined
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Oklahoma us
I am working on an 81 cb900c, a previous owner left the starter out of the bike for an undetermined amount of time, and took the bolts out of the stator housing ( I believe it is anyway) I don't think they removed it, if they did they put it back on without bolts. They also removed the rubber hose from the crank case vent. I know some dirt got in due to being open and a mistake of mine. What do I do to prevent any damage?
 
When you say "left some bolts out of the stator housing" - which one(s) are you talking about?
1782165738462.png

And dirt got in where? Just the EGR tube? What kind of dirt and how even?

Get some cheap oil. drain what's in there. Fill, run for a bit, drain. Swap filter.
 
OK - That little bit of dirt on the gear is a non-issue. It'll get eventually flushed into the filter. for a variety of reasons, you're going to want to change the oil now and again really soon after it's run for a bit.

That it the left side crankcase/advancer cover. Stator is on the right side next to the clutch. You're likely going to need replacement gaskets
Point cover part number 30372-425-000
Cover gasket part number 11395-425-306

You'll also need 6 M6X32 Flanged bolts and 2 M6X40 flanged bolts from a Honda auto dealer (fastest/cheapest - no shipping)
32's are part number 96001-06032-07
40's are part number 95701-06040-08

 
Thank you very much! My biggest worry was dirt from it sitting open for a good bit. Do you think it is safe to try and crank over as is before changing the oil? I am also concerned that some dirt got in the open vent pipe above the oil fill. The vent pipe is missing.
 
Thank you very much! My biggest worry was dirt from it sitting open for a good bit. Do you think it is safe to try and crank over as is before changing the oil? I am also concerned that some dirt got in the open vent pipe above the oil fill. The vent pipe is missing.
Nevermind about cranking it, seems like a bad idea without the bolts.
 
Don’t crank until you drain the old oil and refill with new. You want to take a good look at that oil and make sure there’s not a ton of glitter, see if water got in there, etc. old oil can tell you a lot about the engine.

Drain and fill with some “testing” oil. Get a jug of Rotella T4 at Autozone. It’s a diesel 15w30 that does a good job for a first flush. Once you’ve determined that there’s no leaks and the engine is running properly you can put the good stuff in there.
 
Don’t crank until you drain the old oil and refill with new. You want to take a good look at that oil and make sure there’s not a ton of glitter, see if water got in there, etc. old oil can tell you a lot about the engine.

Drain and fill with some “testing” oil. Get a jug of Rotella T4 at Autozone. It’s a diesel 15w30 that does a good job for a first flush. Once you’ve determined that there’s no leaks and the engine is running properly you can put the good stuff in there.
Makes sense about oil change. Will 15w40 cause issues? I thought it was supposed to be 10w30. Just want to double check. Thanks!
 
Makes sense about oil change. Will 15w40 cause issues? I thought it was supposed to be 10w30. Just want to double check. Thanks!
It’ll cause exactly zero issues, particularly this time of year. When it starts getting this hot, I switch to 20w50 - the 10w40 and 10w30 is based on rational person operating temps, not the TX/OK summer heat.
 
It’ll cause exactly zero issues, particularly this time of year. When it starts getting this hot, I switch to 20w50 - the 10w40 and 10w30 is based on rational person operating temps, not the TX/OK summer heat.
Lool no kidding about temperature
 
It’ll cause exactly zero issues, particularly this time of year. When it starts getting this hot, I switch to 20w50 - the 10w40 and 10w30 is based on rational person operating temps, not the TX/OK summer heat.
thanks, is it important for the oil filter to be oem, or will something like a k&n filter work? do you have a recomendation?
 
thanks, is it important for the oil filter to be oem, or will something like a k&n filter work? do you have a recomendation?
I’ve used Hi-Flo aftermarket filters without issue, I’d imagine K&N would be no different. If your local Honda dealer has it in stock, OEM might actually be cheaper than paying shipping. I’d check on the off chance they do. Regardless of which one you use, make sure to swap the orings out as well.
 
I’ve used Hi-Flo aftermarket filters without issue, I’d imagine K&N would be no different. If your local Honda dealer has it in stock, OEM might actually be cheaper than paying shipping. I’d check on the off chance they do. Regardless of which one you use, make sure to swap the orings out as well.
Thanks!
 
Update: Engine seems like it is in acceptable condition. Quite a bit of wiring needs rewired or repaired. Only one light was working, the main headlight is not. It was wired to a toggle switch on the frame. Wire to oil pressure sensor Is broke, don't know where to reconnect it. That my update.
 
Update: Engine seems like it is in acceptable condition. Quite a bit of wiring needs rewired or repaired. Only one light was working, the main headlight is not. It was wired to a toggle switch on the frame. Wire to oil pressure sensor Is broke, don't know where to reconnect it. That my update.
Do you have the FSM for your bike?
 
This bike has a 4 wire harness that goes past the starter, appears to go behind the left side crankcase/ points cover. Two, a black and green wire seem to be unplugged and broke. Although they seem to be fiber covered. This doesn't seem to fit with the rest of the bike. Any thoughts would be helpful.
 
This bike has a 4 wire harness that goes past the starter, appears to go behind the left side crankcase/ points cover. Two, a black and green wire seem to be unplugged and broke. Although they seem to be fiber covered. This doesn't seem to fit with the rest of the bike. Any thoughts would be helpful.
Pictures might help someone recognize it.
 
This bike has a 4 wire harness that goes past the starter, appears to go behind the left side crankcase/ points cover. Two, a black and green wire seem to be unplugged and broke. Although they seem to be fiber covered. This doesn't seem to fit with the rest of the bike. Any thoughts would be helpful.
Photos would help, but if it’s the lighter green color with insulation that used to be reddish that’s the oil pressure switch. Other one is likely the neutral switch.

In that FSM, there should be a wiring diagram. You’re going to want that - I guarantee that wiring has been hacked to hell and back.
 
Photos would help, but if it’s the lighter green color with insulation that used to be reddish that’s the oil pressure switch. Other one is likely the neutral switch.

In that FSM, there should be a wiring diagram. You’re going to want that - I guarantee that wiring has been hacked to hell and back.
I will definitely be a while sorting wires . I tried posting the photo but it said it was to big of a file. The wire from the oil pressure switch wire is broken as well, looks black. Reaches a black wire that is one I was describing, thanks. Then there is a green one. Not sure there. It just seems unplugged.
 
Those look right. Hopefully, you’ve got a multimeter - if not, get one.

Check continuity from the green one to the red/green wire on the back of the neutral indicator light. The black one to the blk/red wire on the back of the oil pressure light.

Those two broken ones should lead to a terminal block nearby. It should be the same one that the pickup wires from that unbolted engine case.

If you’re using your phone for photos, take a screenshot of the photo and it’ll downsize enough to fit the 5MB limit. I have a bad feeling the ignition on this one has been ripped up chasing a problem.
 
Those look right. Hopefully, you’ve got a multimeter - if not, get one.

Check continuity from the green one to the red/green wire on the back of the neutral indicator light. The black one to the blk/red wire on the back of the oil pressure light.

Those two broken ones should lead to a terminal block nearby. It should be the same one that the pickup wires from that unbolted engine case.

If you’re using your phone for photos, take a screenshot of the photo and it’ll downsize enough to fit the 5MB limit. I have a bad feeling the ignition on this one has been ripped up chasing a problem.
i'm a bit worried about the timing as well. do have a multimeter. pilot box is completely gone, found that out when trying to find neutral indicator light. engine wont turn with starter button.
 
i'm a bit worried about the timing as well. do have a multimeter. pilot box is completely gone, found that out when trying to find neutral indicator light. engine wont turn with starter button.
Neutral safety interlock. The solenoid switch won’t close without the bike in neutral. You can bypass this temporarily by jumping the green/red from the solenoid to ground.
 
Neutral safety interlock. The solenoid switch won’t close without the bike in neutral. You can bypass this temporarily by jumping the green/red from the solenoid to ground.
The same green red the goes to the neutral indicator light? if yes, you may have just helped me figure out my wiring diagram some! i'm not good at reading them.
 
Screenshot 2026-06-25 081140.jpg
There's a couple of Green/Red circuits on that chain that control this safety feature. They link together at the diode (called a rectifier on the diagram). Either one of them needs to close to ground for the starter button to work.

If you're trying to work off your phone on the wiring diagram, here's a link to a highish-res version. Spend $2.98 at UPS or FedEx office to have this printed on 11x14 - trust me, it'll make this a lot easier. https://www.cb750c.com/publicdocs/CB900C_Manual/CB900C_80_Wiring_Diagram.pdf

Before you get too deep into this wiring repair stuff here, make sure you've got the right tools and supplies to do it right and not continue what is likely an underlying hack job that will just create a variable later that will be impossible to troubleshoot.

- Real ratcheting wire crimpers
- Some of these guys to make splices way easier and cleaner https://www.amazon.com/Kuject-Conne...al+wire+connectors&qid=1782394099&sr=8-6&th=1
- De-pinning tool
- Replacement terminal blocks and pins from https://www.vintageconnections.com/ You can also order correct wire colors by the foot from here as well.
 
Thanks! I always enjoy a good excuse to buy new tools! I was going to use crimp connectors and heat shrink tubing wherever the wire needed spliced. But the connections you sent look very nice thanks. I will keep the terminal blocks in mind if any come up.
 
Thanks! I always enjoy a good excuse to buy new tools! I was going to use crimp connectors and heat shrink tubing wherever the wire needed spliced. But the connections you sent look very nice thanks. I will keep the terminal blocks in mind if any come up.
Both Sparck Moto and Vintage Connections are high quality suppliers owned by member @Sonreir, which is why they are one of our sponsors. And so you know, we only do link exchanges with sponsors - we don't take advertising money from anyone. We offer them space here in exchange for a link to VHT on their website.
 
Both Sparck Moto and Vintage Connections are high quality suppliers owned by member @Sonreir, which is why they are one of our sponsors. And so you know, we only do link exchanges with sponsors - we don't take advertising money from anyone. We offer them space here in exchange for a link to VHT on their website.
Thanks! I will be happy to use them when the need comes up.
 
Thanks! I always enjoy a good excuse to buy new tools! I was going to use crimp connectors and heat shrink tubing wherever the wire needed spliced. But the connections you sent look very nice thanks. I will keep the terminal blocks in mind if any come up.
Yeah - those little solder/shrink connectors are way easier, more secure and take up far less room in a wiring harness. I use a heat gun on them, but a small butane torch works as well. Whatever you do, avoid the blue/red BS plastic/tin crap from the box stores on any connector.
 
Yeah - those little solder/shrink connectors are way easier, more secure and take up far less room in a wiring harness. I use a heat gun on them, but a small butane torch works as well. Whatever you do, avoid the blue/red BS plastic/tin crap from the box stores on any connector.
Time to hide my harbor freight crimp connectors.. lol. I will be buying those solder connectors. I am interested to try them. Thanks!
 
Does anyone by chance know if the headlights on a 1981 cb900c are supposed to get 12v when the bike is on and not running?
 
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