New Here, Restoring a '78 CB750A

JTech

CB750 Member
Messages
8
Reaction score
2
Points
3
Location
Georgia
Hey everyone, I'm brand new to the CB scene and grabbed this CB750A for a low price in untested, unknown condition, just under 5K miles. I got to take a look before buying and aside from a lot of surface rust and aging parts I think it will run again soon. Full disclosure, I have worked on cars and helped with a buddy's bike in the past, but this is all pretty new to me. I've been doing tons of research and wound up reading through forum posts here a lot, so I figured I should hop in and say hello.

The carbs are in great shape, it looks like they were probably rebuilt or at least cleaned before the bike was put up. It's nearly complete, only missing side panels and a couple of minor things like a plug boot. Because of the overall amount of corrosion I will be tearing it down to the frame to have sandblasted and repainted/coated and rebuilding. It will be a resto-mod as I don't plan to restore to factory style at all. Auto-transmission bikes are an interesting oddity; we'll see how I feel about it when it's up and running.

It has the original kick start so I know it's not seized (not surprised it won't fire with it), but the electric start isn't working. Even after rebuilding the starter solenoid and bridging it, there's no click and the starter motor doesn't turn. I haven't taken the starter cover off yet to determine the condition of the wire on that end, but based on everything else the issue is probably wiring. I've cleaned a lot of crappy connections so far, so much so that I almost want to make a new harness for it. Almost. Otherwise, will be sourcing a new starter soon.

Any tips, tricks, common things to look out for, anything at all would be greatly appreciated. I've never done a project like this before, so any help is welcome. Thanks!
 

JTech

CB750 Member
Messages
8
Reaction score
2
Points
3
Location
Georgia
I’m looking at one… sat for years and not running. Following. Keep us posted!
I got some more parts off today. The current goal is to confirm engine will run before stripping down entirely, but I figured I would go ahead and start pulling things off while waiting on replacement parts.

I confirmed that the starter is bad, and similarly expensive to attempt a rebuild rather than replace with a secondhand unit. Currently still looking at my options there.

The points cover is heavily corroded, probably the worst thing on the bike. Screws too, of course. They disintegrated with very little effort so I'm stuck until I can get those off, because without that I can't reinstall the cam tensioner once it's off. Can't remove the starter without removing the cam tensioner.
 

dirtdigger

Moderator
Staff member
Messages
3,503
Reaction score
112
Points
63
Location
Black Hawk SD
Make sure its tdc cylinder 1 before taking the tensioner off. Also the starter wont engage if its not in neutral I believe but not sure. If anything in the transmission is bad there are basically no parts available, other than that they are basically the same as the standard motor.
 

JTech

CB750 Member
Messages
8
Reaction score
2
Points
3
Location
Georgia
Make sure its tdc cylinder 1 before taking the tensioner off. Also the starter wont engage if its not in neutral I believe but not sure. If anything in the transmission is bad there are basically no parts available, other than that they are basically the same as the standard motor.
Tensioner isn't off yet since I have to worry about getting the points cover off first; will do that though.

I made sure to be in neutral and gauge light works to confirm. Here's hoping the torque converter is good since everything else seems to be bad. Jumping the starter straight to battery should also bypass any other requirements and it still didn't spin.
 

dirtdigger

Moderator
Staff member
Messages
3,503
Reaction score
112
Points
63
Location
Black Hawk SD
I will try to find where I read it but I think the torque converter actually is the same as in one of the honda cars but seals, clutches, gaskets etc are non existent.
 

dirtdigger

Moderator
Staff member
Messages
3,503
Reaction score
112
Points
63
Location
Black Hawk SD
Take the starter cover off and see if the wire is broke going into the starter itself, have seen them almost broke off. Also take a small hammer and give the starter a tap while trying to engage it.....sometimes the brushes could be stuck or corroded and a little tap can knock it loose.
 

jpdevol

CB750 Enthusiast
Messages
92
Reaction score
78
Points
18
Location
26187
Another simple thing to verify a problem with the starter motor is to ensure the engine cases have a good ground and complete continuity to battery negative - otherwise current will not flow through the starter motor itself. If power in and ground are good, then it's time to see if the starter itself has an issue.
 

JTech

CB750 Member
Messages
8
Reaction score
2
Points
3
Location
Georgia
Take the starter cover off and see if the wire is broke going into the starter itself, have seen them almost broke off. Also take a small hammer and give the starter a tap while trying to engage it.....sometimes the brushes could be stuck or corroded and a little tap can knock it loose.
I did check the wire before anything and didn't notice any looseness, breaks, or anything. Meant to check continuity but didn't have my meter sitting there to remind me. Tapped the starter with a mallot several times before/during retesting and never got a sign of life. I've ordered a replacement, hopefully will see it soon.
 

JTech

CB750 Member
Messages
8
Reaction score
2
Points
3
Location
Georgia
Another simple thing to verify a problem with the starter motor is to ensure the engine cases have a good ground and complete continuity to battery negative - otherwise current will not flow through the starter motor itself. If power in and ground are good, then it's time to see if the starter itself has an issue.
I cleaned up a lot of the grounds, not only for starter/ignition parts but across the board, and tested continuity where I could, everything (so far) has turned out alright with just a little cleaning but I'll have to check in on this when I have more time to go over it.
 

JTech

CB750 Member
Messages
8
Reaction score
2
Points
3
Location
Georgia
Quick update:

I confirmed the starter is bad. It's completely seized up so I opted for a replacement rather than rebuild because I found a used/working OEM unit cheaper than a rebuild kit. I already have the new starter and will be installing it tomorrow.

My setback was the points cover. It was terribly corroded. The screws would just not budge and stripped out with almost no force. I've hit them with PB every time I've gone to work on the bike in the past week. I wound up carefully putting a hole through the (already ruined with pinholes) points cover with a screwdriver and using pliers and cutters to pull it off. I have a new cover on the way already. The bottom screw finally gave way and came out; the top screw decided to snap and leave its back end in the hole, so I have to deal with that now, but I finally got access. Some corrosion found its way in, so I cleaned up the points and everything. It appears to be in an operable state though far from ideal. Now I can finally time the engine to remove and reinstall the cam chain tensioner and replace the starter.

I'll also be getting a new oil filter kit (cover, filter, gaskets, bolt), solenoid, coils and caps, and a couple of odds and ends in the mail tomorrow. I plan on smacking it all back together and with luck will finally be able to get her going for the first time in at least 20 years!
 

JTech

CB750 Member
Messages
8
Reaction score
2
Points
3
Location
Georgia
In the latest news, the starter is fixed and the electric start button works. I also fixed the killswitch, it was turned the wrong way and the bearing wasn't seated in the grooves so it just slid around rather than "clicking" between off/run/off. Cleaned up the contacts of course, so that should be good. Fresh oil, a bit of cleaning here and there, and gapped the points to 0.35mm.

I still don't have spark, though. I realized that I'm dropping from 12.26v at the battery to 10v at the coils, points, etc. so my next task will be to find what's causing that; I suspect the regulator could be bad since it's been exposed without a side cover for however long the bike sat, but I'm sure the wiring itself is going to be problematic here and there as well. I tried to use one of the turn signal bulbs as a test bulb for timing the engine just right but it wouldn't light up, and the LEDs I have just stay lit the whole time since the power draw is so low.
 

JTech

CB750 Member
Messages
8
Reaction score
2
Points
3
Location
Georgia
Alright, so I got spark the other day. Cleaning and reseating connections and rechecking the gaps, and she got something. It's not consistent though, so I'm probably going to end up doing a lot of rewiring. Great.

I got her fired up on starter fluid a few times. Revved high immediately every time so she only ran for a grand total of maybe three seconds, but she ran! I got a Rick's combo rec/reg installed too. Figured it couldn't hurt.

I haven't been able to fire her back over since, and it's probably the carbs. It's always the carbs, right? So, yeah. Carbs and electrical. SOS.
 

jpdevol

CB750 Enthusiast
Messages
92
Reaction score
78
Points
18
Location
26187
Yep, cleaning, setting carbs, checking for leaks @ intake manifolds and synchronizing are standard procedure in restoring the old beasts to operation....
 

dirtdigger

Moderator
Staff member
Messages
3,503
Reaction score
112
Points
63
Location
Black Hawk SD
If you have so many things corroded it may be worth the little bit of money to get a new wire harness to eliminate all possible wire issues. The aftermarket harnesses are good quality and fit well. I have used them from 4into1 and cb750supply. Cleaning grounds as you go and the wiring leaving only components to be potential issues.
 
Top