Blood Sweat Tears and Grease => HELP! 1-800-CAFE-HELP => Topic started by: dohcer on May 15, 2017, 00:56:50

Title: 1980 cb750 project question
Post by: dohcer on May 15, 2017, 00:56:50
Fairly recently I purchased a 1980 CB750 custom off of CL. Performance-wise, the bike seems to run OK. Cosmetically, it was dropped, but as far as I can tell, the only damage was a broken hand break lever, and a small dent on the tank. The previous owner had also cut the pipes off so it sounds awful and unnecessarily load. The headlight stopped working recently, which I think is due to a grounding issue, but when I pulled apart the wiring system I couldn't find anything out of place, so it's probably the fuse I'm guessing.

My plan was just to ride it over the summer and get to know it's quirks/idiosyncrasies, but it's just not really that fun to ride as it is right now, so instead of riding it this summer, I plan on starting a rebuild, mechanically and cosmetically.

I'm mechanically-inclined, i.e. I work temp at a machine shop, but mostly on CNC mills and lathes. Occasionally I'll hop on the Bridgeport for some small volume (one or two pieces) job. That being said, I don't really have experience with engines other than lawnmowers. I wanted this to be foremost a learning experience, but before I go and start disassembling the bike and pulling apart the motor, I just want to make sure that I'm not in over my head. Space, tooling and time are not a factor. Money somewhat is, but this project doesn't have to be finished tomorrow, so I can acquire whatever parts I need as I have money available.

Basically, I just wanted some advice on how to approach this project. If there are no oil leaks or any noticeable issues with the engine, is it worth pulling it apart to clean it, put new seals, etc? Also, I would like to paint the engine too. Cosmetically, I'll probably be going the cafe route, a more aggressive stance, but slightly modified for a more comfortable ride. Fork swap for dual disks from an '80s Goldwing might be something else I would like to do.

I'm new here so I'll be checking out more of the threads, but it'd be great if someone could give me some advice on how to approach a new project, keeping in mind that this is my first time.
Title: Re: 1980 cb750 project question
Post by: hillsy on May 15, 2017, 01:40:51
If the motor is not leaking oil, burning oil, blowing smoke or making nasty noises then chances are its OK and pulling it apart will be an expensive waste of time.

Get a compression gauge and check the compression, check the valve clearances, cam chain tension, do a full service etc and call it good.

Sounds like you should first of all get some mufflers and at least make the bike more pleasant to ride. You'd be surprised how something as small as this will change your view on the bike / riding experience.

The last thing you want to be doing is blowing the bike apart into a million bits because there's a really high chance you will just lose interest and it will end up as a CL ad in 6 months in several boxes.

Title: Re: 1980 cb750 project question
Post by: J-Rod10 on May 15, 2017, 01:50:30
Does the bike have the stock air box?
Title: Re: 1980 cb750 project question
Post by: hillsy on May 15, 2017, 02:17:27
I guess at this point we need pics to see what you're working with.
Title: Re: 1980 cb750 project question
Post by: dohcer on May 15, 2017, 15:32:37
I've attached a few pictures. As far as the airbox, I'm not entirely sure but I'd assume that it's stock. Hopefully the photographs will help.

So for now I'll focus on:


Any recommendations on a compression testing kit? I'm guessing the HF one is out of the question...
Title: Re: 1980 cb750 project question
Post by: ncologerojr on May 15, 2017, 15:40:24
First, get a manual for the bike. It's a necessity for you to go any further.

Start by doing a good tune-up per the manual. Like stated above, valve clearances, points, plugs etc.

I 2nd getting a compression tester. They're not very expensive, and worth every cent. If you get good readings on all cylinders, don't dig in unless you want to do some engine upgrades.

I would also 2nd getting some sort or mufflers or an exhaust system on there asap.

Next make a plan, do some research and go for it. Set small goals, be patient and you'll be fine. People get into trouble when they jump in and feel overwhelmed. You'll make mistakes, and do things twice or more, but you'll always learn from it. Like I said, just be patient, this isn't rocket science.

I built an '81 a while back, here's my thread, maybe you'll find something useful.
'81 CB750k Tracker - Build #3

Title: Re: 1980 cb750 project question
Post by: coyote13 on May 15, 2017, 15:50:12
Yep, get some mufflers if you want it to run well, then go through the FULL tune up procedure as outlined in the manual.  My bet is it just needs a few small adjustments to give you many many miles of smiles.  Looks to be in good condition aside from the exhaust.  If you have the coin, a good 4-2 or 4-1 would be a great investment for performance (and for the sake of your eardrums)
Title: Re: 1980 cb750 project question
Post by: 88SS on May 15, 2017, 19:22:46
I'm sure it's running lean with the baffles off. Definitely get some pipes/mufflers. Check all the electrical/charging system. Before you even think about pods, you should think about a different set of carbs. The keihin carbs are particularly sensitive in every way

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Title: Re: 1980 cb750 project question
Post by: dohcer on May 15, 2017, 23:25:48
Thanks for the replies.

I will be doing the valve clearance check, compression check and cam chain tension adjustments in the next week or so.

As far as pods go, is there a noticeable difference in performance when upgrading from the stock carbs? Are pods more of a aesthetic thing? From the little I've read, upgrading the carbs on these DOHCs and putting on the pods seem to be quite a hassle, and if it's more of cosmetic thing, then I'd rather stick with stock carbs/airbox.

I think the main thing is the muffler. I like the 4-1, and found a company about 20 minutes out that makes a pretty nice exhaust system. For around $325, you get the full exhaust system, headers and all. ( The MAC 4-1 sells for a little cheaper at around $300, so I'm not sure. They look about the same. I might go with the MAC.

After I do the basic tune-up, and know that the mechanical side of the bike is in good working order, I would like to:
(1) chop frame and weld on rear hoop and seat pan.
(2) make fiberglass cowl or purchase pre-made cowl and seat.
(3) reroute new, smaller battery and electrics in welded box under seat pan or if I go with a larger cowl, store electrics in there
(4) remove dent from gas tank
(5) paint frame, engine, tank and cowl.
(6) reassemble

After these things, I might look into replacing the front fork and rear shocks.

Sound like a good game plan? Am I missing anything?

Title: Re: 1980 cb750 project question
Post by: esmoojee on May 16, 2017, 00:50:24
Nice bike, I would definitely go with the delkevic over the mac.  Do the tune up and get the exhaust on and youíll be a lot happier.  These bikes are pretty easy to work on.  I would download a factory service manual.  Itís way better then the aftermarket books.  I wouldnít go with pods unless youíre switching to Keihin cr specials.  Keep the air box if youíre keeping the stock carbs.  Fork swap is pretty easy but you will lose some height.  Cognitomoto makes some parts to swap in modern forks.  Also, check the bulb in the light, if itís still good maybe a fuse or a loose connection.  My connectors on my horn loosened up and I had to clamp it back down.  They are pretty old after all.  Good luck with the build!
Title: Re: 1980 cb750 project question
Post by: dohcer on Aug 12, 2017, 22:23:55
There is a popping sound coming from the engine, but this is most likely the exhaust. Could be the carbs too, running to rich, right?
Since I have the tools and some time, I will be pulling the engine apart for inspection, cleaning and then painting.
I've already ordered the O-rings for the carb rebuild. Is there an engine rebuild kit for the 1980 cb750C?
Title: Re: 1980 cb750 project question
Post by: esmoojee on Aug 16, 2017, 22:15:18
Itís hard to diagnose without seeing your bike. Are you using the air box? Did you swap the exhaust or do you still just have the headers without the muffler? That can cause popping.  Did you check the valve clearances?  We need more info.  As far as a rebuild kit, you can buy gasket kits but in order to rebuild it depends on other factors.  Are you changing the valve seals?  If so go with viton valve seals.  Are you getting compression on the cylinders?  What are your compression numbers?