1980 CB750C Cafe Project "Green Onions"

[Vortices]

Hey all, figured I'd start a build a thread on my new to me CB750C. I bought it non running and stock as hell for 1500 bones in 2021, for an extra 500 the seller threw in some "high quality cafe conversion parts". I was hungover and didn't notice that literally all the parts were cheap Amazon shit. Anyways I started off with a carb clean and some cosmetic work. I had an '81 CM400T which was my main focus as it actually worked.
Unfortunately I don't have any pictures of the 750 before the paint job, frame chop, bar swap, and stock light delete. It's not too hard to imagine though, it was white, crusty, had some lichen growing on it, and had the stock seat.

 

[Vortices]

After spending my time polishing up the CM400 and selling it, and then tackling a full suspension overhaul and upgrade on my Volvo 240 Turbo I finally had some breathing room to work on the hog. I managed to get it running decent enough to get around the block and decided that it was time for some cosmetic upgrades.
I started by using the cheapo Amazon seat as well as the hoop and rear light that came with the bike.
I've always loved the look of older cafe bikes and the small rear cap/fairing piece, so I decided I'd make my own.
I grabbed a piece of the CM400s rear fender I'd chopped off and kept for the duck tail of the rear fairing.

I cling wrapped the seat and, using some discarded fiber glass and carbon fiber from work, got a pretty good shell formed.

I had to add some kevlar strips for some extra rigidity and, after much sanding and priming and sanding and painting, it turned out not too shabby.

For the rear hoop and light setup I glued the light inside the hoop and stuck some reflective gold heat tape inside for extra refraction. I then filled the inside of the hoop with a two part clear epoxy resin and filed and polished it to the shape I was after.

The end result is pretty much what I was after, the only issues with this design is if the light ever breaks I'd need to fully build a new housing as the light is very much encased in resin.
For now I like it.

 

[Vortices]

Over the course of improving the bikes aesthetics I diagnosed the crank no start issues.
My first order of business was a new battery, new plugs, some new gas, and a quick spritz and scrub of the carb bowls, jets, and float valves.
This got her running, not super well, but running.
I then replaced the throttle cables as they were old and gummed up and a quick oil change after it had run for a bit.
The issues continued with the bike, the most prominent being an increase in idle speed as the bike warmed up.
It would start and idle fairly well, but as it got closer to operating temp the idle would increase until it would sit at about 4k when all warmed up. Not great on the ears, and super annoying in traffic.
Yes, I changed the carb boots, no it didn't fix the issue.
After only getting about two months of riding in and maybe a handful of those rides being issue free I decided it was time to try my hand at carb rebuilds.
The nice thing about this platform is that they're super popular so, using a step by step guide off these forums and parts from Randakks, I set off splitting the four carbs apart and tearing them down.

I discovered that basically all of my air diaphragms had tears in them, the float valve seats were gummed up and a bit corroded, the felt seals on the choke and throttle shafts were barely hanging in there, plenty of gas boogers flushed out after the ultrasonic bath, and every o ring was degrading/rock hard.
All o rings, air diaphragms, felt seals, and float valves were replaced with new bits.

All in all I had a great time rebuilding the mechanical computer and I'm very excited to get this thing synced up and running properly.
Attached for your convenience is the guide I used for the rebuild.

https://www.cb750c.com/publicdocs/SeanG/Honda_Carb_Manual_revG.pdf

 

[Vortices]

I really enjoyed the cafe bars and the rider position on my 400 so I'm going to try and emulate it on the 750.
If money were no object I would commit to a gsxr fork swap or something but this bike isn't my only project so I had to settle for some cafe bars and dropping the front end a little bit.
Originally I had some flat bars but I felt like I was riding a Harley, hence the swap.

Attached is the before and after, from flat to cafe.

I ran into a couple issues with the new bars, the main one being that the my turning limit stops became the tank and the bars, and secondly the throttle, and clutch cables were now too long.
I solved the turning limits by drilling and tapping the original stops and installing some bolts to find tune where I needed the new stop range to be.

The need for shortened cables was solved by just buying a set of shortened cables after absolutely butchering my first attempt at building my own.
I also opened up both the left and right switch sets and cleaned the contacts thoroughly, my start switch body had a crack in it which I solved with some aircraft grade adhesive and a small piece of aluminium.

 

[Vortices]

I tried my hand at tire replacement and after inventing a couple new swear words I managed to wrestle a brand new pair of Dunlop D404s onto the wheels.
I don't have a balance stand so I took em down to the local shop for the bead seat and balance.

I also took some time to scuff out some corrosion on the rear axle and clean up all the spots that are hard to get to when everything is back on the bike.

 

[Vortices]

I picked up this front fairing off marketplace for cheap, and although I'm not super sold on it I had some free time and extra paint kicking around and it turned out pretty good.

My only issue is that the top edge of the fairing sits kind of forward and gives it an almost "Harley Roadglide-esque" look to the front.

I'm gonna see if I can trim the windshield to make it a little more streamlined as I would like a more uniform classic cafe racer profile.
I even extended the headlight with some aluminium bracket extensions to try and offset the fairing shape.
I'm not 100% sold on the look.
I'm considering just flipping it over and running no windshield.

 

[sergei]

it turned out very well and neatly

 

[Vortices]

Great progress these past few days!
After cranking and not starting she finally caught and idled up great.
I swapped the old plugs out with some fresh ones, and made sure to spritz some wd40 in each cylinder just to aid with lubrication as it hadn't run in about 8 months.
The carbs were completely empty of fuel after the rebuild and I think it took a while for the ACF-50 I had generously added to be replaced with nice, new, combustible gasoline.
I was ready to do a crab sync but it sounded and looked pretty much dead on at idle, so I guess my bench sync was good enough.

I've got two more hurdles and some odds and ends before it's fully on the road. One being checking the valve clearances, the other being a dynamic timing check, but the days of actually riding this thing are definitely in sight!

 

[Vortices]

I had a couple electrical gremlins that appeared after reassembly; one was a headlight that didn't want to turn on, a brake light that wouldn't illuminate, and horns with no honk. After some digging I discovered the headlight was a simple case of the contacts in the rebuilt switch being misaligned. The brake light and horn not working is a deeper issue as they are all on the same circuit. My fix for this is going to be to transition the circuit off of a bodged together Amazon gauge light setup as it runs leds, and to replace the Amazon flasher relay with an OEM Honda one. I picked up some super mini incandescent lights from a local electronic hobby store and am going to make a little panel to house them in.

I also tackled a job I should have done much much earlier, that being checking the valve shim clearances. I bought a shim kit with a bunch of different sizes and although I got all the clearances back in spec (some were very tight) there are still a couple that are right on the edge of the limit (0.002"- 0.005") as per the Honda factory manual. I'll address these in the future but for now they're in range.
I also spritzed all the electrical bits in the alternator with some cfc free contact cleaner.

 

[Vortices]

It's been an interesting mix of emotions over the last couple days, mainly giddy excitement and leaden frustration.
I solved my electrical gremlin issue by removing the Amazon led equipped speedometer and building and installing a custom incandescent panel. All of my indicator lights now work and my brake light fuse doesn't trip anymore.
Hurray for street legal!
For the panel I just bent some aluminum to fit between the tach and speedo and installed some coloured incandescent lights from a local electronic hobby shop. Spliced and connected into the original bike harness via soulder and a molex connector for ease of removal.

I slapped the front cowl on, double triple checked my to do list, made sure my rear wheel was cotter pinned and the front was torqued and took it for a test drive.
Round 2 of this thing will definitely be a bit more low profile but for now it's fine.

The test drive went ok, the bike stumbles at low rpm and would die at a stop even with the clutch pulled, so that's not ideal, but it revved really smooth and sounded very very good.
I'm thinking I may need to give the clutch a small adjustment as it could be dragging a bit and causing the stall.
Unfortunately my issue of base idle being proportional to engine temp is still present. As the bike gets warmer the off throttle idle climbs higher and higher all the way up to 4k.
Even if I manually twist the grip to close throttle it sits high.

There's a couple things I think it could be but any help is appreciated. My order of operations will be;
1.) Spray starting fluid around the carb boots with bike running to find potential leaks, they were quite loose after the test ride but I didn't have time to go for another ride as I have a big work trip.
2.) Swap out the aftermarket fuel filter for a proper unit. The one I have is a billet, cleanable filter but it's kind of bulky and a stock filter with a 90° input would fit nicer on the carbs.
3.) Look at the spark advancer unit, and make sure it's not stuck, probably just remove, clean, and lube it for peace of mind.
4.) Remove the carbs and check choke for freedom of movement, check operation of throttle butterflies, check hot idle adjustment arm, inspect carb sync screws, inspect the mixture settings and give the top cap internal surface a quick polish.

My thoughts are that this is a spark/timing issue as the bike has trouble starting and just decides to idle high when hot.
Other potential issues I could see is the top caps are not very smooth and the needle valves are sticking.
Again if anyone has had an experience like mine I would gladly accept help.

 

[Vortices]

Alright, I have solved my issue and it turns out it was operator error.
Listed below is what I did and what I found.
1.) I removed the carbs and re read the manual, this revealed that I had set the mixture screws too tight at the Honda manual setting of 1-3/4 turns out from bottomed. The true setting should be 3 turns out. Honda leaned these bikes out to meet emissions standards and they prefer the richer mix.

2.) I inspected the spark advancer unit and to my relief it was functioning just fine, no binding in it's travel and snaps back into place.

3.) I discovered my choke arm wasn't cracking the throttles when closing the choke so when I was trying to start the bike cold I was basically sealing the carb at both ends. A quick bend with a screwdriver got the arm to where it needed to be.

4.) This one was the real kicker. When I was trying to start the bike it wouldn't idle (obviously, it was cold). In my inexperience I increased the idle screw, this seemed like a no brainer. After digging around on the forums I discovered that the idle screw is actually the HOT IDLE screw and should not be adjusted until the bike has been warmed up.

5.) While I was researching high idle issues I discovered that the ideal cam shim clearance should be .004" - .006" and not the Honda factory specified .002" - .005".
I measured all my gaps and found that I needed eight new shims which I got off eBay for relatively cheap.
Yes, I made sure to always turn the engine using the stator bolt.

After reassembling everything and double triple checking all my hardware I was ready to fire it up.
Ladies and genitals I am pleased to announce I will be riding my 1980 CB750C DOHC to work tomorrow and as much as humanely possible until the rainy season starts.
Test ride was successful, I'll be doing a quick carb sync and timing check tomorrow but other than that she's mint!

I also repainted my tank and put an aircraft grade clearcoat on it to avoid hazing from gasoline.

 

[mrtwowheel]

Yeah, you need to have an operating choke with these bikes. First start-up of the day needs full choke no matter the temp. After the first start-up I usually use partial choke to start the bike.

 

[Vortices]

Yeah, you need to have an operating choke with these bikes. First start-up of the day needs full choke no matter the temp. After the first start-up I usually use partial choke to start the bike.

I also realized that my cheapo fuel petcock starves the bike in "on" so she would start with the choke and then immediately die when I pushed it back in. I had to ride around with the petcock in "res" which isn't ideal.

 

[mrtwowheel]

Real nice stripe on that tank. Couldn't see your pictures before. I hate my internet, so I live at McDonald's.

 

[Vortices]

Real nice stripe on that tank. Couldn't see your pictures before. I hate my internet, so I live at McDonald's.

Thanks man! I figured out a sweet little trick for eliminating the little bit of colour creepage under the edge of the tape you get when you paint two separate colours. You paint your first colour, wait for it to dry, tape off where your stripes or design is going to be, then before you apply your second colour do a quick coat of the base colour on the tape edges. Wait till dry then apply your second colour.

 

[Old_Crow]

Thanks man! I figured out a sweet little trick for eliminating the little bit of colour creepage under the edge of the tape you get when you paint two separate colours. You paint your first colour, wait for it to dry, tape off where your stripes or design is going to be, then before you apply your second colour do a quick coat of the base colour on the tape edges. Wait till dry then apply your second colour.

They make a fine line tape that is a smooth plastic material that won't let the paint creep under. Use that to outline the stripe and apply the normal masking tape to the fine line tape. I've also used a wallpaper seam roller to smooth out the edges of the normal crinkled masking tape to reduce the creep.

 

[Vortices]

Some small updates on the ol girl.
I had a crank no start situation which turned out to just be the outcome of adjusting my high idle. When the idle was turned up a little bit my choke arm couldn't bump the throttle open on start, easily fixed with some pliers and a screwdriver to bend the arm a little farther.

This bike has always run a little lumpy and never quite "right". Upon researching these forums I found some great pointers for keeping these old bikes running smooth through small adjustments. I tightened up the spark plug clearance from the higher end of 0.024" - 0.028" down to 0.023"ish. I also tightened the clearance between the pulsers and the rotating nub from the factory 0.016" down to 0.012".

Holy man this thing absolutely rips! Throttle response is much more snappy, engine has a nice growl to it and it makes a hell of a noise when you open it up! No more hanging idle or hesitation during acceleration. I've still got insurance on and I intend to ride it every day I can, seeing as I've got two years of riding to catch up on.

I'll be sure to post some videos in the future.

 

[mrtwowheel]

Great bikes when you master these great carbs.

 

[mrtwowheel]

Gotta say, those Comstars look great on that bike.

 

[Vortices]

Gotta say, those Comstars look great on that bike.

Thanks man! I was gonna switch em out but I'm a sucker for a good set of 5 spokes. I've got an almost matching setup on my car.