Topic: CB750K4. Light frame mod and performance upgrades.

[D-Ral]

Hey guys. I picked up this bike a few months ago and wanted to make some progress before starting a thread.





The middle photo was taken from the seller's garage.

I've been reading this forum daily since I joined about a year ago. Other than that, I don't really know much about motorcycles. However, I worked in a bicycle shop for 5 years and there is a little bit of crossover. Suspension, frame geometry, spokes wheels, etc. I'm not a complete newbie.

Anyway, the plan with this bike in the beginning was just to make it safe and rideable for the summer. Get used to it, and then get creative over the winter. Plan has changed a bit after I bought the bike and started taking things apart.

Although it started easily and ran well in the seller's garage, it didn't come with a petcock or a battery. No throttle cables or throttle sleeve. No turn signals. Both tires needed to be replaced, and the rear wheel was missing three spokes. Also, the right hand control was missing the the back part or lower half, so it had to be started by touching some wires together by hand. Oh yeah. The master cylinder was also pretty frozen.

I decided that other than fix these issues I would also clean and rebuild the carbs. But first I had to get it running in my own shed just for my own peace of mind. So I bought a battery and petcock and attempted to do so.

The first interesting thing I noticed was that the fuel lines were set up to have a petcock on the right side but the tank, which is a 74 tank, is set up to have the petcock on the left. The second interesting thing is that the oem petcock I ordered had only one brass spigot, where as there were two fuel lines to connect! Turns out I ordered the wrong petcock. Oh well. I bought a little plastic tee and rerouted the lines to accept the tank.

It only took a couple tries to start the bike in my shoddy little shed. I think it was probably -5 degrees Celsius.



You can't see the exhaust, but it's there. Ha ha. Anyway,I just wanted to prove to myself that it runs and I should be able to make it run again if I start unplugging and disassembling things.

[MtnPwnd]

Good base to a project. At least you had a whole bike. Haha I didn't get the whole bike so I'm having to figure out what pieces I need and what I don't need. I am doing a complete frame off 9-10 second streetbike though. Check it out! I look forward to seeing where you go with this one!

[D-Ral]

I figured that for my first motorcycle project it would be self destructive to start with something in pieces. This bike has been a mitt full already. I'll post more tomorrow.

[UncleD]

Welcome to the forum. Tons of info and knowledgeable people here.
I'm also building a K4...... also first bike and very very slow.....
Good Luck!

[Frankensteins Bride]

Welcome! Nice starter project. What are you planning?

[D-Ral]

My plans have changed 1000 times. I guess that is why I'm just trying to get it rideable until winter, and then overhaul and design then. Initially I was interested in building a cafe racer. However, after reading builds and looming at photos upon photos, I've also grown to love the stock look as well. I think what is like to do is to modernize a little bit, give it more of a race look, and keep as much of the stock style as possible. I want to make the bike as practical as possible, as well.

My main goal with this project goes beyond the bike itself: to learn. I've never been one to take things apart to see how they tick, but as I grow older, things have changed. Engines still seem like magic to me, and I'd like to change that.

Anyway, here are some photos from my carb rebuild.



My advice: heat gun is your friend. Don't cut the boots. They are expensive. Ha ha.




As you can see, hindsight is 20/20. I struggled for a while trying to get these buggers off. Tugged and pulled and tugged and pulled. Read as many forum posts as I could, and for some reason I decided it was worth it to start cutting through the rubber boots. Once I started, I realized I'd be there for a month trying to cut these with my stupid saw. But once I started... Anyway, back to the forum for help.

In the end, I hooked up some ratchet straps. One end on the carb plate, one on the frame. I added the tension, and then blasted the rubber boots with the heat gun.  When the cuts I had started in boots 1 and 4 started growing, I started sawing again. Wasn't long before the carbs popped right off. Boots 2 and 3 were still intact, 1 and 4 cut in half. If I had just heated before cutting, I'm sure I'd have all boots intact and no rubber powder mess all over my engine. Oh well. That's how you learn, right?

[70CB750]

Welcome!  Nice project.

[D-Ral]

Everyone loves photos, so there's a few of some dirty carbs. Came apart easier than expected. I mangled one main jet trying to get it off, but I replaced all jets, needles and gaskets anyway. Seemed worth it.

If you want to remove your needle jet, here is a good way:




I don't think it's necessary to remove it, now.



Fair bit of vanished and cruddy gas in there, but the hardest part to clean was the old gasket. I soaked the carb bodies in solvent, scrubbed with a combination of wire brush, green scotch brite, and dental pick. If you don't have any acetone for the old gasket, your wife probably has some nail polish remover lying around. Same #$%*.





Spent waaaay too much time adjusting float height. But I think by the end of it I had it right.

After assembling the carbs on the rack, I noticed that the brass flap on the 3 choke slide wasn't staying closed. The spring that holds the tension on it has flipped around. I was able to reach in and adjust it, but couldn't regain the original tension. Hopefully this won't cause an issue.

The carbs were stocked with 110 mains, but the rebuild kit came with 105, so that is what I installed. I'm pretty sure that I read something about people switching to 105s in Hondaman's book, anyway.  We will see how it goes.

[D-Ral]

Last year I bought a parts engine and a bunch of random parts from a guy who had been in an accident on his bike and was just looking to get rid of stuff. Those parts began to come in handy pretty quickly after buying the bike. Found a gas cap that just needed a gasket amongst the pile of parts. Who knew a cap could be made up of so many parts?






There are a few small dents in the tank, but nothing major. Bottom of the inside I the tank has a little bit of rust. I think I'm going to shake the tank with drywall screws inside to loosen up big chunks of rust and then attack it with the works toilet bowl cleaner. It's discussed at lengths on this forum. I'll let y'all know how that goes.

[D-Ral]

Rebuilt the forks with new seals and oil. Was going to replace the springs with progressives, but I wanted to experience 1974 technology first.



Here's a shot of my workspace. Bike won't fit into my basement, but at least all the stuff I remove from it can be rebuilt or worked on in the comfort of my house. My wife loves the smell of gas/solvent/oil in the morning.

[D-Ral]

One of the fork legs was pretty rusty up around the circlip and seal. The fork tube has very small bit of pitting, so I cleaned it up with steel wool.

[deafcaferacer]

My wife loves the smell of gas/solvent/oil in the morning.

Something's wrong with my eyes; I don't think I read that properly. Say what?

[D-Ral]

Yeah, that was a joke. She definitely doesn't enjoy the smell, but I think she prefers it to me talking about the bike all of the time. That's half the reason I started this thread.

It doesn't actually smell like anything anymore. Just a bit like rancid fork oil. But when I was cleaning my carbs it danked a little bit, but I just cracked the windows.

[D-Ral]

So far I've ordered a pile of parts online, mostly from www.vintagecb750.com. Prices are fair, shipping is cheap, and they are Canadian, so stuff comes fast. Had to get new brake pads, throttle cables, speedo/tach cables, right hand control, fork seals, smaller gauges, some wiring bits, exhaust gaskets, etc. Nothing too exciting.

Although I was planning on 'just making the bike rideable,' for this summer, I couldn't help but change a few things with the front end because I had it all apart. I still have it apart, for that matter. The front brake master cylinder was so rusty that I couldn't squeeze the retaining ring out, so I robbed one from my parts collection, disassembled it, and cleaned it. I haven't installed the rebuild kit I ordered yet, I'll document that when I do. I have several brake calipers lying around, so I mixed and matched to make one with the best parts I had. The original brake line is all cracked and cruddy so i ordered up some ballin' stainless steel line from Slingshot Cycles and a new oem metal b line from eBay. It came pretty quickly. I'm waiting to get my front end back together before I set the brake back up.

I also ordered some tapered bearings, which turned out to be a PITA. I should have ordered an Allballs set, but instead I got one from another company, but I came without any spacers. I thought that meant I didn't need them, so I installed them without. Of course, it didn't work, and when I tried to remove the lower bearing from the steering stem, I broke it. Go figure. I have another on the way now. The eBay company I ordered it from is offering to refund my money if I return the broken parts, so that's nice.

Here's some of the stuff I'm using on the front end:







And here is something totally unnecessary. And an Allen bolt kit. Got these from Bullit Custom Cycles, and will hopefully look smokin' with those new mini gauges.

[D-Ral]

I took apart my airbox to replace the filter, and noticed that the foam gasket was all torn up. I replaced it with one from another airbox. While I was at it, I couldn't help but wonder how important it is that the rubber velocity stacks are shaped correctly. All of mine are deformed around the bell of the stack. Looks like someone pressed down on the horn area with a thumb and it stayed down. I seems to me that this would be a reason to replace the stacks, as I figure air would be would be pulled into the system irregularly. However, if I don't have to replace em', I won't. They aren't cracked. What do you guys think?

[D-Ral]

Installed my master cylinder last night. Ended up flinging circlips and springs all over my basement, but after I found them they went in smoothly. Stuck a piece of bicycle brake housing in the rubber stop hole. That works well.

Now I'm trying to figure out how to assemble my brake pads. The outside pad was easy, but I'm having a difficult time figuring out how to attach the moving pad to the piston.



This is what I have. The set screw in the pad does not tighten against anything. S I pulled out the manual, and it mentions a 'pad seat,' but shows nothing in the diagram. Checked the parts fiche, and yep, there it is. I rooted through the cookie jar of old rusted parts, and found this thing:



Fits nicely in the open end of the piston, anyway. But i still don't understand how this goes together. So what am I missing, here?

[D-Ral]

Okay, thanks to some help from some Instagram folks, my caliper is assembled. Didn't realize that the pad just floats upon the rounded seat. Cleaned up the pad cavity in the caliper and sanded the paint off the sides of the pad, added some grease, and everything moves smoothly.

[D-Ral]

I'm reinstalling my rear wheel, and I'm having a hard time sliding the wheel forward in the dropouts because my right side chain tensioner is not the proper one. It is not wide enough an doesn't fit around the frame properly. Anyone have an extra?

[D-Ral]

Smashed off the broken bearing race off my stem with a chisel. Got my cheap plastic eBay blinkers yesterday. I'm just waiting on the bearings and a drive chain adjuster to get this thing rolling.

Heading out on vacation for a couple weeks on Sunday, so these things should be waiting for me by the time I get back. Some friends of mine and my wife are flying to Vancouver, renting an RV and travelling down through Oregon and Washington. If anyone has an advice on fun stuff to see/do, let me know!

[D-Ral]

Well, the bearings came in and went on fine. Should have ordered the all balls kit to begin with. Rigged up the front end between packing for my vacation, and here is how she sits now:





Wiring is half done. Woo hoo! Anyway, wife is hating on me, so I'd better put my phone down. Back at it in two weeks.

[D-Ral]

Back from vacation.

Over the last week I've gotten everything back on the bike. Chain, sprockets, most of the wiring, etc.

Here is my current problem:

The bike starts up right away. Choke is all the way on. If I even bump the choke open a little bit, the engine will rev out of control. It races and fluctuates a bit, but in the end I have to hit the kill switch because I'm worried ill either go deaf or blow the engine apart.

Could there be a leak somewhere in the boots? I have bench synced the carbs but have not vacuum synced then yet.

[D-Ral]

Had some friends over yesterday helping. That last photo was a serious wtf moment. Ha.

[Johnie]

Just want to be sure you did turn down the idle screw...just wanted to mention it. If the idle screw is all the way down then it sounds to me like during the bench sync you possibly set the slides to high causing the over revving of the motor. Many guys have the carbs off and use a flashlight to bench sync those slides. As soon as you see the least bit of light tighten the slide and go to the next. I saw one person use a paper clip to set slides, but in my case the bike ran to fast using the paper clip slide measurement. You have made a lot of progress...you are getting close to dialing the bike in...good luck and keep at it.

[D-Ral]

Thanks dude. I didn't mention that we had unplugged everything in the earlier bucket and troubleshot everything for an hour wondering why it wouldn't start. Then we realized the key switch was I the wrong position.

ANYWAY, we did adjust the idle screw down, that's when we realized that it only took bumping the choke lever to send everything off the charts. I did bench sync, but I used a drill bit. How many threads should be showing on top of the slides? I read many warnings about having at least a certain number, but I have many threads showing on top of the last nut.  I was pretty sure I could bottom out the slides with the idle screw the way I had it, but maybe I was wrong. I'll have to tear the box and carbs off the bike anyway, so we will see. I will try the flashlight method. Maybe I have the slides in backwards?

[Johnie]

The part of the slide with the cut out should be located to the back. I would use the flashlight sync method as then you can get those slides down pretty far and equal. Then use that idle screw to get what you need on start up. I have a spare set of carbs here and will take a pic for you. The slides are all the way down and there is minimal threads showing...give me minute to load up the pics...