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Discussion Starter · #1 ·
Hello, all:
I am a Maryland resident currently working on building a cafe racer out of a 1982 CB750K and 1981 CB750C. This is my first build and I'm a new rider, so although I generally work through the process on my own, I'm hoping to use this site to avoid stylistic or mechanical oversights on account of inexperience. I bought the 1981 CB750C first, then bought the 1982 CB750K after realizing it was an easier starting point (confirming what I read on this site).

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Discussion Starter · #4 ·
Welcome.

What is the mechanical status of the two bikes?

As a new rider what are you hoping to achieve with this build?
The 1981 ran fine, but had 43k miles on the odometer (speedometer was not working so I don't know the actual mileage) and looks like it was laid down (apparently damages are limited to scratches on the right side and none to frame). The 1982 runs great to my ear, but my buddy (I'm unsure of his credibility/knowledge) said he could hear a knock when idling. Also, there was some white smoke from one exhaust pipe, but this got better the more it was run. FYI, this is the status after cleaning the carburetor and airbox/filter. When I bought it, it was not running and had not run in many years.
 

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Discussion Starter · #7 ·
stylistic oversight number one: you bought the custom classic version and not the sports model to build on, the wheel sizes are all wrong, frame and suspension components are all marginal and the bikes weigh a ton.
... the last parts are more mechanical, although my eye goes straight to things like alloy rim upgrades and the such so in a way mechanicals can really bolster the stylistics, but that's just me.
I agree that the Custom version is not ideal, so most of what I'm using is from the CB750K. I'm not installing the wheels/tires until the end to avoid damage during the build. I have the Custom's 19" comstar for the front and 18" comstar for the back, as well as new 400-19 and 450-18 Firestone Deluxe Champion tires.
 

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Discussion Starter · #8 ·
ive always loved the styling of the k models, like the sport f the best. i would put a little effort in getting the k model riding and looking good. i know my answer doesnt help in your quest for a “cafe racer” . the c model is a awfull starting point . your using the k front attached to the c frame and rear? i would use the k frame, plus you have wire wheels. the k might look ok with rearsets, different bars, 4-1, plus other performance upgrades, good luck and carry on
Ultimately, the only parts from the C model that I plan on using is the frame, and that could change if I find that it differs from the K, but I don't want to cut up both if I don't have to. Based on research and direct comparison (I have them both in front of me), the frames are exactly the same shape and only differ in the location of a few attachment points (like fuel tank, side panels, etc.), but I'll switch if I find out I'm wrong.
I like the wire wheels but the rear is 17" and I already bought the 18" rear comstar and tire, so I'm going to see how that comstar setup turns out first. Rearsets are on my list and I already have different bars and a 4-1 exhaust, but if you have any other performance upgrades in mind, I would appreciate your input.
 

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Discussion Starter · #17 ·
This is not a roadworthy, let alone performance, motorcycle. Parked is it's highest and best use.
View attachment 105357

This is an '80's Honda performance bike.
View attachment 105358
You don't need bigger or longer you need proper.
I can’t see the rear suspension on Spencer’s ride, so are you talking about tires now? If so, I’ve read that low profile tires are best for cornering on tracks, whereas bigger ones perform and ride better on regular roads.
 

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Discussion Starter · #18 ·
It's not the battery specs you need to worry about, it's always a matter of: is the charging system compatible with the change in battery technology. Your bikes have dirty old charging systems that were adequate for an ancient technology lead acid battery, which is very forgiving of over-voltage.

LED lights?
Yes, LED lights. I thought AGM batteries worked with the stock charging system because they are still lead acid. If not, do you have suggestions on how to update the charging system? Just an aftermarket rectifier, or is it more involved?
 

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Discussion Starter · #20 ·
I don't know what you have been reading but if a bike to ride is your goal then you are headed the wrong direction.
The frame mod doesn't allow for wheel travel and the correction for that is not longer shocks. In Ontario, where I am, the tires you bought aren't road legal. Look at the shape, does it look like it is made for cornering? You swapped out substantially lighter wheels, with a much greater selection of tires, for just about the heaviest thing Honda built. Putting the battery higher doesn't lower the c of g. Removing the air box from the bike brings in a host of new issues. Worry about a new pipe when you start scrubbing those.
Unfortunately that all makes sense, so I need to think about how I want to deal it all. FYI, I removed the airbox temporarily and plan on putting it back. The 750C had a mouse nest and the 750K had a dead snake.
 

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Discussion Starter · #28 ·
Read all this and tell me if it sounds like your old stock motorcycle charging system is going to do the job.
Sounds like overcharging will be an issue. It also sounds like an ill suited charging system will just shorten the lifespan of the battery, not damage the bike. Do you agree?
 

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Discussion Starter · #29 ·
You seem like you are genuinely interested in putting together a good bike as opposed to lawn art and that will get you help here. Stop looking at or reading about these dopey Pipeburn POS motorcycles they have nothing to do with putting together a great riding machine. Look up Steveo's Ducati Imola build, it is a good example of how a pro completely changes the cosmetics of the bike and never loses anything that makes the SS a top notch performance motorcycle.

The other thought to keep in mind is when you are riding with a group is often the time when someone will want to try out a bike that another rider owns. As an example if you and I are together, me on my snotty, beat up, little 620 Monster and you on a Pipeburn special, we aren't swapping. Not just because I don't want to ride yours but also because I can see you know nothing about motorcycles and my Monster isn't your clunker. I don't want to find out you can't ride it but think you can.

Your framework is the real issue at this point. Put the wire spoke rims back on with the normal tires. Cut out your hoop. Take one shock off, put a ratchet strap between the seat pan and the swing arm. You should be able to tighten it down until the shock bottoms out. That's the travel you need. There are a couple ways to correct it. Airtech may have a seat pan you could use to cover the structuring.

Shocks, fork internals, tires and braided brake line upgrades would be money well spent. Wear and maintenance issues should be attended to. And then just ride it until you know something about the bike.

Progressive and Hagon are good suspension vendors. You can ask for tire recommendations. If you are a new rider don't go with clip ons, better a superbike bar. You want to be upright and comfortable when you are learning to ride, it's a survival thing. You want to e able to see what is going on around you because you have no muscle memory until you get many miles under your belt.
I checked out Steveo's Imola build and that is on point, for sure. Are there more pictures somewhere, as I only found the one. So I have two bikes: the build is 1981 and the other is 1982. Although I am swapping parts for the benefit of the 1981, I'm going to keep the 1982 operational and fully address any necessary maintenance and repairs. That being said, I may ride the 1982 until I feel comfortable enough to start transitioning onto the 1981 build. The ratchet strap idea for testing the swing arm is great. I built the cowl (seat pan is still in progress) and don't see myself ditching it because I like how it turned out and want to apply my personal touch, but I can adapt it as necessary based on the swing arm range test.
Also, I mentioned earlier that a buddy thought he heard a motor knock in the CB750K. I have uploaded two videos to YouTube: one of the bike starting and one of a road test. It would be great if I could get some input on if there is a knock or not (hopefully the video upload worked).
 

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Discussion Starter · #34 ·
Sounds like typical Honda clutch throw-out bearing noise. Pretty normal. You can replace the bearing - no big deal, but it will come back.

Easiest way to test is to pull the clutch lever - if it goes away or the sound lessens - there's your answer.
That sounds like it may be the case. When you say "no big deal", are you talking about the level of effort for the work, or are you saying throw-out bearing noise doesn't lead to or indicate issues that should be dealt with?
 

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Discussion Starter · #36 ·
Let him try it, how would you know it’s not enough clearance anyway? We all know old eyes aren’t reliable.
Do the suspension test and let us know (y) you Made the cowl?!? Looks real cool for sure. Always a fan of personal touching haha
Thanks. I know it doesn't look like much, but it was hard for me to create strait lines and get everything smooth and symmetrical. I should have built this later in the project after getting more comfortable with welding, but I didn't so had to use bondo and epoxy to clean up the awful weld job. What I learned is that it is very hard to tell if lines are strait when the surface you are looking down is three different colors.
 

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Discussion Starter · #37 ·
I'm basically saying that if you replace the CTB the noise will come back pretty quickly.

Your CTB only needs replacing if you have serious gear change issues.

It was a 'problem' with the Honda 4s for years, but it's only a noise, not necessarily a real problem.

Don't worry about it.

Have your ever heard a Ducati dry clutch? - sound like a hand full of bolts being shaken around in a tin can, and then amplified - they still worked fine.

Address the other issues with the bike, before worrying about this is my opinion..
Roger that. The motor we are talking about is a 1982 with 23k miles, and I am in the process of deciding whether I want to use that one or the 1981 motor with 43k miles. Nobody noticed this sound in the 1981 motor, but the exhaust system on that bike was screwed up in many ways, so who knows what sounds will come to light once a proper muffler is installed. The 1982 was shifting great so I think I will go with that one based on your input.
 

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Discussion Starter · #40 ·
So you melted a bunch of bubble gum weld onto the frame with little to no penetration anywhere, and then filled it to make it look decent when in fact it is about as strong as hot melt glue. What's the plan for this bike are you selling it?

What you should have learned is that you don't learn to weld on your motorcycles.
I have not done anything to the frame that could influence structural integrity. I’m talking about welding the cowl together, which is bolted to the frame. I did have to weld small tabs to the frame to bolt the cowl down, but these are positioned outside of load bearing areas.
 

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Discussion Starter · #46 ·
No, that was structural. If it were just for attaching the fender it would have been flat narrow steel rather than the formed.
It seems too thin to do much structurally, but I may be wrong. I haven’t put much thought into it because by the time I bought the second bike and realized that was even there, I had already welded a thick steel L bar across in that area. Sounds like that is more important than I thought so I’ll look into it further and beef it up.
 

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Discussion Starter · #47 ·
It won't pass the ratchet strap test, you don't even need to try it.
Don't know what the cowl is, unless you're talking sleds but, carry on with your personal touching.
I did the test with the shocks from the 750k and found that the tire radius (of the 750k wheel with moderate tread wear) was 5/8” too much. In other words, the rear hoop would need to be moved 5/8 inches to prevent contact.
I measured the travel of the rear shocks at 3.5”, fully extended, but there is also a 1” rubber bumper (so 4.5” of travel w/o bumper). I got 15-7/8” from wheel center to rear hoop when shock is fully extended, and 12.5” when shock is completely depressed. Also, the spring rate of the shocks are on the second to lowest setting, with 3 stiffer alternatives available.
To keep the rear hoop, I thought of a few things (or combination things) that I could do: 1) replace 1” bumper w/ 2” bumber; 2) select a stiffer shock setting; 3) adjust the position of the rear hoop. What portion of the shock travel is actually used during normal driving conditions, and how bad of a situation am I in if the shocks bottom out? In other words, are these shocks designed to accommodate a wide range of riders and riding styles without adjustment, or is the owner expected to adjust the shock settings so that the full 3.5” of travel is used?
 
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