CB750 K Cafe Racer build – Post 1 (2 August 2021)
For those who don’t know the Honda CB750 K first coined the term Superbike over 50 years ago. It was originally revealed as a concept by Honda in the late 60’s and shortly after (in 1969 to be precise) full production bikes hit the dealer floors. They were an instant success and won countless accolades and races. They essentially blew the competition out of the water at the time. There is some serious history in this vintage of a bike and we’re fortunate that one of our clients brought this one in to our workshop for some attention.
The purists won’t agree with the direction for this build. But that hasn’t stopped us before. There are several reasons why this one won’t be a restoration, and lends itself to a custom build instead. Mainly, the fact that it has been modified in the past, although quite poorly.
The mission for this build is simple. Make it beautiful and functional, all before it’s 50th birthday. We’ll be modifying this one extensively and improving the performance of it considerably. But we’ll still retain that old-school nostalgic look that the CB750 K series bikes are renowned for.
CB750 K Cafe Racer build – Post 2 (16 August 2021)
Over the last two weeks we’ve been working on assessing the CB750K and how well it runs. This will determine whether it needs any serious work to the engine or not. We started the process by doing the fundamentals and checked / adjusted the valve tappet clearances, timing electrical (ignition coils, condensers, points etc.), and so on. The bike also returned very consistent compression readings of 130psi (+/- 5psi) across all cylinders. Not bad for a bike that is about turn 50.
If you’ve just purchased a new bike and are wondering what needs to be checked then take a look at our Tech Article 4 on Where to start when it just isn’t starting. With all the positive signs we needed to create a muffler insert so that we could hear the engine run (without losing our hearing). This took on our usual process of creating an insert that is also captured in our Tech Article 1 Part 2 on building your custom muffler insert. This is a 1972 bike an here in Australia that means a noise limit of 100 dB(A).
This bike was able to be started but was far from being able to idle on its own. We decided to get a set of new Mikuni RS34 Carbs for it to ensure it breathes a whole lot better and kick things off in the right direction when it comes to upgrading the performance. Check out a photo of them installed down below.
CB750 K Cafe Racer build – Post 3 (23 August 2021)
After getting the bike idling and ensuring it will have many years of high rpm we decided to move onto our first challenge for this build. That is, removing the rear wheel and replacing it with a modern one to increase traction and achieve a more modern sports look at the rear. We’ve opted to use a Ducati Monster rear-end with a 160mm wide tyre as the substitute. Fortunately the wheel directly fits within the rear swing-arm and all that is required is a) matching the existing 750K axle diameter to that of the Ducati, and b) aligning the front and rear sprockets.
Task a) is the easier of the two, and requires us to machine an axle sleeve to go over the 20mm CB750K axle. The sleeve will have a wall thickness of 2.5mm and bring the total effective diameter up to 25mm to suit the new wheel bearings. We’ll then need to centre the rear wheel and machine some axle spacers to keep it there. Here are some progress photos:
Task b) involves aligning the front and rear sprockets. Now that we’re using a wider wheel at the back the front sprocket no longer runs true to the rear. We’ve checked clearances between the swingarm, the frame and anything else that it could strike and fortunately the only thing required is to offset the front sprocket so its in line. We’re still looking at our options on how to do this, but the first step will probably involve printing one out to size. The other important consideration will be the chain size. We’ll need the front and rear sprockets to match up in size for the chain.
CB750 K Cafe Racer build – Post 4 (6 September 2021)
We’ve made further developments with the front sprocket in the last week or so and decided to test fit everything. Using our 3D printer we managed to print one to size to simulate the actual alignment between front and rear. Ducati sprockets with the same pitch diameter come in only a few select sizes so we’re going to match the front teeth to the rear. This will allow us to use a 525 chain that should be ample considering the power output of the CB750. In this setup we went with an 11mm offset. It’s likely that we’ll get this custom made by a local sprocket place that specialises in this kind of work.
We also managed to strip the bike further down and get started on some custom fabrication. First up we’re looking at getting rid of that massive oil can on the side of the frame. We’ll lean it up and place it directly behind the carbs that should a) prevent any debris from the rear wheel flicking into the carbs and fouling the filters, and b) removing some visual weight from underneath the riders seat. We know we can get similar oil cans online, but prefer our own style. This involved shaping the can to the profile of the frame and to add some style bead rolling the seen face. Still yet to do the internal plumbing but we’re getting there.