1981 Honda CB750C Cafe Project

BigNickel

New Member
Hey everyone,

New to this site and the whole cafe racer culture, so I thought I'd start a thread to get some feedback and learn from others.

I just picked up a 1981 Honda CB750C off a friend of mine who was planning to build it up but had too many projects on the go. It has a little over 30,000km on it and runs solid. I've been wanting a CB750 for many years now but it was never a good time. When I ran into this one I had jump on it. It's a DOHC model...I originally wanted an early model over the DOHC but I couldn't pass it up either way.

I have a few solid ideas in my head as to where I want to go with the bike. I'm going to sit on it for a few weeks before doing anything too drastic so I don't have to do work twice. My friend gave me some 90's Suzuki Katana front forks and complete rear end with it, so I've been entertaining the idea of trying to swap in the modern components. I'm looking for any tips or info for working on these DOHC models. Nothing major to report yet, just wanted to get the thread started up.

Here was the bike when I pulled it in the garage. It was missing the seat, side covers and mufflers. Not a huge deal as all of those will be taken off regardless.

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I took off the crash bars, windshield, rear tail light and fender assy

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You can swap the swing arm from a cb1100f and it will accommodate a much wider wheel and is a direct bolt on as well as a 900f swing arm. As for the katana front end, how long are the forks? You don't want to have them too short which will mess with your rake and trail. Another popular swap is cbr f2/f3 wheels and front end. Also if you want any other type of dohc tank on that bike then you will have to make your own crossmember to bolt it to because the c tanks are shorter. Not a big deal just a heads up. We're you going to keep your carbs and air box?
 
Some of those suggestions and ideas are exactly the type of stuff I was hoping for from some folks with experience here. Thanks so much for the pointers.

Doing a quick measure, the Katana forks are an inch or two shorter. I'm not 100% sold on using them, I just got the stuff with the bike as my buddy was entertaining the idea of using it. I'd love to do an inverted swap using the stem from Cognito moto but I'd need to find a really good deal on the GSXR items for that to be possible. I'll look into the 900f swingarm and go from there.

As far as the carbs and air box, I would love to run pod filters to clean it up but I've been reading the horror stories with getting your bike to run right afterwords unless you switch carbs. Eventually I'll go new carbs and pod filters but for now are there alternative air boxes you can get that look nicer than the stock unit?

Cheers
 
Iim sure that you would greatly improve on the handling with the katana front end. The stock front end is terrible on the c model. Yeah the stock carbs are very difficult to tune without the air box. I don't really know of any alternatives to the stock box. I wish I would have kept my stock carbs and air box. I switched to mikuni bst34ss carbs off a mid 80s gsxr. They work very well with pods but I'm not sure if there is an actual performance gain from them. They are a lot cheaper than smoothbore carbs and you get the reliability of cv carbs.
 
J-Rod10 said:
Steel Dragon Performance makes an aftermarket box.

Do you have any experience with these air boxes? I ran into them a while back while searching online. I've yet to see anyone with one or reviews on how the bike runs with them (ie: does it keep your bike running as smoothly as stock?) They look much better to me, so I'd love to hear they work as I'd probably order one.

Cheers
 
I believe that the C model is raked out a bit more than the F due to the placement of the front axle on the forks, this may somewhat compensate for a change in front end geometry if you go with the shorter front end. I do know that on my 900F when I jacked up the rear shocks for more ground clearance the resulting minor geometry change made the bike twitchier in the corners. Also these bikes don't have a lot of ground to begin with, something to consider if you are going to lower one. Too bad there isn't someone making custom triples to adapt modern forks to these girls.
 
CarbsAndCylinders said:
I believe that the C model is raked out a bit more than the F due to the placement of the front axle on the forks, this may somewhat compensate for a change in front end geometry if you go with the shorter front end. I do know that on my 900F when I jacked up the rear shocks for more ground clearance the resulting minor geometry change made the bike twitchier in the corners. Also these bikes don't have a lot of ground to begin with, something to consider if you are going to lower one. Too bad there isn't someone making custom triples to adapt modern forks to these girls.

It would be a massive expense but I believe cognito has their stem for the DOHC models to get the modern inverted GSXR forks on there. I'd entertain that swap but it would make my project go way longer as that would eat up every penny of budget. I picked up the bike for a very fair price so I was going to try and make something cool of it. Getting some better suspension on it is my #1 priority as I actually want it to be good to corner with. Any ideas are appreciated. I'm thinking of trying to get the Katana front and rear on and see how it goes. I can always change it if things go south. I do recall seeing someone who did this swap online somewhere that's why I still have hope. The forks aren't a ton shorter. I know ground clearance is an issue but me having super short legs, I wouldn't mind getting it lower than stock but high enough that it's not an issue.
 
BigNickel said:
Do you have any experience with these air boxes? I ran into them a while back while searching online. I've yet to see anyone with one or reviews on how the bike runs with them (ie: does it keep your bike running as smoothly as stock?) They look much better to me, so I'd love to hear they work as I'd probably order one.

Cheers
I do not. They are about the only option, however. Unless you can make one.
 
The Steel Dragon box looks heavy, and it isn't really an airbox it is just a 4 stack filter. I can tell you by looking at it that it will not be easy to jet the carbs and it will not increase your performance.

I'm gonna buy another one of these damn bikes next season just I can make a decent airbox available for them. At steel dragons price, I think I can do it, and it damn sure will deliver better performance than theirs.

Make one. Just be sure it has as much air space as you can provide within your visual goals, and start with a single small hole on the intake side and slowly enlarge it until the factory jetting responds the best. There's obviously more to it but r and d is just that.
 
I've thought about making one. I'm a CNC Plasma table programmer and operator so I can draw up and cut my own pieces if needed. Even a similar size air box doesn't overly bug me, just something nicer to look at and more suiting to the bike once it's made to more like a café racer. I do love the simplicity and look of the pod filters and have debated getting carbs that will run properly with them even if it doesn't generate much power, more or less to clean it up, but I also don't mind the air box look if done right as well. I want the bike to run as this will be used and not just for Instagram haha. It's a shame parts are so slim for the DOHC's.
 
There are plenty of parts, you just have to know where to look. Cycle exchange has some nice exhaust systems, and go fast parts. A good source for mods is on the cb1100f.net site. Lots of valuable info there.
 
The Steel Dragon boxes are very good quality, I had one on my SOHC 750 before pulling the motor. You will however have to do a little tuning if you change from stock, regardless of what you change to. If you go the pods route, it can be a little trickier to get just right, but it can be done. Buy good filters, not the cheap Emgo ones. K&N, UNI, or the like are whatcha want. I believe Cycle X and Dynoman both carry applicable sizes.
 
generalyen said:
There are plenty of parts, you just have to know where to look. Cycle exchange has some nice exhaust systems, and go fast parts. A good source for mods is on the cb1100f.net site. Lots of valuable info there.

+1
 
If you replace the CV carbs with regular ones then you have a lot more options for air filters and will have an easier time of tuning them.
 
I would have loved to try out some direct lift smoothbore carbs but I'm not made of money (unfortunately ). Another drawback to direct lift is that it doesn't automatically adjust for altitude. So if you were wanting to go over a pass or anything like that in theory your air fuel ratio would not be optimal. That's where cv carbs are an advantage. I know that honda racing used the keihin cr carbs for they're race team, but they also were not experiencing thousands of feet of elevation change either. But like I said mikuni slingshots work great on these bikes with pods. You just have to be creative with the insulators because they carbs don't line up.
 
Small Update:

I finally gained some energy and motivation to keep working on the bike. Winter is pretty much over so I can open the garage door and feel some nice warm air. I've removed all the electrical and the gas tank. I sat in my garage looking at the bike as it sits and I think I've finally came up with the foundations of what I'd like to see happen.

Just before the new year this air box by Cognito Moto came out: https://cognitomoto.com/collections/new-products/products/cb750-air-intake-box-sohc-and-dohc

I'm going to order this shortly along with the Benjie Dimple tank/seat combo. Tank/Seat options that don't take a highly skilled fabricator to make look nice are limited for the DOHC, but I do like the look of this particular combo from Benjie. I'll order the tank in the mild steel option. I also think I've made up my mind on a GSXR front end and Danmoto GP exhaust.

Hopefully another update soon with some parts and more progress photos. Ordering parts right now is rough since the Canadian dollar is so bad.
 
I stripped down the bike to the point where I need to get parts in to start fitting everything up.

I ordered a seat off ebay. It's more of a brat style as I want to be able to take my wife out with me. I'll end up fitting a proper "hump" seat afterwords as well so I can swap them out as needed.

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