Aussie 1982 CB750 DOHC - First Build

Off the top of my head...

So the story goes that Honda was forced for some reason to use the Keihin CV carbs on their DOHC racing bikes but still wanted to run velocity stacks. Their engineers figured out a way -- by blocking the air cutouts and one main jet, along with increasing the main jet size. There is a downloadable manual which documents the process. There are a ton of different solutions so you don't have to do it exactly as they did -- only follow the same principles.

There are a couple of threads about the topic on CB1100f.net (A bit of a long read -- #1 is the megathread).
1. http://www.cb1100f.net/modules.php?name=Forums&file=viewtopic&t=70514&postdays=0&postorder=asc&start=0
2. http://www.cb1100f.net/modules.php?name=Forums&file=viewtopic&t=73027&highlight=
3. http://www.cb1100f.net/modules.php?name=Forums&file=viewtopic&t=72244
4. http://honda-cb750-s.456789.n3.nabble.com/1981-CB750C-Carbs-Pods-vs-Airbox-td4043715.html

I'm planning to do this mod to my bike...

Link to manual:
http://www.cb750c.com/Genesound/carbdocs/RS1000manualV1.pdf
Page 54-55
 
Fantastic, that's going to take a lot of the trial and error out. I'll go have a read now. Thanks bananas!
 
Love it bananas! Thanks very much for posting that stuff. Have you done much research into which stacks you'll be using?

Edit: and also how are you planning on blocking off the jet?
 
Got my goodies including this super sexy rear rotor, I thinking I'm going to be pretty close with fitting the CBR caliper over it, just have to do a little machining tomorrow then I'll know where I stand.
yjamunys.jpg


Fitted bearings for rear wheel and swingarm tonight too, now that they're in I can measure up for the spacers and really start to work on alignment and geometry.


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So am I, I'm leaving the welding to a mate who has far better skills than me.


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I'm thinking of filing down a piece of stainless and inserting it (or welding if necessary) into the two ports. Went with K&N Filters for peace of mind..
 
I've just finished putting my carbies back together, I blocked off the air cutoff valves but haven't touched the other required block offs yet. I think I'll get it up and running and then revisit the final setup for the stacks.
Unfortunately I had just bought a set of #115 jets before I knew of the honda project so will have to order another set of #130's in my next order from the states.

Here's the VB52's in all their glory:
y4yjanyr.jpg


Plan for the night is to machine a surface for the new headlight bracket I just got and tweak a few little things that have been annoying me lately.

Matt
 
Milled in a face for the headlight bracket, will swap out screws for bolts when I get a chance.
Tomorrow will be the rear hoop fit up.
It feels good to keep ticking things off the list. Even if only minor.
yqenety5.jpg
 
My next update starts with me not realizing the difficulty of welding alloy to stainless. My top shock mount was alloy and the new piece of frame is stainless so there lay my problem. I needed a stainless upper mount, so what better way to spend my night.

Here's what I started with:
a4atavy4.jpg


And here's what I ended up with:
7y2adune.jpg


And a comparison of the old and new:
qanune4y.jpg


Also got my new frame piece welding in today:
gevuhyne.jpg


I'm still waiting on one more seal and then I can put the engine together. It'll just be fiddly bits keeping me amused until then.

Cheers

Matt
 
the shock mount looks good so far but you need bracing in the vertical plane, you haven't addressed that
the 2 seat tubes is just going to be constantly flexing and will break or crack eventually we are talking about loads well over 1000lbs working on there
 
Thanks xb, I'll look into that. There's no instructions for this so I'm learning as I go.

Matt


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xb33bsa said:
the shock mount looks good so far but you need bracing in the vertical plane, you haven't addressed that
the 2 seat tubes is just going to be constantly flexing and will break or crack eventually we are talking about loads well over 1000lbs working on there
Click to expand...

Hey XB, I'm in a similar place myself. Would you know of any good literature for frame bracing?
 
i have a tony foale book on the evolution of frame and suspension design that has great info an 'splain-ataitin
check his website http://www.tonyfoale.com/

as far as bracing with a far forward mounted single shock the loads are pushing down on the swinger pivot and up at the top shock mount totally diffferent directions than with the original twin shock
and the loads quite high
in this case the idea is to stop the seat tubes from bowing upwards/constantly flexing, with as direct a connection to the pivot area as possible
the dohc frame looks like a natural for some tubes that go from upper mount down to the frame right behind the upper rear engine mount
 
I've hit a speed bump lately with missing parts. Have solved a few of the problems but am left searching for an oil control orifice which sits between the crankcases... Bummer
 
Thanks evil, nice of you to say. I'll want to put a few hundred rides on this mount before I come close to even recommending it or selling it.
 
Searching for a missing oil control orifice which fits between the two crankcase halves. It went off to the vapor blasters fitted and never returned which is a real bummer.
If anyone knows where to find one or even pictures of one it would be a fantastic help.
Part number is 11620-425-003
 
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