I built this 81 Honda CB750 from the ground up. I had an extensive build thread on another forum but it’s gone belly up.
So here’s a basic run down.
I got the stock bike in trade on a 5.0 swap into a Datsun pickup. Test fired and figured good enough to tear into it.
First was to jig the frame cutting away everything but the neck. That way I retain the stock VIN # for DMV sake. The backbone is 1 1/2”x.120 wall 4130 cromolly. The rest is 1”x.120 wall DOM.
I built the girder from scratch using brass/bronze bushings n washers I sourced from McMaster Carr. The fork is 1”x.120 DOM with thin wall trusses for rigidity. Took a bit of tuning to get the geometry and rake and trail where I wanted it. The spring was sourced from a 50r80cc quad. Wasn’t sure it would be enough but works great!
Front wheel is a nordisk 21” off my 87 husqvarna 500xc. I also used its Bembro caliper and master. The rear wheel is the stocker off the CB, with its drum brake.
The tins were take-offs from my sportster. The king and queen seat were fabbed with the idea of making a wide rear seat for the old lady to ride along. In hindsight I wish I had made it a bit more slim...
I had it pretty well together n took it for a maiden voyage. It ended with me learning to fly and a seized engine. Long after the concussion wore off and my flesh grew back, I put another crank and pistons in it. I ran it in the garage a bit before a rod came apart n scored that crank! It sat for a year with me flat pissed off at it.
I eventually stumbled into a cb900 shafty engine complete though without carbs. All the research I did pointed to the innards of the 900
fitting in the chain drive 750 cases. They did! The 900 exhaust cam had some chunks out of the lobes(mabey pitting) and had been run that way. The 750 cam was very similar in spec but had more overlap.
It wouldn’t run for crap with the 750 carbs so I built steel 1into2 or 2into4 manifolds and bought a set of mikuni vm36-4s. I got the 1to2 cable setup from cycleX. I wouldn’t recommend it as it’s been problematic and still causes it to hold at 2,000 often. Figuring the jetting was a nightmare eventually accomplished by a drill bit. It runs on the rich side at low rpm but great above 4,500. it will foil plugs if I ride it around much under 2,500. I literally just ordered a couple pair of 340 and 330 jets(I think I’m at 350ish currently).
The exhaust is over 30 foot of 1 1/2” exhaust tube. I included some simple baffles and a small open chamber between the pairs. It’s only 1 1/2” long so probably don’t do a whole lot... the baffles neither, it loud!!!
Well that’s roughly were it’s at.
So here’s a basic run down.
I got the stock bike in trade on a 5.0 swap into a Datsun pickup. Test fired and figured good enough to tear into it.
First was to jig the frame cutting away everything but the neck. That way I retain the stock VIN # for DMV sake. The backbone is 1 1/2”x.120 wall 4130 cromolly. The rest is 1”x.120 wall DOM.
I built the girder from scratch using brass/bronze bushings n washers I sourced from McMaster Carr. The fork is 1”x.120 DOM with thin wall trusses for rigidity. Took a bit of tuning to get the geometry and rake and trail where I wanted it. The spring was sourced from a 50r80cc quad. Wasn’t sure it would be enough but works great!
Front wheel is a nordisk 21” off my 87 husqvarna 500xc. I also used its Bembro caliper and master. The rear wheel is the stocker off the CB, with its drum brake.
The tins were take-offs from my sportster. The king and queen seat were fabbed with the idea of making a wide rear seat for the old lady to ride along. In hindsight I wish I had made it a bit more slim...
I had it pretty well together n took it for a maiden voyage. It ended with me learning to fly and a seized engine. Long after the concussion wore off and my flesh grew back, I put another crank and pistons in it. I ran it in the garage a bit before a rod came apart n scored that crank! It sat for a year with me flat pissed off at it.
I eventually stumbled into a cb900 shafty engine complete though without carbs. All the research I did pointed to the innards of the 900
fitting in the chain drive 750 cases. They did! The 900 exhaust cam had some chunks out of the lobes(mabey pitting) and had been run that way. The 750 cam was very similar in spec but had more overlap.
It wouldn’t run for crap with the 750 carbs so I built steel 1into2 or 2into4 manifolds and bought a set of mikuni vm36-4s. I got the 1to2 cable setup from cycleX. I wouldn’t recommend it as it’s been problematic and still causes it to hold at 2,000 often. Figuring the jetting was a nightmare eventually accomplished by a drill bit. It runs on the rich side at low rpm but great above 4,500. it will foil plugs if I ride it around much under 2,500. I literally just ordered a couple pair of 340 and 330 jets(I think I’m at 350ish currently).
The exhaust is over 30 foot of 1 1/2” exhaust tube. I included some simple baffles and a small open chamber between the pairs. It’s only 1 1/2” long so probably don’t do a whole lot... the baffles neither, it loud!!!
Well that’s roughly were it’s at.
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