1979 CB750F Tracker (maybe?)

Popeye SXM

Also used for MX
Great project
Years of moto have left me with bad knees and a not-so-good back
I know that feeling very well, I started racing MX at aged 6. I am well in to my 40,s and feeling it, still lovʻin bikes ;)
 

jordandogtown

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That's the plan. I'm going to be using a similar design of the SOHC - two parallel tubes running from just under the tank all the way to the back. Design-wise it should be pretty simple without having to worry about shock mounts
 

jordandogtown

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Popeye SXM said:
I know that feeling very well, I started racing MX at aged 6. I am well in to my 40,s and feeling it, still lovʻin bikes ;)
If that's the price we have to pay, so be it. I wouldn't trade it for the world
 

enterlance

New Member
Go for the CR carbs! I have a 80 CB750F and just bought CR31 from Japan as a Christmas gift for myself. Took them a month to ship and $720+shipping, but man does it look beautiful.
 

jordandogtown

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enterlance said:
Go for the CR carbs! I have a 80 CB750F and just bought CR31 from Japan as a Christmas gift for myself. Took them a month to ship and $720+shipping, but man does it look beautiful.
That's my plan exactly! I got tired of the CV carbs real quick. Have you had a chance to ride with them yet?
 

jordandogtown

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I took a few minutes to work on a (very) rough mockup for the subframe rails. I'll be attaching them in the same manner as the stock, and adding in a few more gussets, so I think they'll be plenty strong enough. Tubing is 1" x 0.125" - same diameter as the frame but a lot thicker. I have a tendency to overbuild, but the rails will be hidden by the seat anyway.

I want to give the bike an extremely slight "stinkbug" stance, with the rear just barely higher than the front. Right now, the bottom frame rail is 2.5o in relation to the table top. This mock up has the new sub frame at the same angle. I'm hoping if I get the frame in the right places, I can adjust the suspension to fine tune the stance



I had to get a sense of the bike with the wheel and seat in place, although this is a pretty pitiful attempt. The bottom of the seat pan will sit on top of the subframe rails, so it's about 1.25" higher than it should be, and needs to be trimmed at the white line, and who knows if the wheel is where it should be - but it was fun and definitely got my wheels turning



Note: a bike on a table is so much better than a bike on the floor. I've been missing out
 

The Jimbonaut

Well-Known Member
I know nothing about modifying frame geometry, but if you can keep that tank position on the frame how it looks in that second photo then you're right on the money man. Love the lines - like you say, drop the seat rails an inch or two and you're in business.
 

adventurco

Nick Ol' Eye
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I think the "seat" portion of the pan needs to be aligned with the far rear point of the tank. It looks awkwardly high up at the moment, even with the 1.25" drop that needs to happen. Maybe even a bit lower, so then your actual butt pad will be level with that angle at the rear of the tank. Just my .02
 

mitchellsk

Member
Looking good. I would drop the pic with the tank into Photoshop and then move the seat around til it looks right.

Even if you print the pic on a printer and cut out the seat pic you can do the same thing... move the seat up & down.

Sent from my XT1635-01 using DO THE TON mobile app
 

jordandogtown

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I agree with all of you, and thanks for the input.

The seat is definitely way too high, but I didn't want to cut into the fiberglass until I knew exactly where I needed to start trimming. The seat pan should be even with the bottom rear "point" of the tank, and the 0.5" cushion will be the only thing extending up into the lines of the tank (I forgot how uncomfortable the seat will be until I typed it out :-[). I knew the mockup wasn't close but couldn't resist.
 

jordandogtown

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The Jimbonaut said:
I know nothing about modifying frame geometry, but if you can keep that tank position on the frame how it looks in that second photo then you're right on the money man. Love the lines - like you say, drop the seat rails an inch or two and you're in business.
Thanks man. Unlike some I love the bottom line of the tank on the DOHCs, so I want to show it off instead of try and hide it. The front is moved forward and down about an inch, with the rear raised about 1.5". I think if I can get the bottom curve to flow into the frame rail coming down it'll tie everything together pretty well
 

The Jimbonaut

Well-Known Member
I'm with you 100% on these tanks. I switched out the F tank for a K on the 750 I'm working on at the moment - it's a bit boxier than the F - but I really dig their shape. There's a guy on DTT that's got a great build going on (the Frankenhonda) and he's also got his tank flowing into the tail section. Tough to get right I'm sure.
 

teazer

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Measure the seat height above the table and then from seat height to footpegs to get an idea of how much it has to come down. Allow for seat pad etc of course. At the moment it appears to be about 4 inches too tall, but that could be an optical illusion.
 

teazer

Well-Known Member
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And measure the fork rake to see if it's the same as stock or much altered.
 

jordandogtown

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teazer said:
Measure the seat height above the table and then from seat height to footpegs to get an idea of how much it has to come down. Allow for seat pad etc of course. At the moment it appears to be about 4 inches too tall, but that could be an optical illusion.
Oh it's way too tall in the second picture. I was just using it as a "close one eye and stand way over there" mockup than anything concrete. Just to get the general idea.

The sub frame will actually be the same height as the cross bar that holds up the passenger seat on the stock sub frame. So the seat will only be a couple inches higher in the middle, and the same height out back. I've got a set of F3 rearsets I'm going to use that will also be about two inches higher and a little further back, so the riding position shouldn't be too terrible
 

enterlance

New Member
jordandogtown said:
That's my plan exactly! I got tired of the CV carbs real quick. Have you had a chance to ride with them yet?
Currently just assembling my bike. I would let you know how do they do once I got a chance. I bought 4 velocity stack filters instead of pod filters to keep the unfiltered raw racing look.
 

jordandogtown

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Right on, I like the way you think. That's my plan as well. This bike will be such a fair weather/sunday driver that I think the velocity stacks will be fine
 

VintageMBike

New Member
BTW dont get your CR carbs from DCC, get them from Dynomat. They jet them as best they can to your setup, and they are about $200 cheaper.
 

jordandogtown

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VintageMBike said:
BTW dont get your CR carbs from DCC, get them from Dynomat. They jet them as best they can to your setup, and they are about $200 cheaper.
Thanks for the heads up, I hadn't really shopped around much yet.

Do they come with the same plastic velocity stacks that are included from DCC? Ready to bolt on (other than any jetting changes)?
 

VintageMBike

New Member
jordandogtown said:
Thanks for the heads up, I hadn't really shopped around much yet.

Do they come with the same plastic velocity stacks that are included from DCC? Ready to bolt on (other than any jetting changes)?
Yeah.

The guy over there, Buzz, asked me what my setup was. They jetted the carbs and sent them to me. Slapped them on and had zero problems. They know what they are doing over there!!

Here is a link:

http://dynoman.net/carb/keiCR.html#cr31
 
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