1992 CB750 Scrambler Build (first time builder)

[Mcbuckets13]

I started pulling my bike apart about three months ago and haven't had time to get back to it with school and work. I have a 92' CB750 Nighthawk that I was given for free (can't beat that). Bike was pristine with 50k miles on the clock having taken a few trips to Arizona by my uncle. I rode it all summer of 2016 but have always abhorred the asthetics of it. Looking to make it into a bike I'd be proud of, I started looking at other builds and love the Scrambler style. I have started gathering parts and now need more time!

I have the Mac 4 into 1 pipes, a motocross bar with a 2 inch rise, and a k-series tank (which is awesome) but bot much else. I've already pretty much chopped the frame where I want (I won't be using a hoop).

Next tasks include welding the frame, making the seat pan, and tucking all the wires. However, I am having a problem figuring out what the best way is to run the air into these carbs. Originally (and without much research) I grabbed four Emgo pods before I saw how they restrict airflow into the outside diameter of the carb intakes and am now in search of a better solution. I am currently thinking about the K and N filters or a two carb setup using some Mikunis that I saw on here but that new carb setup is a little pricey. I need more ideas. I also need to work the new gas tank to fit just right over the frame backbone. It currently sits atop it and it bothers me. I gotta visit a metal shop to bang a cave into the center of the bottom. I'm also thinking about some superbike forks and am looking into that now too. If not out of budget, I would like to consider that too. Any ideas or suggestions are welcome! I will update this as soon as I get more time!

 

[Mcbuckets13]

I started pulling my bike apart about three months ago and haven't had time to get back to it with school and work. I have a 92' CB750 Nighthawk that I was given for free (can't beat that). Bike was pristine with 50k miles on the clock having taken a few trips to Arizona by my uncle. I rode it all summer of 2016 but have always abhorred the asthetics of it. Looking to make it into a bike I'd be proud of, I started looking at other builds and love the Scrambler style. I have started gathering parts and now need more time!

I have the Mac 4 into 1 pipes, a motocross bar with a 2 inch rise, and a k-series tank (which is awesome) but bot much else. I've already pretty much chopped the frame where I want (I won't be using a hoop).

Next tasks include welding the frame, making the seat pan, and tucking all the wires. However, I am having a problem figuring out what the best way is to run the air into these carbs. Originally (and without much research) I grabbed four Emgo pods before I saw how they restrict airflow into the outside diameter of the carb intakes and am now in search of a better solution. I am currently thinking about the K and N filters or a two carb setup using some Mikunis that I saw on here but that new carb setup is a little pricey. I need more ideas. I also need to work the new gas tank to fit just right over the frame backbone. It currently sits atop it and it bothers me. I gotta visit a metal shop to bang a cave into the center of the bottom. I'm also thinking about some superbike forks and am looking into that now too. If not out of budget, I would like to consider that too. Any ideas or suggestions are welcome! I will update this as soon as I get more time!

 

[AlabamaCB750]

Because 513 pound scramblers are great.

 

[Mcbuckets13]

Because 513 pound scramblers are great.

Lol, indeed. I'm just going for the look. Not really sure what the "point" of a scrambler is or what it is supposed to be, feel, look like, or evoke a sense of and frankly don't care. All is good and fun my friend. Didn't know this bike was that heavy either. Wow, I woulda guessed like 450 or so.

 

[AlabamaCB750]

Lol...I'm just giving you shit for fun! I'll be the first one to tell you it's your bike, and you do exactly what you want to with it. Don't let anyone talk you out of your vision, or to change your mind just to conform to the "norm". Just be aware though, duel sport, or "scrambler" tires are not your friend on the street with a bike like this. But beyond that, dream big and go your own way. Make it your own. And, welcome to the forum!

 

[AlabamaCB750]

Oh, and one more thing, pod filters look cool, but if your new to jetting, you might be better off to stick with your stock air box for a while. Don't ask me how I know...��

 

[Mcbuckets13]

Ha, yeah thanks bud. I figured this lesson out the hard way like the natural knucklehead I am. I'm pretty much balls deep now though. I was thinking about a two carb setup I saw on here or I also just found a nice mod on the Nighthawk forums to use one filter to maintain constant velocities feeding the four carbs. All of this is pretty new to me too. I am sure it is noticeable. Thanks for the advice btw.

 

[AlabamaCB750]

After some good advice on a couple of different forums, I finally got it through my head to work out the asthetics first and not to worry so much about increasing the power right off the bat. Engine tuning is something that is a long hard earned process. Decide your basic layout first, and worry about horsepower later. There is no "magic" design to quickly get what your after. Be original. Ask questions. Make sure the money you spend on parts really makes a difference. Don't jump the gun. You'll thank yourself later!

 

[AlabamaCB750]

Scramblers, by the way, were off road versions off existing street bikes before duel sport bikes existed. Honda, for example, took a CB350 and added beefed up forks and rear shocks, high pipes and semi-knobby tires and made a bike that was passably off road capable. It became the CL350 with these modifications. But even then, at 328 pounds, it was still a handful on anything more than a dirt road.

 

[Mcbuckets13]

Awesome. I never knew that. I figured (with little thought) that it was a just a made up style to emulate the Triumph Scrambler (of which I don't know what looks like) lol. Thanks buddy.

 

[Mcbuckets13]

Update, have been messing with bike as time allows. Tank I am fitting does not fit exactly right. The problem is that I want the bottom line of the tank to flow with the seat rail of the frame and the bottom of the tank hits the top down tube of the frame. I beed roughly a quarter inch more. Thus, only conclusion I have is that I need to chop and weld tank...problem is I don't know how to weld...I grabbed a junk tank from the junkyard (btw I highly recommend Cycle Service of Bear Delaware if you're in the area) and took to welding it as practice. Long story short...if you're gonna weld thin metal like this with a mig...use argon gas and solid steel wire. I first tried with flux core and it burned very quickly through the metal. I have one pic of practice with the flux and one with the gas and solid wire. Next update, I hope to show you finished final tank. And after that fabbed up seat pan. Oh, I also grabbed some of the k&n pods and will let you know how they run with stock jetting and a mac 4 into one exhaust.

 

[Mcbuckets13]

Ok, I abandoned the welding gas tank part of the project. I ended up just taking a ball peen hammer to the underside of the tank and babied the metal in to where it needed to be an 1/8th inch or so to clear the downtube. Am now fabbing up the brackets to hold the tank on. I ground down the old tank brackets and am going to weld an 1/8th inch piece of steel flat bar stock to the old location and the fit the new pins to the bar where I want it. Then am going to weld some 16 gauge plate to the top of the frame where the old rear gas tank bolt went and gonna bolt the new tank to that. As of now I am thinking I'm gonna cut off the old tank bracket that runs across the seat rails and just use a big plate of 16 gauge. I don't think this bracket is structural. Will update soon! The stock plate on the bottom of the tank on the rear actually sits right on top of the seat tubes perfect when I put the tank where I want it, only thing is I have to make grommets that are about a quarter inch taller so that the tank is exactly where I want it.

 

[Mcbuckets13]

Tank is now on and I just got the rear seat rail exactly how I want it. Looks kinda stick huh? I didn't wanna use the hoop as I'll be putting a fender on the back and really wanted the lines to look right...

 

[amc49]

Biggest issue I see there is that there does not seem to be a lot tying the 2 sides of shock input into a box strong enough to contain the load. I would have made that crossbrace at the front that it looks like the seat plate bolts to much bigger and thicker and with some more triangle going down on both sides. It needs to be deeper in the near vertical it is in.

Made even worse on that frame because Honda did not take the frame rails that normally go up to join the horizontals all the way up, instead they pulled them inwards to attach to one tube only and going further forward. Meaning the horizontals now have a longer length to flex in, making it easier to happen. I know why they did that, it makes the steering head to swingarm pivot stronger across that distance but it weakens the actual shock input stiffness area. More than one set of forces to contend with there.

I've seen something similar to that in the past and the shock loads had the two sides of frame moving up and down independently of each other. The bike was impossible to drive on a slightly bumpy road at over 40 mph. How I got it for free.

Just some thoughts and I could be full of crap, yours of course and do what you will................

 

[Mcbuckets13]

Thanks AMC. Definitely good advice and a lot of things that I did not think about. The frame originally did not connect at the back, rather it was just the two seat rails jutting out with the tabs you now see welded together in the back, the only thing back there holding it all together was that horrendous tail section that jutted out about a foot...lol that sucked, had to get rid of it. Other than also cutting off the grab bars and other mounting tabs for various components, I didn't modify the frame. I do see what you are saying with the shock mounts on the triangles and those vertical bars just being mashed on and ten tacked. I am looking at it now and think I might box out the rest of the triangles and that entire area with some 12 or 14 gauge steel. Would definitely be a little stiffer, but I also plan to run a sheet of 16 gauge plate across the top front section anyway for the seat pan, that should add a little more stiffness there too. The rear seat pan area is only going to get a box below it with a pan on top, so not much more stiffness there. Maybe even thinking of running a tube of 12-14 gauge rod between the rails now...any input is appreciated from anybody...after all, a wise man will listen to any knowledge or advice but it is up to him to discern what is right and best huh? Thanks AMC

 

[Chris R]

Any update?

 

[Mcbuckets13]

No update as of now, I started to mock up the seat pan and box out the triangular parts around the spring mounts, but haven't had a lot of time since then. I did just get a garage to finish the project in as I've been using my pop's house for now and he works out of it so...also just changed careers and am out on the road driving trucks for next 30 days. Will update soon, I promise. Looks like I already lost this summer, looking forward to the next...

 

[Grampy]

Hi
how far have you got with this project?

 

[Mcbuckets13]

Still short on time, have to grab a generator to run my welder now. Still out on the road driving trucks. Was in process of building seat pan and boxing out the frame around the rear shocks. Things left to do are seat pan, seat upholstery, lights, battery box, and paint. Also, need to tuck/shorten wiring harness. Oh also, am gonna be making a rear fender and rear wheel water guard.