79 CB750K RCB Clone-ish. On the road.

Re: 79 CB750K RCB Clone-ish. Engine done!

It's no wonder I'd not heard of or seen the bad ass RCB as only 10 were ever made.

Question, the RCB ran CV carbs with stacks. Isn't a cv carb a cv carb? Why then, so much grief over ditching the airbox and going with pod filters??? Carpy emailed me once that proper jetting is all that is required for the dohc to run well without the air box...
Re: 79 CB750K RCB Clone-ish. Engine done!

flatcurve said:
I got the Hagon's an inch over and they look good. I haven't sorted my front forks yet, so I don't know how much of an effect they had on the stance.

You got a picture?

I do plan to build a swinger then a frame in the long run... But that's after I spend $2K for a lathe, $2K for a tig welder and about $600 for an air compressor. Which means in about 5 years time...

Regardless I need a set of carbs for this bike, I rebuilt my set for it and had to canabilise some parts from my other bike due to a few lost bits on my account when I moved. I figured the GSXR would be a good mod for this one when I had to put my CB carbs on my highway bike.

I trimmed my fairing today, I will try to get a picture of it sitting on the bike up later.
Re: 79 CB750K RCB Clone-ish. Engine done!

That's overly simplistic, but partially true. Jetting is certainly a factor but vacuum offset is the bigger issue in running CV carbs 'open'. A CV carb is built to kinda smooth out the throttle response of the engine and a perhaps less than prudent throttle wrist. Change over to a mechanical carb of the equivalent flow and jetting and you'll note that the response is far crisper, more 'right now' as opposed to a CV carb 'catching up' if that makes sense.
As far as the RCB and other race engines are concerned, I'd bet that there are several factors at work:
1) Likely the engine has a higher static compression, the inverse of which *should be* a higher vac. signal to the carb.
2) I'm certain the cams are much more aggressive ie; it's likely a heavy breather.
3) I guarantee that rack of carbs is far FAR from a regular set of CVs. I've seen several external vacuum reference setups on race bikes, I've seen external piston offset pumps used too....it's only air after all.

I imagine that part of the reason for their use on some bikes is to ease the violence upon cracking the throttle. I run a set of 39FCRs on my Triumph and the difference is night and day.
Previously it was a fast but civil bike to ride, after it was a slavering brute looking to kill me if I don't give it respect and pay attention. Crack the throttle in the first three gears and don't....the fucker will leap right out from under you.

but I like'm like I like my women.....crazy and dangerous. Always a better ride
Re: 79 CB750K RCB Clone-ish. Engine done!

Nice! My zx9r was like that. I swear the bike had attitude and personality, if that is possible for an inanimate object. But yeah, the search goes on for a rack of GSXR carbs!
Re: 79 CB750K RCB Clone-ish. Engine done!

My shadow is just like a woman, bitchy and dont want to start to save her soul but if your lucky enough to get the choke and throttle just right without flooding it hang on!

So, with the fairing trimmed to fit:

Re: 79 CB750K RCB Clone-ish. Engine done!

Garage Rat said:
You got a picture?

I do plan to build a swinger then a frame in the long run... But that's after I spend $2K for a lathe, $2K for a tig welder and about $600 for an air compressor. Which means in about 5 years time...

Regardless I need a set of carbs for this bike, I rebuilt my set for it and had to canabilise some parts from my other bike due to a few lost bits on my account when I moved. I figured the GSXR would be a good mod for this one when I had to put my CB carbs on my highway bike.

I trimmed my fairing today, I will try to get a picture of it sitting on the bike up later.

Not with the shocks installed. I test fitted them when I was tearing down the bike to see how they'd look. It's bare frame right now. Next time I'm working on it if it's not too much bother I'll see if I can rig them up again.

If you're running an airbox, I've got two complete sets of CV carbs for the 79 and a box of random 1st gen DOHC carb parts. One was recently rebuilt, but will probably need to be cleaned again as it's been sitting off the bike for two years. The other was pulled off of a wrecked bike. Never ran that rack, but I'm pretty sure none of the slides were scuffed up. Holds gas too. So I'm sure they're serviceable. If you're interested, let me know. I'll give you a good price on either of the racks, and if you want the box it's yours for postage.
Re: 79 CB750K RCB Clone-ish. Engine done!

I don't want to run the air box. I spent too much time wiring and relocating everything. I know of a mod that uses a pop can cut in to quarters to block off the intake of the carb that goes under an air filter.
Re: 79 CB750K RCB Clone-ish. Engine done!

biker_reject said:
It's no wonder I'd not heard of or seen the bad ass RCB as only 10 were ever made.

Question, the RCB ran CV carbs with stacks. Isn't a cv carb a cv carb? Why then, so much grief over ditching the airbox and going with pod filters??? Carpy emailed me once that proper jetting is all that is required for the dohc to run well without the air box...

Is any carb the same as all others of the same design? Is a CR special the same as a rinky little carb on my lawn mower? No. there can be huge differences between similar looking carbs in terms of jetting, air bleeds, air flow etc.

Carpy is only half right about jetting. In principle it's just a matter of jetting, but DOHC carbs happen to be very hard to jet correctly and appear to need all manner of changes to the air circuits to avoid really bad A:F ratios ate certain parts of teh rev range.

GSXR flatside CVs reportedly are a magic fix for DOC performance motors and they are easier to jet - so I hear.

RCB had a very different motor to the DOHC street motor. Honda used modified DOHC motors in the 1980 RCB because the change of rules made them change from prototype to production based engines which is why there are so many RCB replicas based around DOHC motors. And that's perfectly OK.

As for jetting the RCB, I'm going to make a wild guess that Honda has more resources available to get that right than the rest of us combined. Just because they managed to get them right doesn't guarantee that we will have the same success.

Re: 79 CB750K RCB Clone-ish GOT MY RCB FAIRING!!!

You're asking for pics of MY shit? On YOUR own thread? Sounds like a sure-fire hijack, but... Sure thing. I shouldn't shoot my mouth off if I'm not ready to show the goods! And I'm stuck on a freakin' greyhound bus right now (finally, they have wi-fi) so what else have I got to do, but distract myself from the fact that if I'd kept my bike on the road instead of tearing it down to "re-build" so many years ago (well. really it was because of my messed up back and all of the pain-killers, I didn't want to kill somebody ... else, and no way in hell I was gonna part with the bike...) if I had kept the bike rolling, I'd have saved enough on greyhound tickets over these past years to buy just about any classic bike I could dream of - or trick out my CB750F with all of the bells and whistles, 1123cc overbore, Ducati sport-classic wheels, Dyna, micro-squirt, yadda yadda yadda ....

But here's the thing, I don't have a recent picture, but I do have some from ... last (?) summer. Mocking up the fenders, signal lights, seat, plus the rebuilt and polished front end, etc. So, without further ado, lemme try to link something here from my photobucket.

Last summer, or the one before (?)

Here's a view or two of the 'F tank while it was getting banged into shape.


Here's something of the controls with a drag bar I had set up. Note the cut down and tapped fork caps with valves from Princess Auto. Then a pic of an inverted superbike bar, the "clubman on the cheap" I recall seeing on my father's and his pals' CB's when I was a kid. A good pal sent me a pair of clubmans which don't fit the DOHC triple, but I've cut them in half, and intend to braze a tube over the middle section between the clamps. And by the way. The VF750F Sabre had adjustable bars which, if you picked up two sets, would make a great set of adjustable tarozzi style clubmans for almost nothing in comparison to a real set. I also like the idea of "one inch"nominal copper pipe acute angle (135 degree bend) elbow fittings. Not on the shelf in your local home depot, but they're out there. I'd like to braze up a set from pipe fittings and chrome 'em. But, I'm using the pair that were a gift. Because ... well that's the kinda shit that says "I get you" ... I don't know about you guys, but once again nobody left any bike parts under the Christmas tree for me. Christmas 2012? THEY'RE getting bike parts from ME. Ahem. Pics of the top clamp:




I know you might be thinking "Why not just start with a 'C model in the first place?" Here's the KZ fenders I fitted up to the oem fender mounts. I think they look more like an old 'K. The wide KZ "ltd" type fender will look tight on a fat 160/60 tire.


This pic is several years old, but it's got the bobbed 'F seat cowl which I made entirely from the stock cowl and a bit of ABS cement, with some judicious cutting. Paint was just for mock-up, of course I put filler on it ... right before tossing it on the "discard" pile. Still, interesting to know you can modify these cowls with ABS cement from the plumbing aisle! I picked up a half sheet of 1/8" ABS for ten bucks, can be heat bent, glued, etc. If you look at scale model building sites, (Not MY kind of hobby, but useful fabrication perspectives from their camp.) Want another cowl for your stock seat? Use the mounting area from the stock cowl, and just ... play with the plastic!


Now, let us draw some comparison of front wheels, my new wire wheels vs the comstars.


The wire rims have been found lacking as fitted, especially the goldwing. But, I think that in the pics here I've laced up a steel 18" rim on the goldwing hub, almost as wide as the blank alloy one I have (which I carved out of the rear comstar) steel one is a 2.15" where the commie alloy would be a 2.5". So, this is a mock up only, hence the loose spokes. (And, a note about spokes: Buchanan's lists cutting & threading your old spokes for a smaller rim @ about twenty bucks. Good thing these are all stainless. I've already polished the slight tarnish off of them all in a drill chuck using fine steel wool & 600 grit. Just saying, you don't HAVE TO spend a hundred and fifty bucks per wheel on new spokes!) If I were hung up about weight, well then the GL/KZ wheel-set would have fit a set of wearable tires I had on another set of comstars just nicely. But succumbing to peer pressure - "I put eighteen inch wheels on the motha-fucka - so when I roll around this town it looks like I hova hova hova....."

http://www.youtube.com/watch?v=CJQU22Ttpwc (2min+50sec)

First, '77 GL1000 fr hub with rim off of the rear '77 KZ750B (aka KZ1000 hub), which you see in the second photo. Both hubs have now been severely polished, (KZ hub surface was like 40 grit sandpaper, now is almost like glass ... getting there!) and the rotors have been half-way drilled. BUT, I'm right now sourcing a set of CBR900RR rotors for the front, and the rear KZ hub fits the front rotor off of the '82 CB750F. And yet, I like the rotors on the CB650 special from last month's "Motorcycle Classics". Made and owned by Derek Pauletto of Calgary's Trillion Industries, it incorporates the idea which I'd lusted after but never thought would fly, that of cutting out the center bolt pattern of the old rotor then drilling a new bolt pattern a half inch out from there, and bolting a newer, smaller rotor onto that. NO IDEA how to cut the damn thing, right now I'm going through cobalt drill bits like cigarettes just drilling the first few dozen of 252 1/4" holes in each '77 GL rotor....
That rear hub is getting a 160/60 slung around an 18" by 4.50" steel rim I found cheap on fleabay ($80), or -IF- I get the balls to do it, the front hub gets the old comstar rim with holes drilled in it, and the rear gets an alloy rim cut out of an '89 kawi which is also a 4.50" and seems to have not too thick of tri-spokes to carve out. If I feel like throwing all caution and good sense to the wind. Incidentally, the steel rim is cheaper, and probably lighter due to the thickness of the alloy in a cast rim. (If you were to look at the stock 19" rim off the gold-wing, it would remind you of disposable aluminum cook-ware! I'd feel safer with some more meat on it, even if it does wind up heavier.) And speaking of weight. The comstar rear hub weighs as much as the whole KZ wheel. KZ hub is half steel, but still lighter. I don't think I need that big of a cush-drive, but I like the conical hub. I'd like to cut holes in the inner face of it, and find some bright varithane skateboard wheel material and cut new cush rubbers out of that. Flourescent blue or shit like that. Lord knows, this hub could still shed a few pounds!

Enough "ado about nothing". Here are the wheels.


(((Which reminds me. Does anybody need a 19" by 1.85" alloy front rim, or an inner tube for a 19" rim of that size? I could swing you a deal!)))

Here's some seat cover inspiration courtesy of my '82 C70 passport.

Seat foam will be a low flat banana seat in the "Wrenchmonkeys" style, if I can afford them I'll stuff it with those blue Dr Scholl's gel insoles. Love 'em under my feet. Under one's arse would be ... just that much better. OR, and I hesitate to first clarify that my first "wife" worked in a marital aids emporium the summer she turned eighteen, which was where I learned of this material, okay? But ... how about that "Cyber-Skin" stuff? One could buy a really big ... dong, or something. Maybe a great big fat ASS made out of the stuff, one big enough to park one's big fat ass on top of? I've heard of silicone butt-cheek pads for skinny old men with no ass cheeks left, for sticking in the back pockets of your jeans so as to forestall onset of "decubitus ulcer" (DON'T look that one up, trust me!) Yoga matts have good foam. But I want my seat to make waves like a water-bed every time the bike moves. Seriously though, I like the LOOK of the "wrenchmonkeys" seats, but I'm sure they ride like ... ass.

The seat pan I moulded from bondo. (I dicked around with cutting up the old one. But they're "delrin" aka glass-filled nylon. Can't be glued 'cause of the nylon, can't plasti-weld them 'cause the filler makes a weak spot in the weld and it will crack pretty soon down the line. I wanna "duck-tail" style seat cover, like the '69 "sand-cast" had. "Honda" logo on the back of the cover, all of the OEM details. Then I want a rack with loops for saddle-bags, in the old chromed round tubing style of course.)

The spare top clamp cut out to make the brazed together alloy clip-ons which held up beautifully on the cheapo "welds" but which cracked in the casting when I "tested" one. Going with clubmans which also have to be brazed at the right width, but the clip-ons were a good project, and practice for the rear-sets which look a heck of a lot better than the "lattice" peg plates on the bike in the pics, (not completed at the time, assembled for a mock-up only, and found to be wanting in the peg position department, and promptly became scrap for the new set.) The bastardized clamp, (shudder):

I've gotta get some newer pics uploaded. And once I've spent on some fleabay bits this summer, a semi-final assembly stage will lead to a build thread (of my OWN ha ha.)

As you can see, nowhere near the state of completion you've got going on. And now that I am garage-less once again, the project is out in the elements....

But hey, thanks for asking for a peek. It helps me to take stock, if nothing else! Especially since these pics of my bike are so old, and yet the bike looked better then than it does right now! Ha ha. I'll update my photobucket right away and find some of what I've been tinkering with more recently.

Some great RCB clone build threads on my home forum, www.CB1100F.net these days. Check 'em out! And yeah, don't go on what Steve Carpenter says about DOHC Honda CV carbs. This issue has been handled in GREAT depth over on the CB1100F-orum. Dyna results showing flat spots, etc. MANY fueling options tried over there, after all it's ALL DOHC Supersport/Bol D'Or/Super Bol D'Or over there. Apparently, they don't want pics of 'K models in most forums, but somehow they tolerate a heck of a lot of CB1100R content. The custom forum is an offshoot of the 1100 one, but to me personally the 'K should have been lumped in with the supersports, heck they should have kept in all in one place and if Glenn had the whole thing to do over today he'd probably have kept them all under one roof and set up the off-shoot for all of the "Franken-Bike" builds proliferating over there in recent years. I've discussed it with him, and the cafe "issue" raised there as well, and Glenn (the site's sole creator/moderator/proprietor) is firmly in the "It's okay to be cafe" camp. SO, if you can somehow ignore the site's (other) trolls, you'll find a very ... "cafe positive" environment, lumped in with an enormous pile of DOHC information. Just not a whole lot of ... "tastefully" styled bikes. Not to this 'F-er.

Re: 79 CB750K RCB Clone-ish. Engine done!

As long as you have several mounting locations you could use expanding rubber sleeve nuts. They're used to mount a lot of bike screens, don't see why a handful wouldn't do the trick and help fight cracking and fatigue around the mounting points....
Re: 79 CB750K RCB Clone-ish. Engine done!

Have you made the mounting frame yet or are you planning to make it out of 1/4" flat strap? That is really heavy. We use 1/8" alloy and save a ton of weight or make up a tubular mount which is also fairly light.

You could also copy the design ideas in a CB1100R fairing mount.
Re: 79 CB750K RCB Clone-ish. Engine done!

So as far as the brackets for the fairing are concerned, I made them off the tank. It seemed like a good idea. I used some U bolts and welded them to the sides then made some brackets off there. I also have a bracket that clamps to my down tubes. Its plenty sturdy!


Now more of the tank stretching.



Traced the right skin over the left to adjust for the taper of the tank:


Trimmed piece beside the weld. I had to make some releafe cuts so I could bend it up and better match the lines of the tank. I still have more fiddling to do with it.


Re: 79 CB750K RCB Clone-ish. Stretttccchhh the tank.

So, while drilling some holes for my wind screen, it cracked... I need to stop drill it now and just leave it. I cant afford to try again when it comes from Germany...


To hold the screen on I am re using some screws and rubber grommet nuts I took from a scrap sled hood for the same purpose. It works great, I just need new hardware.

Better picture of my mounting hardware:


Yes, the screws will be counter sunk in the end.
Re: 79 CB750K RCB Clone-ish. Stretttccchhh the tank.

I'm going to try to trade my Redwing Hammerheads for an old hack frame.

The plan is to put an old racing sled hood on, but which one?



In the next few weeks, I will also be trying to finish my seat and mount my Hella lights. I've got all winter and next to no dollars any more...
Re: 79 CB750K RCB Clone-ish. Stretttccchhh the tank.

I'd re-think that fairing bracket. When you drop the bike or it slips when it's being parked, there is a high possibility that the bracket will tear a chunk out of the tank. I like the idea from a shape and angle perspective, but not so sure about the crashworthiness.

Get a drill bit from Gustaffsons. They sell a special drill with a different cutting point to normal drill bits. If the crack is small, just drill the end of the crack with a 1/16" drill bit. We do that on racers to save the screen for a few more weeks/months of action.
Re: 79 CB750K RCB Clone-ish. Stretttccchhh the tank.

I have a complete dual headlight assemble for a 86 GSXR, ill make ya a good deal if ya need it. love the way this bike is turning out.
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