Topic: A TON of 750 questions from a noob

[kev.rohr]

Hi All, been lurking awhile, first post here.

Some background on me so you know who I am - I was an ASE certified auto technician for 10+ years. so I'm no stranger to a shop. I'm not at all afraid to rebuild a motor (done plenty of them, so yes, I can read a mic and I know what plastiguage is). I understand motorcycle suspension and can rebuild carbs, set points and redesign an EFI system. I've been riding forever, and I've taken apart every bike I've owned.

Anyway,
I just picked up a 1976 CB750 for $375 (not a bad price at all!)



So I'm considering what all I'm going to do to it, or actually, what all I can or should do to it. Definitely gonna be a top-notch cafe racer, showroom top to bottom, but meant to be ridden! I'm thinking I want to keep it old-skool looking, so no USD forks unless it's low-key (no gold anodizing!), but I want to modernize how it runs and handles.

I'll be building a 550 for a friend before I get to my 750, so any input on either model would be much appreciated, all the following questions apply to either bike. These will be my first true cafe racer builds, I want to do them up right.

The more I read, the more I'm learning, and I've been all over this forum - there's so much information, it's almsot overwhelming! I would be forever grateful if some of you guys could give me an overview or a primer maybe on things that are commonly done to these 550's and 750's. Chime in on any or all of the ideas below to add, clarify, correct, inform...

Engine (750):
12,000 miles on the clock, but I don't know yet if the speedo or odo work, so I can't say for sure.
This is what I'm thinking about right now, so lots of questions here. I went through the carbs today and tuned it up. It's purring like a kitten, with lots of throttle response. But these things are "soft" and slow. I'd love to wake that motor up and get it snorting like a more modern bike. 67 HP in a 500lb bike. meh.

There's a lot of blowby, so I'm anticipating going through the top end anyway, and an oil leak somewhere on the bottom, so I'm sure I'll be cracking the case and maybe doing the bottom end while I'm in there. I'm bracing myself for having to punch the cylinders (I'm expecting taper or out of round) which means replacing the pistons. I've read about a big-bore 836 kit? if I'm replacing pistons anyway, it seems to make sense to do this. I was looking at the specs: 9:1 compression ratio, small bore, big stroke (compared to modern sportbikes). What does the 836 kit do to the compresion? Is there a short stroke kit? Long rod or piston kit to bump compression? Or do people shave the head to get higher compression? Do I want higher comperssion? What about the cam?

What do people do to these things to bring them to life? And if I do whatever to it, will the stock kehin carbs keep up, or am I also looking at some CR carbs or something else? Will the clutch hold up? I've read about a short shift kit to bring the gear ratios closer together?

Without spending an absurd amount of money, what's a good formula to bring this motor to life? It doesn't have to be drag race crazy, I just want to breathe some life and fun ride-ability into it.

On to other questions (there's so many of them!)

Suspension:
• Cartridge upgrade? I hear it's essential. Or a fork swap? (non-USD, or at least old-skool looking USD forks?)
• Progressive rate springs? I've read that it's more desireable to keep the linear-rate springs? What's everyone's take on that? Preload adjusters to go with the springs?
• rear shocks/springs? Common swaps?
• Is anything commonly done to that ugly, heavy swingarm with those un-necessarily complicated shock mounts? One could ditch a lot of weight back there. Right now, the rear end sits noticably lower than the front. I'm not even sure the correct rear shocks are on there - they're not long enough. Now that I think about it, the front forks are a mile long - I bet for some reason they're fully extended or really close to it (with full weight on them).
• Wheels? (Gotta keep the spokes, but I've read about different diameter spokes?) If I build it to handle like a modern sportbike, and I'm dragging knee on this thing at 100MPH+, can these old wheels handle the stresses?
• Larger more modern sportbike tires? What fits?
• I've read that the weight distribution is more towards the rear - I think there are some mods to shift this to the front?

• Dual disc front brake conversion?
• Newer calipers/front rotor(s)?
• Anything need to be done to the rear drum brake? (besides obviously going through it) Is there a disc brake conversion?
• mechanical tachometer - is there a conversion to electronic?
• Points to electronic ignition?
• Alternator - I've heard these are weak and don't keep up. What upgrades/mods are out there?

And I keep seeing gas tanks with this more rounded profile:




What are these tanks from? I've looked at a bunch of different years and models of CBs, but I can't pinpoint anythign specific?

I see a lot of people cutting knee recesses into the tanks to modernize the look, but it doesn't look like my tank has a shape which would allow that - it's very square-ish, and if I were to cut the tank profile in towards the rear, it introduces a clearance problem with the stock kehin carbs. Any good ideas/common swaps? to get a tank with a bit more shape to it?

I know this is one hell of a first post, and I'm sure I'll figure a lot of this out as I go along, but I'd appreciate any tips and guidance.

TIA all!

[Retro Rocket]

Welcome Kev.  The best way to get more power from these old bikes is in the heads. The heads are quite restrictive stock and wake up with good porting. I think IMO that the Mikuni RS34's are a better carb for the street and are about the same price as CR's , they have accelerator pumps {CR's don't} and are a more tractable set up and  can be bought to fit our bikes. The 836 kits are 10-5 to 1 so they do raise compression. There is a vast amount of hot up parts for our bikes and this forum has Big Jay {APE engineering} Mike Rieck {head and porting expert...Maybe the best there is} and countless other experts and is definitely the right place to come...Those tanks are from Benji's cafe racers, there have been good and bad reports on the quality of his tanks, some are built using the base of the original tank {steel} and the rest shaped in fibre glass. I am not a big fan of this type of construction.  Anyway welcome and good luck with your bike and remember....we like pictures.....

[brandEn]

Welcome! Looks like you have a good platform to start with. Those tanks are from Benjis Cafe Racer and most of them are fiberglass. I went 836 on my build and I am more than satisfied with the output. Most of the experts with these bikes will tell you if you want to get all the HP possible you need to let it breath better by putting on big carbs, a free flowing exhaust, a good ignition and a good valve/port job. I am looking forward to see what you put together!

FYI Cycle Exchange, A.P.E., and Dynoman Performance are some of the best online retailers for high performance engine parts.
 
edit... Retro Rocket beat me too it

[oldk6guy]

all of your questions can be answered here http://cyclexchange.net/

[Retro Rocket]

all of your questions can be answered here http://cyclexchange.net/

No, all questions can be answered here, most parts can be bought from Cycle X, but not all.....

[kev.rohr]

Howdy!

Burn the seat. 

Already in the trash!

And don't throw away the floorboards! They are actually sought after by some folks.

Really!? I never would've guessed! There's a heel-toe shifter and brake pedal to go with them. Glad you mentioned it, they would've ended up right next to the seat!

@ the others:
I like the sound of 10.5:1 with the 836 kit! Assuming I'll have to punch the holes anyway, this sounds like the way to go. Thanks, I'll hit up some of the head porting threads and see what I can find, and have a look around that other website.

[Logan80]

Really!? I never would've guessed! There's a heel-toe shifter and brake pedal to go with them. Glad you mentioned it, they would've ended up right next to the seat!

I'd love to find a set of floorboards like that for my bike. Never looked for them so not sure how hard they are to find, but I bet they are comfy for distance riding!

If you decide to get rid of them free, or real cheap, let me know, maybe we can work something out...

That bike will be a fun project, mine was in similar shape when I got it, with 44k miles on it and sitting for 15yrs. I've discovered a lot of little problems, and a few big ones in the mean time, I managed to get it to fire up just to prove to myself that it ran before I put any real money into it. Now it's in 3 boxes and on my garage floor. Herniated disc in my back put a quick stop to all my projects. Surgery was 4 days ago, hopefully I can get back into action soon!

[Hinomaru]

Welcome to the forums.

Consider this reference book for your CB750 project, it has tons of useful information: 

http://sohc4shop.com/catalog/product_info.php?products_id=29&osCsid=5b662f8b99053c182b9ac944634b8ae4

[754]

 If you narrow the rear of tank, carb linkage will hit. Changing to 77/78 PD carbs, will get rid of clearance (to tank bottom) problems

[GammaFlat]

The K6 was arguably one of the slower of the CB750s.  Out of the box, it had something like high 40's / low 50's in terms of horsepower.  If you endeavor the performance of a late model sport bike... well, you're in trouble.  You're not going to get drive-ability, cornering, 10 second quarter mile times and reliability as you would with a new R1 no matter what you do to one of these old bikes. 

If you care about drive-ability and reliability, I think you're possibly closing in on 100 horsepower.  Talk to MReick or HondaMan about this. 

There are certain thresholds.. that once crossed, cause you to need to do "more still".  As an example, once you get around 75-85 HP, you'll be thinking about connecting rods.  mmmmm, maybe we should start thinking about working on the crank.  Getting near 100, you're probably spinning some big numbers (RPM), so we ought think about some titanium parts.  You get the point.  Depending on what you want to spend, you can get some substantial (for these bikes) horsepower. 

My K6
I'm going to be happy with 75-ish.  I've got 1 over (750cc as opposed to 736cc) pistons.  I'm not doing anything exotic on the top end.  Just porting and chamber work.  I'll also be running an "F0" cam which has a bit more lift than my factory stick.  I think I'll end up with a bike that has a completely different complexion than a factory K6 and it will be "spirited" but will probably fall short of what you want.  On the other hand, I'll likely be spending considerably less than your project. 

[oldk6guy]

Going off of your list of questions i really think you should check out cycleX.  they have a rear disc conversion,  they have sprockets that will move your chain over to allow for larger tires,  you can lace an old harley 40 spoke rim to your hub in the rear which you can fit a larger tire on.  you can take the entire front off of a gl1000 goldwing which will give you slightly larger front forks. 37mm vs. stock 35mm. this will also give you twin discs up front and the larger master cylinder.  you can also make alot more power with some of the other piston kits, 836 is ok but there are 850 and 915 kits with 10.5 or 12:1 compression.  i believe the cycleX cams are drop in type, you may want to call them to verify this.  they also sell a high output charging system.  once you break in to this 35 year old engine be prepared to replace almost everything, you can't do this on the cheap.  valves, guides, head studs, gaskets, machine shop work, cam, bigger carbs, this all adds up very quickly.

[Retro Rocket]

you can lace an old harley 40 spoke rim to your hub in the rear which you can fit a larger tire on.

They are 16 inch, not what you want if you are trying to get a good handling bike.....

[kev.rohr]

Thanks, all

@k5streetfighter: I agree, I'm seeing too many $$$ already, but that's the nature of the beast. I'll give those guys a call and talk to some other builders too, before I get into it, to figure out a good balance of dollars vs. performance for me.

@GammaFlat: Maybe I overstated myself when I talked about dragging knee at 100mph   I don't expect performance of a modern sportbike, that's what my Kawi ninja is for! But I want to get it moving better, for a more enjoyable ride. I'm very familiar with the thresholds to which you refer - I've crossed them many times.

@754: A saw that too. I'm not too fond of the stock tank's shape. It's sounding like I might be going with different carbs, so that'll be a non-issue (I hope!)

@Hinomaru: Looks like a good book, but wow, $85!

@Logan80: I might throw these up on ebay, just to see if they'll fetch anything. If I find they're not worth selling, they're yours. Make me an offer if you like, they're worth very little to me. The only reason they're not off the bike already is 'cause I don't have any rearsets for it. Good luck with recovery, hope you can get back building soon.

At this point, I'm just poking around at my options. I have plenty of time on this; I'm building my friend's 550 this winter, so my 750 will wait until after that. I told him his 550 was "practice" lol!

Are the 550's the same way as with the 750's, open up the heads to wake it up?

Here's a video of it running, after going through the carbs. Can anyone tell me if this amount of blowby is normal?



To me, it'a an indication of worn compression rings/cylinders. (But I don't know these motors that well!) It doesn't burn oil or blow any smoke, and seems to run strong, so I'm betting the valve guides, seats and valvetrain are good. If I don't go balls-out, I'm hoping I can get away with new pistons/sleeves, a cam (timing chain of course) and flowing the heads, hopefully coming in at less than $1000.

[BobbyR]

Kevin, you may want to check in with Mike Reick, MREICK. I think he had one was proven 100 Hp on a dyno.  You will also have to do some frame stiffening. 

[kev.rohr]

Thanks all for the tips - I'll get in touch with those members when the time comes to tear into the motor.

UPDATE: Build thread here:
http://forums.sohc4.net/index.php?topic=92265.0

[bender01]

That blowby looks like moisture in the engine. The few Ive gotten back on the road after sitting all did this. Youill need to run it awhile till it goes away. The airbox will stop that raspiness but you probably know that. That seems like a great running engine to me!
 Welcome

[Spanner 1]

+1 about the blowby.... don't worry about it . Just ride the bike 100 miles and then see what's coming out of the breather tube ( I bet no smoke ).... be sure the 3k maintenance items are all done first.... good luck.

[johnrdupree]

• Is anything commonly done to that ugly, heavy swingarm with those un-necessarily complicated shock mounts? One could ditch a lot of weight back there. Right now, the rear end sits noticably lower than the front. I'm not even sure the correct rear shocks are on there - they're not long enough. Now that I think about it, the front forks are a mile long - I bet for some reason they're fully extended or really close to it (with full weight on them).

The rear sits lower than the front because the PO put lowering blocks on the swingarm (the chrome thing the bottom of the shock is bolted to).  Ditch that and the rear of the bike will come up.  Since the PO was trying to chopper-ize that poor CB, you may want to check to see if the fork tubes are original, there's a chance it has extended tubes.

~john

[gnarlycharlie4u]

you want to wake this bike up?

talk to MReick about head porting. that should be #1 on your list if you're really trying to inject some performance into the bike. that way later mods can be used to their fullest potential.
then get some later model carbs with the accelerator pump.

Finally if you feel that's not enough, get an oversize piston kit and matching cam.

all the other stuff like lightened alternator rotor, crankshaft, rods, etc is totally up to you. they have some advantages but they also have their drawbacks and aren't for everyone.
I would recommend some Carrillo rods if you're going with an overbore piston kit though.

[kev.rohr]

The rear sits lower than the front because the PO put lowering blocks on the swingarm (the chrome thing the bottom of the shock is bolted to).  Ditch that and the rear of the bike will come up.  Since the PO was trying to chopper-ize that poor CB, you may want to check to see if the fork tubes are original, there's a chance it has extended tubes.

~john

Those aren't stock, huh? (I have a lot to learn!) Funny I was looking at them saying to myself that if the shock were bolted up without those, it would raise the rear!

you want to wake this bike up?

talk to MReick about head porting. that should be #1 on your list if you're really trying to inject some performance into the bike. that way later mods can be used to their fullest potential.
then get some later model carbs with the accelerator pump.

Finally if you feel that's not enough, get an oversize piston kit and matching cam.

all the other stuff like lightened alternator rotor, crankshaft, rods, etc is totally up to you. they have some advantages but they also have their drawbacks and aren't for everyone.
I would recommend some Carrillo rods if you're going with an overbore piston kit though.

Thanks, I'll get in touch when the time comes to tear into the motor.

And on the blowby - thanks, it makes complete sense. Maybe I don't need to replace pistons after all. During the test ride, I noted that the speedometer works, so it really does only have 12,000 miles on it! Well, that leaves me in a quandry as I was all psyched up to bump the displacement and compression and throw a bigger cam at it and make it into a snorting hotrod! Meh, maybe I'll do it anyway, just cause I can.