Topic: My CB750 Drag/Cafe Project Bike

[moshjack]

I originally posted this on advrider.com, but I figured you guys over here may have quite a bit more information to help me out  


I'll be using this thread as a way to document my legalization of the most recent acquisition of my motorcycle collection. I'm in my early 20's so everything is being done on a budget with this one. I've acquired boxes of random spares with this bike, and have already begun buying the necesary electrical goodies. Ok, so here it goes:

The bike:



Frame: 1978 CB750K
Engine: 1969 CB750(not sure which letter)
History:

This was the last incarnation of a drag bike raced by my previous service manager at my last motorcycle shop. He had a couple of these but this was his fastest. As it stands today, it is a gutted out CB with 1250cc (supposedly) motor and high comp pistons. Needless to say, I've already sprained my ankle kickstarting the thing! It was stripped down and built up from scratch to be as fast as he could make it. Aftermarket swingarm, no rear shocks, he even gave me the wheelie bar. There is no battery, no wiring harness, no lights, only an ARD Magneto firing the plugs; and a kickstart to sprain your ankle on.
The exhaust system was custom made (as far as I know for the bike in 1979 and is a 4 individual pipe megaphone system with ZERO baffling, pure straight pipe (hurts your neighbor's ears at idle.)
The carbs are aftermarket CR Smoothbores that Dave (previous owner) took great pride in.
The bike basically sat covered up in the shop for 25 years. It has NO corrosion, the paint is mint, and the thing fired up in one hour of fiddling.

Yes, it is a race bike, it even still has the slick on the back.

The Plan:
Make it street legal! I have 2 major plans in mind. Plan A involves a complete teardown to the frame, painting everything White/Black/Red and putting a cafe fairing and rear shocks on it. Plan B involves leaving it vintage style and just cleaning it up real nice and maybe even leave the solid mount rear.

Both plans require lights to be added on. This bike had the stator and flywheel removed as well as the entire harness and battery, and I've already purchased a stator, flywheel, regulator, rectifier etc.. and the other electrical parts I am pulling off of a CB1100 parts bike that I acquired at the same time. I will leave the ARD magneto and install a very small battery that will only power the lights. I've never made my own wiring harness before so that should prove to be pretty interesting. I went to college (3/4 finished) for electrical engineering so I have a decent idea of what I'm doing. Any tips are always welcome for creating this harness though!

What is the best way to measure overall displacement? I keep hearing/reading that if this motor really is 1250cc then it will surely explode the next time I start it up. Supposedly it has stronger rods and pistons as well, but no way to really tell without tearing it down.

So..... I will keep this thread as a place to post up fresh pictures of my progress and to hopefully get some help if need be.

Also, what's a good way to NOT sprain your ankle kickstarting something like this?? I can't afford to be on crutches every other month!

Oh, right... Did I mention I got the bike for free?


Joshm

[andy750]

Wow!! Great story and sign me up for more! Good luck with it and take lots of photos!

good luck
Andy

[sinister902]

sounds like this will be a fun rider when done! I look forward to progress

[moshjack]

Anyone have any opinions on what to expect longevity-wise with the motor? Is heat the main cause of death for these big bores?

[NitroHunter]

Hey Mosh, welcome to the site.
Verrrrrrry cool project.
Lots of big bores running around, and yes they get hot.
Drag-tastic street prowler would be my vote.  
Find out everything you can from the PO about what's inside, we can direct you from there. There's a few guys on here who know a little about these things...  
What ET / MPH did this run?

[MCRider]

It has a finned cylinder block, is 1250cc even a physical possibility? I was thinking anything above 1000cc required a special made block. Or if not, heat would be the death of it as parts of the cylinder liner are hanging out in the breeze, causing hot spots.  Ok for 15 runs on the drag strip, not so much for hours on the highway.   OCICBW

I know from personal experience an 888cc is very streetable, heat not a big problem. Street legal is one thing. Streetable is something else.

IF you want a use it as a for real street bike, I think you'll have to dial down the cc s. But a 915 would be a lot of fun. Can be done with the stock block.

Though someone here has a 1000cc they use regularly on the street, I think.

[moshjack]

Well it certainly wouldn't be an everyday commuter bike! I have a Tuono and an XR650L, so I have most of that area covered. I keep in touch with the PO so I'm just compiling a list of questions as we speak! I'll have more pictures in the next day or so. Thanks for the encouragement, ADV forum had me depressed with my plan!

[MCRider]

Well it certainly wouldn't be an everyday commuter bike! I have a Tuono and an XR650L, so I have most of that area covered. I keep in touch with the PO so I'm just compiling a list of questions as we speak! I'll have more pictures in the next day or so. Thanks for the encouragement, ADV forum had me depressed with my plan!

To heck with them. These are just big old toys and its just money. Unless its life threatening to you or someone else, I say go for it.

[NitroHunter]

Common big bore kits ran up to 1080cc. The oversized sleeves' flanged tops touched each other when installed. The minute gap left between the sleeves had to be filled in with silicone to prevent oil leaks, you can probably see it filled in between the fins.

RC blocks didn't have that leak concern - since there were no fins, only ribs - but there was still no going larger on sleeve OD and retaining a useable top flange.

Stroker cranks (+6mm standard) added about 100cc. At least one guy using 3/8" crank and triumph rods one-offed a slightly larger 1213cc motor. Maybe REX (in Aus) has done something larger, then again maybe not. WAY too much R&D & $$$...

So I wouldn't count on 1250cc to be an accurate displacement figure... but even if this motor was a stocker it looks cool.
Of course we're STILL waiting for those numbers from the strip...  

I put many miles on a 1000cc street motor without issue. But yes they do get warm.

[MCRider]

I forgot about stroking it.

[moshjack]

Common big bore kits ran up to 1080cc. The oversized sleeves' flanged tops touched each other when installed. The minute gap left between the sleeves had to be filled in with silicone to prevent oil leaks, you can probably see it filled in between the fins.

RC blocks didn't have that leak concern - since there were no fins, only ribs - but there was still no going larger on sleeve OD and retaining a useable top flange.

Stroker cranks (+6mm standard) added about 100cc. At least one guy using 3/8" crank and triumph rods one-offed a slightly larger 1213cc motor. Maybe REX (in Aus) has done something larger, then again maybe not. WAY too much R&D & $$$...

So I wouldn't count on 1250cc to be an accurate displacement figure... but even if this motor was a stocker it looks cool.
Of course we're STILL waiting for those numbers from the strip... 

I put many miles on a 1000cc street motor without issue. But yes they do get warm.

I just got back from Dave's place (the PO and builder of the bike). He said what he can remember is that the motor was pretty special back in the day and he stands by his 1250 quote regardless of my attempts to convince him otherwise. This was built using carillo rods and certain 1-ring pistons, and it was a stroker setup. He said that it was machined out to the point of there being small "windows" before being sleeved for the pistons, and that it would leak a very small bit of oil around the fins (sounds right on the money Nitrohunter) from that process. He said that this was the best motor he had built, and outlasted his other built motors by far. He couldn't remember every little detail off the top of his head ( He last rode it almost 25 years ago) but he did say that he was able to get into the high 10's when he raced it. That was with late 70's tires so probably a bit better these days with quality rubber. I will probably just end up tearing it down and measuring everything.

More pictures soon!!

[MCRider]

Sounds like he did the max bore, which does leave the centers of the 1-2 and 3-4 pairs in the breeze.

I'd give him the benefit of the doubt that its a 1250 to him but really something less and was rounded up as is often the case. Our 750s are really 736s. The CB350 twin was really a 325cc. With that much of a discount a 1250 is really a 1162.

But it was his pride and joy and no benefit in arguing about it. Just measure it and keep it to yourself.

There's a thread here about a 1000cc Cobra motor someone was rebuilding. When he got it apart there wasn't much worth saving. Hope yours turns out better.

[Retro Rocket]

Though someone here has a 1000cc they use regularly on the street, I think.

I will be doing a build thread soon as soon as i finish sourcing all the parts in need, it will be  a 1000cc engine with one of Mike Rieck's 1000cc billet aluminum blocks and i intend to ride this little beast everywhere....

Mick

[traveler]

Not so sure about these motors so much.....but with the pure synthetic oil we have now, and a GOOD oil cooler, I think you'd be surprised how it could survive.

I bet Mobil 1 "V-Twin" oil and a oil cooler will make it work.

Using Mobil 1 V-Twin in my Modified Harley Big Twin, I am able to get by in the heat of summer WITHOUT an oil cooler with no problems.  100% synthetic oil dosen't break down under heat like old "dino-oil".

As long as the clutch slippage from the oil dosen't become a problem, and no oil leaks pop up from the new oil, you would be fine.

~Joe

[Retro Rocket]

Not so sure about these motors so much.....but with the pure synthetic oil we have now, and a GOOD oil cooler, I think you'd be surprised how it could survive.

I am using heavy duty everything in my build so i am not too worried about the engine holding together, and i will be using an oil cooler. The synthetic oil is no good for our clutches although Voxhonda has just finished building dry clutches for the 750's so i suppose thats an option as well.


Mick

[moshjack]

Ok, so I've put some more hours into this project. I've also decided that I want to leave this as close to original as possible, while adding lights in the most basic way possible. The current plan is going in the direction of a somewhat streetable bike, but still a drag racer. I had a line on a whole 750 motor for cheap to use for the stator/flywheel etc..., but it turned out to be a DOHC motor (my fault for not asking more questions.)



So I started to asses the condition of the major components. I rebuilt the front brake caliper and master cylinder. I need to do the same with the rear caliper and master cylinder as well. The frame is in good shape, not rusted or weakened anywhere I can see. It does have some paint coming off, and was absolutely filthy. It will be sanded and painted in the next month or so. I pulled off the exhaust because I need to come up with some sort of removable baffles to throw in there. In it's current state it will make you deaf at idle. I really like the look of the 4-4 exhaust, and this system was custom made for this bike supposedly. Any ideas on baffles? I have considered rolling/welding some steel into a cone, drilling some holes, packing it with fiberglass, welding a cap on the end and jamming it in there with a set screw X 4.


It is also setup as reverse shift:





Anyone know what to do here? I have three leads and two prongs on the flasher relay. Do I need a different flasher? Or does that third lead go somewhere else?

I've started labeling all the wires I CAN identify via charts and tracing them down the line. Only ran into a few snags but not too bad.

As for the electrical parts, I have a stator/flywheel/everything else under that left side cover on its way to me right now. I got a somewhat complete wire harness from a friend at work for free, and have been simplifying that harness. My plan is to either use this harness with a couple new connectors, or make my own alongside this one with nice fresh everything. Always open to tips/suggestions on what to do here. It seems like what I'm missing is the harness to the handlebar switches/gauge/light assembly. I will be using a universal turn signal switch so I'll most likely end up wiring that part of the harness by hand, but not quite sure about the indicator light bulbs and harness.

A shot of the oil cooler:


The wheels will clean up nicely, I just barely wiped this one down. You can also see what I think are the mounts for the wheelie bar welded on there:


And a picture of the stable. My dad has the 2 BMWs in the back of the pic:


Thoughts on my plan so far? I still need to pick a nice small sealed battery for the lights, not sure what size to go with.

Josh

[KB02]

Flasher relay looks stock. All these bikes had two prongs on the relay and three wires. One just hangs out for the fun of it.

[NitroHunter]

My .02 
For the pipes, roll some expanded metal to about 1.5" for a strait shot through the megaphones, pack the outside with course steel wool from Home Depot. Sounds wicked and cheap to keep repacked. 
Looks great so far.

[Alan F.]

My .02 
For the pipes, roll some expanded metal to about 1.5" for a strait shot through the megaphones, pack the outside with course steel wool from Home Depot. Sounds wicked and cheap to keep repacked. 
Looks great so far.

That's the best cheap baffle trick I've heard yet, nice.

[moshjack]

My .02 
For the pipes, roll some expanded metal to about 1.5" for a strait shot through the megaphones, pack the outside with course steel wool from Home Depot. Sounds wicked and cheap to keep repacked. 
Looks great so far.

Any ideas for an end cap of sorts? I could start cutting and welding from sheetmetal, but that may be tricky to get it the right shape to fit over the end. I was thinking of welding a circular piece to the end of that 1.5" straight through pipe and having 2 bent tabs on the end to put 2 bolts through. Have a better idea? Something in Home depot that may require only slight modification that I haven't found yet? You guys are a great help so far!

[754]

 I think he means perforated metal in the above posts.. you could try Cone Eng.. for baffles.

 Cool thread, I think that I would go back to 76 or earlier tank and fenders and sidecover if you can fit them,, tribute to early 750s..

[lucky]

Flasher relay looks stock. All these bikes had two prongs on the relay and three wires. One just hangs out for the fun of it.

I have the same problem.
The wiring harness has three wires,black ,grey,and green.
The wiring diagram in the workshop manual shows 3 WIRES black ,grey,green.

But the photos of the flasher keep showing two prongs.

What is going on?

WHICH WIRE "just hangs out"  the green or the grey?

You cannot just say one just hangs out and then not tell us which one.

[lucky]

I really like the look of your bike.
 I would try to put the short 11.4 inch shocks on it and it will ride much nicer.
Did you ever solve the turn signal puzzle?
I have the same problem.

[lucky]

The workshop manual shows the K3-K7 with three wires and three prongs on the turn signal flasher.

The 19977 and K8 or (1978) also show 3 wires on the turn signal flasher except for the CB750A.

I just used a 1975 aftermarket wiring harness and so far it all works out Plug and play.
The clutch switch or Starter motor safety unit is still a choice t be made.
I have some old Safety units and i am going to try one of them. Al of those starter motor safety units are distorted and cracked from the heat sink inside but I do not know if it would make them inoperable.

So many CB750's used that starter motor safety unit I am surprised no supplier sells after market units. Maybe forum members need to start asking the suppliers.

[jaguar]

very cool start
Cant really see that motor working on the street but it should be fun either way