Topic: 78 750K full teardown and build plan. Hit me with some reality!

[Delox]

Hey all,

I'm new to the forums and to CB's in general. I am not new to engine building, fabrication, money pits, etc. I've acquired a 1978 CB750K which I am currently in the process of tearing down and rebuilding as my own. The bike has only 13,000 miles on it, but spent a long time sitting.

I was able to get the engine to run and test drive it through all 5 gears. I have torn the engine down entirely for powder coating, and I am researching my rebuild plan.

The engine is the original engine. Here are the basics of my engine build plan:

• I am leaning towards an 836 kit
• mated with a mild, low end torquey cam
• Springs, if fit for the cam
• Porting the heads, if recommend with given cam
• Electronic ignition
• Keihin CR carbs, or opt for stock rebuild/rejet if better suited

I believe that's most of it. Not looking for a crazy build, but figure if I'm tearing it down for cleaning and I'm used to higher power bikes, might as well refresh the old thing.

My real question is about the availability of these build parts for  the 78 engine. There is a LOT of information on here about these bikes, and I've read up on as many of the differences  with 77/78 as possible. The vendors I've looked at show a lot for the K0-K7, but less so for the K8.

Thanks for any guidance!

[Wobbly]

The K7 and K8 are nearly identical. All the parts you want are readily available (unlike the genuine Honda parts). After that many years and (!) building up the top end, you probably need to replace primary chain tensioner and chains with it while you are there. Replace the oil strainer screen (oil pump) and check condition of the chains then. I don't know how old the tires are, but apparently you need a new chain and sprocket. Since the stock brakes are atrocious, improving performance should go hand in hand with adding another front brake. The stock shocks are pretty useless too, and ICONS and new bearings for the front and rear fork are probably a good idea along with new fork oil. I would focus on the basics first, then pay attention to brakes and suspension before modifying this low-mileage engine which, left alone, will probably serve you for many, many years. 
You can refresh und upgrade all you want, the CB will always be outperformed by the newer bikes. So, hopefully you are driven by your love for these bikes and not search for performance (it also would be cheaper to buy something newer than to restore and modify). The one thing where--I believe--the CB 750 is unrivaled, is its legendary reliability and longevity. Both will be compromised.
Due to its low mileage, this would probably be a good candidate for a restore to original condition. But that's just my opinion.

[seanbarney41]

Wobbly, is quite right.  If you are looking for modern suberbike performance, you will not find it here.  However, they are fast enough to be fun imo and can totally hang better than most bikes of this era in modern traffic.  Your proposed engine mods are only gonna get you to around 70hp realistically, and if done correctly.  I would ride it with the engine stock for as long as you can stand it.  It will probably run damn good for a long time but it is likely to start leaking oil, if it has been left to sit for more than a couple years.

[Delox]

Thanks guys,

I forgot to mention the basic maintenance things you went over. Primary chain, cam chain, tensioner, and full gasket kit etc. is all going into it. This post was focusing on the engine mostly, but I definitely plan on redoing/modernizing the suspension and brakes. Thanks for the input on that too.

I have the frame 100% stripped and have added the gordon kit to allow for future headwork, but I'm hoping a good build will avoid leaks. Speaking of which, I should mention I yanked the stock twig-like studs and will be going aftermarket for those too.

I'm not looking to make the bike compete with any modern superbike by any means. I just have a hard time not wanting to put a grand and a half into the engine to get a bit more oomph out of it. As I mentioned, catering towards some more low end torque would make me happy. I've got the engine torn down, cases split and all for cleaning anyway. Hell the sound of the 836's is enough alone to make me want to

Hmm, this is good though, its got me thinking about maybe considering just some new piston rings and a professional carb rebuilding 

[seanbarney41]

Well then, it sounds like you are determined.  Low end torque is not these motors forte' nor will it ever be.  Your plans will net nice improvements in the mid range though.  Intake ports are the biggest plug...a port clean-up and good valve job will be the best bang for the buck, even more so with planned 836 and mild cam...

[Rigid]

Low end torque? Get a Harley.  Top end and braking, get a 600cc sport bike after 1999.  You seem to have no plan other than change everything you can think of with no real vision or purpose.  Thats cool if its what your into.  Just don't expect to be satisfied with looks, or performance, compared to bikes that have already had decades of development in specific performance niches.

[Retro Rocket]

Gday Delox, ignore the above comments mate, there are plenty of us doing exactly the same thing with great results, there are tons of great new performance parts for these bikes and plenty of well respected performance guru's on the forum, the bloke above isn't one of them....   If you are after torque, don't remove too much off the crank if that was part of the plan, just balance it, good aftermarket carbs like the Mikuni RS34's , are great for getting the most out of the engine due to their adjustability and accelerator pump, or the older Mikuni VM29 smoothbores, the popular CR29's are more top end orientated. . Getting a cam for low end torque will leave you down a little on horse power but you can get a respectable amount of torque from these engines when compared to most modern 4 cylinder bikes {unless you go large displacement}. The 70's and early 80's was the era of the torque monsters, thats why you still see lots of early Kawasaki's, Honda's and GS Suzuki's at the strip...

[Delox]

I decided on a CB for this build because I enjoy the looks and it fit the build style I am going for.

As I mentioned, I am not trying to compete with newer style bikes by any means. I simply figure its worth my time while the bike is fully disassembled to cater it towards my preferred powerband/type. I don't often get to rev out bikes in my area, so I'd rather the benefit of some low end for my around-the-town cruising. That's all.

Retro, thanks for all the specifics. I will definitely get some good research out of it.

[PeWe]

I remember the difference when I got my CB750 K6-76 tuned...WOW, what a sound and acceleration when twisting the throttle!!
- Russ Collins forged 836 (Later on replaced by Action fours pistons with lower compression, this year NOS RC 836 back in)
- Ported head with larger inlet valves (F2 inlet valves 34mm, before 32mm)
- Action Fours SS-1 cam, same numbers as Webcam 41
- HD valve springs
- Lightened alternator
- 4-1 exhaust as I used earlier too. (4-4 in OEM look that can support modded engine would be nice upgrade today)

The bike worked very well with these mods.
4 holidays down south in Europe, 5000-6000km each time during 15-20 days. Cruising speed on Autobahn, French and Italian highways was 150-200km/hour. 2 not so heavy persons on the bike + luggage (clothes, tent, sleeping bags...)

[Don R]

I built an 836 with a mild port job and a mild cam. I was pleased with the torque increase, I notice I can shift sooner and not lug the engine yet it will wind to redline at will. It's got stock 79 750 shocks and I rebuilt virtually everything. It is an early front end and still needs a double disc upgrade, because it's so fast. Don't let the nono's spoil it for you, have a blast.
 They are cautious because so many non-mechanical guys get in over their head and are constantly asking the most basic questions. Some of them never finish their projects.

[HondaMan]

Here's a little "tech info" for you to scratch your head about (BTW, do you have my book?):

1. To get 'low end torque' out of these engines, relative to the way they were originally designed, you must go big-bore, step one. Nothing but going oversquare in the bores will increase the low-end power vs. top-end power. This said: the HP in these engines is all above 6000 RPM, in any incarnation. This is due to their small per-cylinder size.

2. ALL of the aftermarket cams are made for one thing with these engines: more top-end HP. Again, to make the low-end stronger, it takes a cam that opens sooner than the one you have now, closes sooner than the one you have now, and has less 'dead time' between the intake and exhaust lobes. In other words: a pre-1973 cam, if stock. Or, use the Megacycle 125-00 (or -04 in the regrind type) and advance it an extra 3-5 degrees. This cam will not require heavier springs, especially if you advance it: the power will fall off suddenly at about 8500 RPM, and the springs are good for another 1500 above that, as is.

3. Polish the valves. Larger valves are good for more HP here, but not for more midrange torque.

4. Raise the compression ratio. The stock is 9.2:1 on your engine. Using the 836 flat-topped pistons will bring it in a little higher than that. Using the Wiseco forged, 10.25:1 pistons opens a whole can of worms you may not wish to fight with (although the Gordon kit would make those future re-entries into the head easier...). If you are planning to go touring with it, skip the forged pistons (from anyone) and stay with cast, and below 9.8:1 final CR.

5. Intake tract: stay with the stock airbox. This will net more midrange power and higher top-end HP values than "pods", and you won't have any mixture tuning problems. If you go to the CR or other carbs, you're kinda on your own, as it is real hard to make a decent intake tract when the carb bell points right at the frame. Honda sometimes solved it by moving that frame...

6. Use a long-duration, moderate-voltage spark. Long duration is far more important than higher voltage in these engines, and "hot" coils make short sparks. If you go to one of the 3-ohm type coils (Dyna, Accel, et al) then add a 1-ohm primary resistor to each coil and use 10k ohm sparkplug caps, set the plug gap between .035" and .040", and use extended-tip plugs like the ND X24ES-U for stellar results.

The post-1976 heads were the best-shaped inlet ports of the 750  SOHC4 era, but they are not perfect. Some study of your particular one will net some improvements. You will find that your valve guides are finished, as they were cast iron: install some of the APE bronze guides and even if you decide to go to a taller cam, nothing will bind. They cool better, too: make SURE the machine shop sets the clearance at .0008"-.0012" intake, .0012"-.0024" exhaust, even thought they will protest and tell you that you don't know what you're talking about: trust us. This MUST be done, or there will be trouble. Honda originally set the clearances much tighter than even those...

[BPellerine]

okay I will bite,what is the can of worms you are talking about with the weiscos?bill

[Stev-o]

Can't add much after Hondaman's post except to highly suggest an 836cc kit.
I have one in my K4 and worth the money spent.

Here's a link to his book, highly recommended....

http://forums.sohc4.net/index.php?topic=132433.0

[danfr]

Another thing that you may want to consider is an oil cooler. Higher compression=higher temperatures. There's some lengthy threads on oil cooling, so educate yourself and talk to the more experienced forum members.

0.02

[Retro Rocket]

Exhaust length and gearing will also have a bearing on torque...

[HondaMan]

okay I will bite,what is the can of worms you are talking about with the weiscos?bill

On the street, like in casual and commuting use, the forged pistons heat and cool much differently from the cylinders. This then requires lots of cylinder clearance, lest they get stuck on occasion. One common occasion: run the engine to the local gas station (5 minutes) and gas up: the pistons get hot, the cylinder cool fast while gassing, then on startup the oil is pressed off the skirts. Sometimes the pistons even stick, making it hard to start. Get it fired up, the skirts scratch, and here we go...

I see engines that died this way from the old Powroll and Wiseco pistons, with maybe 8k miles on them. Rings are cracked, bores and skirts scratched. Often, the clearances were set at .0025" or even as much as .004" for whatever reasons, and the result was not very good.

[BPellerine]

with all of the people running weiscos and such I have not heard of this problem ,not saying that it did not happen,was told by big jay that I could run my 836 at .0015 because they had a high silicone content.I have heard of piston slap with a large clearance.bill if this is a hijack I am sorry

[setdog]

You mentioned you are already famiar with money pits, and cited 1500 budget.

I would increase your budget, if you can afford to, so by
The time you finish the smile on your face will be that much bigger.

After all you're the one that going to ride it. And hopefully it will be a work of art,
And your favorite bike.