Topic: CB 750 812 and 836 kits

[MauiK3]

I'm sure this has been asked before but I'll ask anyway.
Is there a well laid out analysis anywhere of doing the 812 or 836 change to a CB750 (K0-K6)?
It seems each displacement change has to consider a lot of things like compression, flow, swirl, etc.
So, what do people actually get when they do the deed?
Are the horsepower and torque actually up? If so, at what RPM?
What's the down side?
Are the carbs up to it? It seems there would need to be some jetting changes.
Is the head ok as-is or will some extensive plumbing changes be needed?
Has anyone put one on a dyno to map the changes? I don't think that just boring one out would give much without properly setting the rest of the system up.
If I ever do another resto I thought I might try the increased displacement out but if the results are marginal, why bother? I'm thinking of a street friendly engine, not some track monster with a hair trigger power band.

[ekpent]

When I switched over an 836 Russ Collins piston and cylinder set-up many years ago along with stock rods,cam etc. I noticed a large improvement in torque and power. Still have the bike only with a mild Andrews cam in it now. Took some getting used to because the bike always felt like it was in a gear lower. When I was in 5th I would be fishing for another cause it felt like 4th ! With a cam and headwork and rods etc. you will get more power in the upper rpm's if that is what you like and really let it rip. If you have the extra dollars I say go for it. No replacement for displacement and the little higher compression ratio ia nice on the better pistons like Wiseco.
   As a side note a friend of mine had a Wiseco 811 kit when they used to make those and it ran very well. Pops Yoshi loved the smaller size as well 

[Don R]

  Any overbore is worthwhile if only to round out the cylinders. I believe the accepted order of improvement on a sohc4 is displacement and compression, porting next, then cam, then carbs. Exhaust changes may or may not happen at any point along the way. Jetting is certainly to be considered depending on what year the stock carbs are. Late smog carbs that came with 105's may need a jet/rod change before an earlier carb that came with 120's.
Just my opinion, your mileage may vary.
 Edit, my 836 with a pocket ported head and F2 cam had a nice torquey feel, it didn't seem to need as much rpm to do the job. I later messed it up with a huge cam that made it peaky again. The next time it's apart the F2 or similar milder cam will go back in.   
 

[PeWe]

836 is the most common nowadays and early 80's where I live. Yoshimura provided 812 and 823cc kits.

I always tried to gear to the not existing 6:th gear.  Same today with 1005cc

I started that route with 836 due to worn out pistons and a big scratch in one cylinder. RC 836 was in stock at the tuner followed by ported head, 34mm (F2) in valves and cam. Lightened alternator rotor too.

Same guy that ported it 1983 refurbished it again with 5 mm valve stem kit 2016.
Combustion chambers were opened to fit the pistons back then. Pistons today are usually made for stock chamber diameter.

Stock carbs work fine. I used them with pods. Stock pilots (40) needles raised 1 step from middle (clip one step lower). Main jets were huge, 140 something. I should start with 135 today with same setup.
Edit: Rather open 4-1 included.

[andy750]

If you check my signature you will see I have recorded details of an K2 810cc build (and a K4 836cc build in a separate leaky engine thread: http://forums.sohc4.net/index.php/topic,173213.msg2029836.html#msg2029836).

Specs on the K2 810cc (55,000 miles on the bike):
Brief summary:
CB750K2 with wiseco 810cc pistons
Dynoman 295 cam
New valves/bronze guides.
New rocker arms
Stock carbs: Jetting - 40/120
Exhaust - HM300s
Air filter  - stock airbox with stock filter
Tires - BT-45s
Chain -RK O-ring 102 links
Sprockets - 17/48 (Z1 enterprises)
Ignition - Dyna S
Coils - stock
Plugs - Denso Iridiums

K2 810 gets 71.5 hp at rear wheel according to the dyno.  Could def get more as tyre/chain were not ideal but have not been back to test yet. Top speed so far has been 128mph (GPS recorded). Stable as a rock. The K2 is an amazing bike to ride and way better than stock! 

CB750K4 836cc was completed at end of last summer by Mike so not had time to do the same rigorous testing. More to come!

cheers
Andy

[MauiK3]

I think I'd prefer an overall HP and torque bump through the range. High end screaming isn't much use on the street.

[seanbarney41]

I think I'd prefer an overall HP and torque bump through the range. High end screaming isn't much use on the street.

sure it is...you have 5 gears and a clutch

[MauiK3]

True
It would be cool to see before and after dyno charts.

[seanbarney41]

there have been plenty posted over the years, well, after mostly.  The before info is fairly well documented by Honda and other after market info sources.  Time to do some searching.   ...just stick with a cam on the milder side and you will not lose any mid-range.

If you are really looking for big tire spinning, wheely poppin', street hooligan action, these are not that and it is quite expensive to approach the level of low and mid-range performance offered by slightly later Kawasaki's and Suzuki's.  Please understand I am not baggin' on Honda at all.  I love cb750's.  For me, satisfying my need for acceleration is often more cost effective to keep an open mind to the "other bikes".

[scottly]

True
It would be cool to see before and after dyno charts.

The problem is that no one has ever documented the individual effects of each modification; they tend to make multiple mods at the same time, so it's hard to say which has the most "bang for the buck". An 836 is a 13% increase in displacement, and will provide a similar increase in rear wheel HP. Porting on it's own may provide a similar increase, and camshafts can be a wildcard.

[scottly]

New valves/bronze guides.

Any port work, Andy?

[Terry in Australia]

I went down the big bore kit/ported head/bigger valves/bigger cam/better ignition/CR carbs/exhaust (etc etc) route a few years ago, and while it was fun, it was still a wobbly old CB750 that didn't handle, or stop. All those mods cost a lot of money, and depending on what stage you go to, affect long term reliablity, and don't improve resale value, as most guys looking to buy CB750's want stock bikes now, due to their collectability.

Seriously, you're better off buying a 10 year old sports bike if you want to go fast, and leave your CB750 engine alone. It'll be cheaper to buy that old sports bike, and your CB750 will outlive you.

[disco]

I have a set of Cruisin Image 836cc Pistons in one of my K2's. They're certainly cheap to buy & they do a good job. The machining itself cost more the piston kit! That bike definitely does make more torque than a stock engine. Everything in the motor has been refreshed, but it's still got all stock head, valves, cams etc. It's still a very tractable bike on the street, with good manners.

My other refreshed K2 engine is running 0.5mm oversize CI Pistons. It's good, but the extra torque of the 836cc motor is appreciated. And neither cost me fortune 😎

[PeWe]

DP295 cam give good from low to high. Not a screamer that suddenly wakes up after 6500-7000 rpm.
My bike was a good tourer with 2 persons. Cam Action Fours SS-1. I have used it a few years ago, replaced it with an RC295 profile and got a little bit more from low just before 4000 rpm.

[andy750]

New valves/bronze guides.

Any port work, Andy?

Yes Mike did some mild porting on the head - pics on page 2 (http://forums.sohc4.net/index.php/topic,32093.50.html).

I have had this 810cc bike now for 8 years now and every single time I ride it it puts a smile on my face. For me it was 100% worth the money (very very reasonable and has had the longevity I was looking for. The Dynoman cam has a higher lift so it really has much better acceleration than stock.

[MauiK3]

I agree, stock is better in most cases.
Most folks don't want a heavily modified motor.
I've never entertained changing my K3, it got 2nd over pistons and some head cleanup by Hondaman and runs very nicely. If I did another resto I think I'd just leave the engine as stock as possible again.
If I ran across a salvageable K0 then I'd really have to think about making it as stock as I could.

[Stev-o]

I have the Wiseco 836 kit in my K4, my butt dyno can def tell the difference from a stock bike.
Plus, I walked away from a fellow member running through the gears on a spirited ride, he had a stock K5.

That said, I wouldnt spend the money unless boring was necessary, you're looking at around $600 for it.  [I had no choice, I blew a hole in the motor!]

[PeWe]

The CB750 engine need to be refurbished due to age and use anyway.
Head with valve jobs, cuts. Maybe new guides, valves.
Minor port job by yourself as Hondaman show in his book and on this forum.
http://forums.sohc4.net/index.php/topic,132814.0.html

But really open the ports, area in the seats and just before need professional skills and tools.

These machines are money pits difficult to resist.

[MauiK3]

They sure are. After bringing one back I keep asking myself “what would I do if I found another that
needed help?

[Stev-o]

They sure are. After bringing one back I keep asking myself “what would I do if I found another that
needed help?

You would "help" it!  That is what we do...

[spotty]

I had an 812 kit, mild clean up of the head, up jetted carbs and a mild cam ( i remember RC312 written on a convenient piece of cardboard by the guy who did it for me) it also had completely open drag pipes
as usual no dyno was done but it didn't have a whole lot of extra top end but mid range was phenomenal, you could hold it at 4K ( 60mph in top), bang the throttle open and it took off like a two stroke, awesome fun
don't know if any of this has been any help but its fun reminiscing
incidentally uncle terry bought this motor off me years later with a less than healthy gearbox so its remnants may still exist somewhere ion his shed of infinite joy 

[MauiK3]

Anyone have any good information on what cam would work well with an 812 or 836 kit? I'm thinking solid street performance, not top end screamer.

[andy750]

Anyone have any good information on what cam would work well with an 812 or 836 kit? I'm thinking solid street performance, not top end screamer.

Dynoman 295 cam (http://www.dynoman.net/engine/cams/webcams_honda.html#sohc)

STREET & RACE CAM
1969-76 CB750 F&K
1977-78 CB750K

This cam makes excellent low to mid-range power and torque and top end as well. We have many customers that like them for the street, drag racing, and even with sidecar set-ups.

Requires high performance pistons and valve springs.
Available in either billet or hardweld (core required for HW).

DP295 Cam Card
Specs:
Total Duration: 306° in   296° EX
Lobe Centers: 105°
Lift: .390" IN   .360" EX
Lash: .004" IN    .006" EX

[ekpent]

 Stock cam would probably work fine for that. If you get a cam then headwork would be nice. And so it begins, right PeWe   

[PeWe]

Agree!
DP295 work really fine!
It has high IN lift so better springs needed.
IF Action Fours SS-1 still should be available I should recommend it for moderate tuning.
Beware of the fun domino effect upgrading more and more