Topic: HOW to Engine CB750 '73 Upgrade for street use only

[Bruno]

Hello All ,
I can use some good advice on upgrading my CB750 K3 engine .
The engine is due for overhaul . I live in europe ( lot's of smaller roads with plenty of bends ) and ride with friends who have modern bikes who have way more power than I have  .( It will not be used for closed course racing ,only street use) .
Therefore I would like to gain more power from the engine especially Torque .
Looking for an engine with " linear " power curve with a healty push in lower to midrange if possible.
The engine is completely disassembled so I like to do it only once in a proper way ;-)
My crank and rotor is already balanced and lightened .
What do you suggest regarding piston size , Compression Ratio , Valves , Cam and Rods ?
I have been looking to the 3 main vendors of parts but cannot decide what parts to buy ... I know there is no substitute for Cubic inches but looking for all options ...
836 , 890 ,906 or 915cc size pistons with what CR  (10.25/1 or 12.5/1 ) ? There is so much choice ! don't really want to bore the cases ( Not sure anyone here can do this for me ) .
Carillo or Falicon rods ? ( same price )
Normal valves or oversize or even 5mm conversion ?
Engine head work ?
Price is important but will decide after I hear all options ...

Thank you for your time ,
Bruno .

[Jerry Rxman Griffin aka MuthaF'er]

My take: how deep are your pockets?! $$, $$$, $$$$+

Want to go easier on them then do a 836 and cam with valve springs. No expensive rods to buy and no block work except cylinder boring. $$

Next step do porting. $$ Next would be larger valves and hardware, etc. $$$$

Next step might be a 915 or 1000 with the corresponding sleeve replacement that requires the block boring and cylinder boring $$$$ THEN you're going to need those $$ rods. The CycleX 915 kit will not need case boring.

You can go 12.5 to 1 but don't on a street bike.

[gschuld]

+1

It’s hard to argue the cost effective upgrade of a 10:1 compression 836 engine with a quality valve job and good springs and a performance cam, some porting work is a bonus.  Makes a fun peppy reliable street engine.  If you intend to keep up with lead footed riders on modern crotch rockets, it will be disappointing.   But a solid 70 or so RWHP is plenty of fun.  Perhaps consider upgrading to twin disk brakes up front for better stopping power.

There are lots of options to get higher output out of these engines, but as Jerry suggested, it starts to get far more involved and expensive as you move up the displacement and power numbers.  This is because of all the other upgrades that become necessary beyond the added displacement related parts.
 
OEM rod bolts, cam and primary chains and tensioners, cylinder studs, etc start to become weak links as power goes up.



George

[Tracksnblades1]

Spend a little money on her....she'll reward you.....😎

[wannabridin]

HP costs money, how fast do you want to go?

[gschuld]

Perhaps it would help if we put together a few detailed examples of a few different levels of performance upgrades.  Like you would see in the back of some V8 engine building books.

List all the parts and machine work required, and the dyno results with a summary of each.

So in street gas compression:

Like a stock engine with just a mild street performance cam and springs.

An 836 with cam and springs, stock valves

Same 836 as above but with upgraded rods(or rod bolts on OEM), ported Head, hotter cam(125-75 maybe)

A 915(biggest you can go without boring cases) ported head, HD cylinder studs, cam and primary chains and tensioners, back cut gears, cam, etc

A 1005(popular JE 71.25 pistons) with similar to above

In race gas:

A 1000 plus cc race engine sample.

Through out the forum, these example already exist.  It’s more about assembling what I’d already there into a consistent format.  It might serve to give guys an idea about what certain things take.  I could post some vintage articles on the subject to supplement the examples.  Below is one for the 1000cc crowd😎

Just an idea....🤔

George

[Bruno]

Hello All ,
thanks for the reply' s  I appreciate it .
Yes I know , it comes down to what money I would spend but first I would like to know all possibilities before I give my friends with their modern bikes a run for MY money ;-) .
Porting it will be ,
836cc or more it will be
Lightened and balanced crank it will be
transmission upgraded it will be
CR , cam , piston size still to be fine-tuned ....
please continue and thanks for your help !
Bruno .

[PeWe]

Cam with good bottom torque is my recommendation. I have tested cams that are boring on low and suddenly wakes up after 6000rpm so I almost lost the grip of the handlebar at first test ride. Probably a racers choice. CX-7 with 836cc. (Maybe another thing with 1000cc). That cam made my bike to scream extra loud.

My Megacycle 125-20 with 836 and 4-1 made the tacho needle to pass the red and enter the green on other side really quick! + 10.000... + a surprise high front wheel lift on a country road bump at +130 kph.

Street and inspired country roads riding is much more fun with earlier response. 1000cc gives that but that will drain your wallet! Good rods needed too. No real need to rev over 8000.

I have a cam in today that has RC295 carved in the end under its washer. Numbers I got when timing match RC295, DP295 sold today by Dynoman.
Measured lift @ 0 lash IN/EX 0.391"/0.361" (9,93/9,18mm) duration 259/257, Open/close 25/54 56/21

I used an Action Fours SS-1 cam in the 80's with 836cc, ported head, stock carbs and 4-1
Worked fine, not that hot and no aggressive lobes. That cam should be remade today.
Card say 0.360 lift, duration 270*, open/close 28/62, 62/28
I measured lift /IN/EX 0.361 (9.17mm), duration 254/256,  22/55, 56/24 at 1mm lift and 0 lash.
Lash later set to 0.1/0.15 wich will make in numbers to be bigger and earlier than ex.
My 295 cam has better response from 3500-4000 rpm on 5:th gear, just twist.

I highly recommend the 295 cam with a ported head and matching springs. Bore stock cyl to 836cc, pistons 10.5:1 like JE
Maybe others here have experiences of other good cams that have similar good response from low. (If you have the same idea about the engine characteristics for street and touring.)

EDIT: Cam card numbers are not the only truth!
Cams with same numbers can be totally different. Shape of the lobes
Action Fours explained that very well in their catalog. see attached page

[PeWe]

Do not forget the primary drive hub. Old hard rubbers inside.
I sent two hubs to Germany where they did the job for a good price
http://www.fourever-classic-parts.de/index.php?option=com_content&view=article&id=26&Itemid=31

Primary chains to be replaced too. I used BMW timing chains in my last K2. Probably stronger than stock. Fits direct.
BMW 530 d,  184 HP,  [E39],  Production: 1998/08 - 2000/09,  Engine code: M57 D30 (306D1)
Original partno: 11 31 2 248 729

You can find more about prim chains in this thread
http://forums.sohc4.net/index.php/topic,154183.100.html

I used Honda stock chains with 836 and around 80 whp during 35.000 km in the 80's, mostly high speed touring. Had not got that much more slack. 67.5mm measured as manual say. New 65, service level is 70mm.
Tough decelerations is the hard thing for these chains as I have heard.

I'm sure that forum member MRieck can give you some good advice too, based on his hugh amount of experiences. A big box with needed stuff can be done too. We are many on this forum that have got our engines improved with his help.

[Bruno]

A Big thanks for all that have already replied !
I start to see the trees behind the woods ...
for now ,
Balanced and lightened crank , revised transmission ,
Rods by Falicon,  APE or Carillo ...
Pistons 915cc
CR 10.5:1
New prim chains , studs etc ....
If I find the money ported head with normal or 5mm valves .
How will that sound ? 
Keep it comming !
Thanks ,
Bruno .

[gschuld]

Sounds reasonable.

A 915 with tapered sleeves is as big as you can go without needing to bore out the upper cases.  And you can get away with skipping the porting if you need to. If you want to consider a doing it in two steps, it’s easier to have the best rotating assembly out of the way(pistons on down), so you can upgrade the head with porting, bigger intake valves, later.  With more head flow the ideal cam choice changes, same for carbs to a degree.

It’s most convenient to do everything at once of course, except for the $ aspect.

Bill Benton built a 10.5:1 compression 915 a few years back.  Mike Rieck stage 4 k model head, RC 327 cam, custom bored factory carbs(roughly equivalent to Smoothbore 29s), and made 101.xx hp on MR12 race fuel.  That is special fuel that add more power than normal.  So let’s say 97 RWHP on quality gas.

That’s a LOT of power for a street cb750.  The RC327 cam isn’t what most people would consider ideal for a street bike(Bill built his motor for drag racing)

But with a cam like a 125-75 that engine would be in the same ballpark and be great fun on the street if timed correctly.

Food for thought.

George

[slikwilli420]

Sounds reasonable.

A 915 with tapered sleeves is as big as you can go without needing to bore out the upper cases.  And you can get away with skipping the porting if you need to. If you want to consider a doing it in two steps, it’s easier to have the best rotating assembly out of the way(pistons on down), so you can upgrade the head with porting, bigger intake valves, later.  With more head flow the ideal cam choice changes, same for carbs to a degree.

It’s most convenient to do everything at once of course, except for the $ aspect.

Bill Benton built a 10.5:1 compression 915 a few years back.  Mike Rieck stage 4 k model head, RC 327 cam, custom bored factory carbs(roughly equivalent to Smoothbore 29s), and made 101.xx hp on MR12 race fuel.  That is special fuel that add more power than normal.  So let’s say 97 RWHP on quality gas.

That’s a LOT of power for a street cb750.  The RC327 cam isn’t what most people would consider ideal for a street bike(Bill built his motor for drag racing)

But with a cam like a 125-75 that engine would be in the same ballpark and be great fun on the street if timed correctly.

Food for thought.

George

Im going to disagree with George just a hair on this one. Based on what Mike has proven for flow numbers on a ported head, I would start with a head and move down rather than finishing with the head if funds are tight. All those tasty bits below a stock head is like making a runner breathe through a straw. A ported head on stock rotating assembly should give very noticeable gains and still not tax the engine parts nearly as much as bigger pistons with more compression. Beyond that, George is spot on.

[gschuld]

Sounds reasonable.

A 915 with tapered sleeves is as big as you can go without needing to bore out the upper cases.  And you can get away with skipping the porting if you need to. If you want to consider a doing it in two steps, it’s easier to have the best rotating assembly out of the way(pistons on down), so you can upgrade the head with porting, bigger intake valves, later.  With more head flow the ideal cam choice changes, same for carbs to a degree.

It’s most convenient to do everything at once of course, except for the $ aspect.

Bill Benton built a 10.5:1 compression 915 a few years back.  Mike Rieck stage 4 k model head, RC 327 cam, custom bored factory carbs(roughly equivalent to Smoothbore 29s), and made 101.xx hp on MR12 race fuel.  That is special fuel that add more power than normal.  So let’s say 97 RWHP on quality gas.

That’s a LOT of power for a street cb750.  The RC327 cam isn’t what most people would consider ideal for a street bike(Bill built his motor for drag racing)

But with a cam like a 125-75 that engine would be in the same ballpark and be great fun on the street if timed correctly.

Food for thought.

George

Im going to disagree with George just a hair on this one. Based on what Mike has proven for flow numbers on a ported head, I would start with a head and move down rather than finishing with the head if funds are tight. All those tasty bits below a stock head is like making a runner breathe through a straw. A ported head on stock rotating assembly should give very noticeable gains and still not tax the engine parts nearly as much as bigger pistons with more compression. Beyond that, George is spot on.

Logistically though, if he is going to rebuild an engine, it makes little sense to rebuild a stock engine with factory pistons/cylinder/rods/OEM cylinder studs, non back cut trans gears, etc if then he needed to tear into the engine again to replace all those things to upgrade.  My point was if he is to go half way, do the foundation of the engine first and only ONCE, as you can take another head and have it ported while running your engine, and have the good cam and carbs ready to swap on when ready later.  Easy to do.

Now if there were two engines on hand, you could take the one in the best condition and run that with the improved top end.  Ported head, good cam, carbs.  Then rebuild the second engine as time/funds allow with the extra displacement and strength bottom end and swap the nice top end onto it when ready.

Either way is not as convenient to doing everything at once, but if doing it in two stages, I’d prefer to do build the foundation ONCE and add from there.😉

George

[slikwilli420]

Sounds reasonable.

A 915 with tapered sleeves is as big as you can go without needing to bore out the upper cases.  And you can get away with skipping the porting if you need to. If you want to consider a doing it in two steps, it’s easier to have the best rotating assembly out of the way(pistons on down), so you can upgrade the head with porting, bigger intake valves, later.  With more head flow the ideal cam choice changes, same for carbs to a degree.

It’s most convenient to do everything at once of course, except for the $ aspect.

Bill Benton built a 10.5:1 compression 915 a few years back.  Mike Rieck stage 4 k model head, RC 327 cam, custom bored factory carbs(roughly equivalent to Smoothbore 29s), and made 101.xx hp on MR12 race fuel.  That is special fuel that add more power than normal.  So let’s say 97 RWHP on quality gas.

That’s a LOT of power for a street cb750.  The RC327 cam isn’t what most people would consider ideal for a street bike(Bill built his motor for drag racing)

But with a cam like a 125-75 that engine would be in the same ballpark and be great fun on the street if timed correctly.

Food for thought.

George

Im going to disagree with George just a hair on this one. Based on what Mike has proven for flow numbers on a ported head, I would start with a head and move down rather than finishing with the head if funds are tight. All those tasty bits below a stock head is like making a runner breathe through a straw. A ported head on stock rotating assembly should give very noticeable gains and still not tax the engine parts nearly as much as bigger pistons with more compression. Beyond that, George is spot on.

Logistically though, if he is going to rebuild an engine, it makes little sense to rebuild a stock engine with factory pistons/cylinder/rods/OEM cylinder studs, non back cut trans gears, etc if then he needed to tear into the engine again to replace all those things to upgrade.  My point was if he is to go half way, do the foundation of the engine first and only ONCE, as you can take another head and have it ported while running your engine, and have the good cam and carbs ready to swap on when ready later.  Easy to do.

Now if there were two engines on hand, you could take the one in the best condition and run that with the improved top end.  Ported head, good cam, carbs.  Then rebuild the second engine as time/funds allow with the extra displacement and strength bottom end and swap the nice top end onto it when ready.

Either way is not as convenient to doing everything at once, but if doing it in two stages, I’d prefer to do build the foundation ONCE and add from there.😉

George

Fair enough. I took the "do it all at once" approach and it was quite spendy, which many here can attest to.

[PeWe]

836 will work fine with stock rods and around 80 whp.

Important information about 915cc kit. This is usually 67.5mm pistons which make 900cc (901.75cc), not 915.
Important to verify bore diam if order rings for such kit
"Real" 915cc means 68mm pistons and bigger sleeves that must get case machined. Dynoman sell 68mm kit. This can use even bigger pistons.

Important that the shop know how to seal the cylinder-sleeves since holes will occur between 1-2 and 3-4 when bore for the bigger sleeves. Not fun with constant oil weeping.

My K6 case needed to taste the Dremel to get the tapered sleeves to enter with a small gap all around. My old K2 case did not need further job.

[Bruno]

Very interesting info !
Lot’s of things to think about .
I will have to bite the bullet and do it all in one time ... even the costly porting of the head ...
It will take just a bit longer to get there .
Will have a good look on what piston set to take , Wiseco , DP , CycleX....
thanks !

[gschuld]

I’d very seriously recommend Mike Rieck for the porting work.  It makes sense to source a door head here in the states, have Mike work his magic, and have him ship it to you. 

I can assure you it will be worth it👍

[calj737]

It makes sense to source a door head here in the states, have Mike work his magic, and have him ship it to you. 

I bet Bill Benton Racing has a few quality spare heads that would be ideal candidates for a Mike Rieck Magic Head! You ought to give Bill a PM-

[PeWe]

If Mike will do the head, better to find a head in USA. A good head on eBay cost less than the shipping a head from Europe.
Did that with a crank to be sent to APE by Mike, gearbox too for undercutting.

Alternative is to find a good ported head ready to use.

[Bruno]

Allright !
Next question , not wanting to start a brand war !
Just looking for valuable information .
With all the available rods considering ,
Which ones to choose , APE , CycleX , Falicon or Carillo ‘s for my application ! Again not want to start a brand war here , just good info . I like the Falicon shape though .
Same question for porting , APE , CycleX or Rieck porting . Same as above , not wanting to start a p.... contest , just good plain info  .
If I ever win the lottery I will build several engines with parts and pieces from everyone ! Promised !
Thanks !
Bruno .

[slikwilli420]

Allright !
Next question , not wanting to start a brand war !
Just looking for valuable information .
With all the available rods considering ,
Which ones to choose , APE , CycleX , Falicon or Carillo ‘s for my application ! Again not want to start a brand war here , just good info . I like the Falicon shape though .
Same question for porting , APE , CycleX or Rieck porting . Same as above , not wanting to start a p.... contest , just good plain info  .
If I ever win the lottery I will build several engines with parts and pieces from everyone ! Promised !
Thanks !
Bruno .

I have the CycleX rods in my race bike and would not hesitate to use them again. The ones from APE are a high quality option as well, both competitively priced.

I would never use anyone but Mike Rieck for head work. There is a fair mix of art, skill and science to getting a head right, and he's the one to go to.

[gschuld]

Allright !
Next question , not wanting to start a brand war !
Just looking for valuable information .
With all the available rods considering ,
Which ones to choose , APE , CycleX , Falicon or Carillo ‘s for my application ! Again not want to start a brand war here , just good info . I like the Falicon shape though .
Same question for porting , APE , CycleX or Rieck porting . Same as above , not wanting to start a p.... contest , just good plain info  .
If I ever win the lottery I will build several engines with parts and pieces from everyone ! Promised !
Thanks !
Bruno .

I have the CycleX rods in my race bike and would not hesitate to use them again. The ones from APE are a high quality option as well, both competitively priced.

I would never use anyone but Mike Rieck for head work. There is a fair mix of art, skill and science to getting a head right, and he's the one to go to.

Well, there are others I would use for headwork, but Mike is my top choice😎

George

[MRieck]

I have spare heads on hand.

[Ilja]

What I'm planning to do is to remove my turbo setup (although its working fine, I have two other turbo's which are legal ), use CycleX 836 F2 wiseco pistons with the high "pop-top" dome, my F2 head (because bigger valves, and tighter springs and bigger intakes), depending on final compression (10.5:1) the stock f1 cam or (11.5:1) the DP295 (which I had before, but sold again), some RS34 carbs, velocity stacks in foam filter and a custom 4-4 exhaust like "house rockers super build".
And my ignitech programmable ignition.... my engine already has HD studs and stronger primaries. 

Should be a nice setup for a nice street bike that will last another 40 yrs.

[Cb750 Racer]

What I'm planning to do is to remove my turbo setup (although its working fine, I have two other turbo's which are legal ), use CycleX 836 F2 wiseco pistons with the high "pop-top" dome, my F2 head (because bigger valves, and tighter springs and bigger intakes), depending on final compression (10.5:1) the stock f1 cam or (11.5:1) the DP295 (which I had before, but sold again), some RS34 carbs, velocity stacks in foam filter and a custom 4-4 exhaust like "house rockers super build".
And my ignitech programmable ignition.... my engine already has HD studs and stronger primaries. 

Should be a nice setup for a nice street bike that will last another 40 yrs.

That's a sweet setup I've been waiting to see a build with the "pop-top" pistons I know there probobly already is one on the forum I'm a bit out of the loop. Sell me that turbo setup with a gt20 attached! I have thousands in Draw-through parts but an intercooler is really attractive. I still have that Z1r TC intake manifold if you still want it to Ilja. In terms of porting MRieck has the best on the forum from what i've seen (Sorry Kenny )

[PeWe]

Where have all old ported heads from the 70-80's gone?
It must be many collecting dust and no longer used waiting for a seat and valve job.

[Ilja]

Where have all old ported heads from the 70-80's gone?
It must be many collecting dust and no longer used waiting for a seat and valve job.

I think so... I have at least two heads for every bike I own. XD
... but I just saw this: https://www.ebay.co.uk/itm/Oversized-Intake-Valves-Honda-Four-CB-750-K-SOHC-69-77-34mm/273093318769?_trkparms=aid%3D444000%26algo%3DSOI.DEFAULT%26ao%3D1%26asc%3D20170221122447%26meid%3D013483a1238346a380b88bcdd3d85638%26pid%3D100752%26rk%3D5%26rkt%3D6%26sd%3D273093320079%26itm%3D273093318769&_trksid=p2047675.c100752.m1982

If it's possible to install 34mm valves in the K/f1 heads, then why did they go through all the trouble for the F2 head?
I'm porting my f1 head for RS34 and am installing these 34mm valves, and then I will just run my current "cheap" 836cc flattops instead of expensive pistons.

[MRieck]

Where have all old ported heads from the 70-80's gone?
It must be many collecting dust and no longer used waiting for a seat and valve job.

I think so... I have at least two heads for every bike I own. XD
... but I just saw this: https://www.ebay.co.uk/itm/Oversized-Intake-Valves-Honda-Four-CB-750-K-SOHC-69-77-34mm/273093318769?_trkparms=aid%3D444000%26algo%3DSOI.DEFAULT%26ao%3D1%26asc%3D20170221122447%26meid%3D013483a1238346a380b88bcdd3d85638%26pid%3D100752%26rk%3D5%26rkt%3D6%26sd%3D273093320079%26itm%3D273093318769&_trksid=p2047675.c100752.m1982

If it's possible to install 34mm valves in the K/f1 heads, then why did they go through all the trouble for the F2 head?
I'm porting my f1 head for RS34 and am installing these 34mm valves, and then I will just run my current "cheap" 836cc flattops instead of expensive pistons.

Those 34mm ebay valves won't work in the f2 head....they are to long and keeper groove is different.

[Ilja]

Where have all old ported heads from the 70-80's gone?
It must be many collecting dust and no longer used waiting for a seat and valve job.

I think so... I have at least two heads for every bike I own. XD
... but I just saw this: https://www.ebay.co.uk/itm/Oversized-Intake-Valves-Honda-Four-CB-750-K-SOHC-69-77-34mm/273093318769?_trkparms=aid%3D444000%26algo%3DSOI.DEFAULT%26ao%3D1%26asc%3D20170221122447%26meid%3D013483a1238346a380b88bcdd3d85638%26pid%3D100752%26rk%3D5%26rkt%3D6%26sd%3D273093320079%26itm%3D273093318769&_trksid=p2047675.c100752.m1982

If it's possible to install 34mm valves in the K/f1 heads, then why did they go through all the trouble for the F2 head?
I'm porting my f1 head for RS34 and am installing these 34mm valves, and then I will just run my current "cheap" 836cc flattops instead of expensive pistons.

Those 34mm ebay valves won't work in the f2 head....they are to long and keeper groove is different.

Yes, the F2 head also already has 34mm valves... I wanted to keep those valve sizes so was planning on buying special F2 pistons for conversion back to N/A.

But now I can just upgrade the F1 head (=K) with these valves so it's also 34mm, and install that head to get back at normal N/A compression with my current pistons instead. 

[PeWe]

My K6 head got F2 in valves as an upgrade