Topic: CB750K6 -76 project back to life with more updates

[andy750]

Looking forward to hearing your impressions Per. Its a good looking tire thats for sure.

[PeWe]

Hi Andy.
I'm ready to test when the winter is gone, most sand on roads removed  and around +15C

[andy750]

Thats a good plan! I am waiting for the snow to stop, the inch of salt on the roads to be washed away and then I am good to ride no matter the temp (heated gear helps!).

[PeWe]

Salt washed away too!

[PeWe]

The new rear wheel finally on the bike today. It was new over a year ago when I laced it.
3" wide to get proper tire fitment of 120-130 wide tires.

I have a new set of Michelins waiting 100/90-19,  130/80-18.
That rear will fill the gap  better;)

Stock 2.15 can only use max 4.0-18.
My old 120/90-18 was on a stock rim. That's why its that flat and really tired!

Test ride in a perfect riding weather! Sunshine, blue sky and not windy. Only 15C in the shadow, sun warmed up.

Last weekend was rainy and windy. Perfect head swapping weather!
Head removed on Friday, another earlier looking K2 head,  34mm F2 IN valves with lash caps/28mm Kibblewhite K EX valves back on and cam timed on Saturday. Carbs synched on Sunday.

This head is shaved 0.5mm so I advanced the cam a little. Punch marks side by side for a few crank degrees earlier.

Engine sounds harder now Compression tester show 215 PSI.
No pinging when twisting the throttle thru 4000-5000rpm area.

Previous head 205 PSI. (392 with 33.5/28.5mm valves)

Its guide heights/valves not sunken enough made stem reliefs for keepers to pass into valve stem seals.

Bike seems to have better torque after the swap. Will be fun to see how the dyno graph will show the butt difference.

Will try one step bigger main jets after throttle test in higher speeds where it felt lean around 180-200kmh. Reduced a little made it to be happier.

Below the cam advance and K2 head that finally has got ti retainers. Risky with F2 in valves.

I understand the good thing with running lash cam timing.

Last dyno May 2022 showed no decreasing hp at 9200 rpm.

*Cam timing August 31-2022* *** 392 head ***
0 lash: 1mm lift:
IN 27/54*    9.98mm lift
EX 57/22*   9.38mm lift 

Running lash 0.10/0.15mm @ 1mm lift:
IN 24.5/51*    9.82mm lift
EX 54/17*       9.15mm lift


Above work well.  (Another  timing around 1 degree later worked fine too)

Tried more advanced, less power when IN open at 29.5* almost 30* at 0 lash.

Also tried more retarded when IN open at around 24* at 0 lash, acted like less compression.  The fun response at 4000-5000 rpm suffered.
#####################



This head use F2 IN valves, Kibblewhite 28mm K  EX valves.
Chamber volumes close to previous head.



The old K2 head's port design

The higher compression with cam advanced a few degrees. Both heads have very similar chamber volumes.
#

[grcamna2]

Hello Per,how do you like the 125-75 Megacycle? camshaft;is it more a high midrange rpm power improvement ?
I like your new 3" rear rim,fits your 120/80 rear tire;this change can cause the bike to handle well ?

[PeWe]

The 125-75 cam has a very even power from low to top.
No specific rpm where it suddenly takes off.
I thought it should suddenly wake up abruptly at 7000rpm. Nothing like that.
Nice idle at 1000-1200 rpm.

Edit again:
When this bike had 836cc, a CX-7 cam was used for a while. I remember that engine was much louder than before with an almost lost of handlebars around 6500-7000rpm when cam kicked in.

I have afterwards got thoughts about carbs that were not correctly jetted. It might have been a lean needle area, main jet kicked in fully around 6500-7000 rpm.
A milder cam have not same need of fuel as the CX-7 which is not far away from a 125-70/75.

I have 2 dyno runs I have attached here and in the Dyno thread.
101whp (1005cc)  October 2020 (the head used then is mounted now, thinner base gasket).
October 2021 getting 104whp, 970cc.

Edit:
It has slightly better torque now. 104.7whp May 23 2022

Thanks about your wheel comment!
Yes, it behaves better, especially compared with the old tire.
I guess next tire will be better. 130/80-18 will increase the circumference a little and fill the fender. The engine really need one gear more.

Smaller rear sprocket will probably remove some fun.
Today 18:48. 18:43 an alternative. Clutch might complain...

18:48 and 130/80-18 tire will give even numbers in kmh.
4000 rpm -100 kmh
8000 - 200
This in theory, CB750  gearing chart I found on this forum.

[PeWe]

A better photo with both alu rims on. Front last year, rear last week.


A UFO tried to beam the bike up, but I grabbed it in time

[Medyo Bastos]

The new rear wheel finally on the bike today. It was new over a year ago when I laced it.
3" wide to get proper tire fitment of 120-130 wide tires.

I have a new set of Michelins waiting 100/90-19,  130/80-18.
That rear will fill the gap  better;)

Stock 2.15 can only use max 4.0-18.
My old 120/90-18 was on a stock rim. That's why its that flat and really tired!

Test ride in a perfect riding weather! Sunshine, blue sky and not windy. Only 15C in the shadow, sun warmed up.

Last weekend was rainy and windy. Perfect head swapping weather!
Head removed on Friday, another earlier looking K2 head,  34mm F2 IN valves with lash caps/28mm Kibblewhite K EX valves back on and cam timed on Saturday. Carbs synched on Sunday.

This head is shaved 0.5mm so I advanced the cam a little. Punch marks side by side for a few crank degrees earlier.

Engine sounds harder now Compression tester show 215 PSI.
No pinging when twisting the throttle thru 4000-5000rpm area.

Previous head 205 PSI. (392 with 33.5/28.5mm valves)

Its guide heights/valves not sunken enough made stem reliefs for keepers to pass into valve stem seals.

Bike seems to have better torque after the swap. Will be fun to see how the dyno graph will show the butt difference.

Will try one step bigger main jets after throttle test in higher speeds where it felt lean around 180-200kmh. Reduced a little made it to be happier.

Below the cam advance and K2 head that finally has got ti retainers. Risky with F2 in valves.

I understand the good thing with running lash cam timing.

Measured cam with 0 lash at 1mm (0.04") lift and 1.25mm ( 0.05")

Finally at running lash too.
Lash IN 0.10mm, EX 0.15mm (0.004/0.006")
20/47.5*  lift 9.80mm
49/12.5*  lift 9.12 mm
Cam: 125-75 as before.

This head use F2 IN valves, Kibblewhite 28mm K  EX valves.
Chamber volumes close to previous head.


The old K2 head's port design

The higher compression with cam advanced a few degrees. Both heads have very similar chamber volumes.

Good head is hard to find, eh?


Sent from my iPhone using Tapatalk

[PeWe]

I have a good matching 392 head that need its guides/seats adjusted for 10mm lifting cam.

The K2 head that is on now work fine. A CB750 enthusiast will notify its old for the K6.
Both heads recently refurbished/ported.

[Medyo Bastos]

Great looking stuff for sure! Those wheels eould look great on my bikes


Sent from my iPhone using Tapatalk

[MauiK3]

With that much power it must feel like a rocket ship.
How about vibration, is it about the same?

[grcamna2]

That's looking real good PeWe 

[PeWe]

Thanks for your comments guys!
The engine runs fine without specific vibrations.
I guess the balanced APE crank with Carillo rods + lightened alternator helps quick response with less vibrations.

When carbs are right it runs smoother. The carbs jetting has changed with displacement and compression. Needles affect from very low rpm.

Rims and spokes from
https://www.central-wheel.co.uk

19 x 2.15 Non Valanced alloy rim
18 x 3.00 Non Valanced alloy rim  (MT profile).
Stainless spokes.
Rear rim use shorter spokes that feels sturdier.

Front tire a little slim, 90/90-19.
I have a new 100/90-19 waiting to enter with new rear 130/80-18.

I often think about modern bikes with much more power when I accelerate on the fun country roads.
I have no need of more power when speedo needle reach 190 kmh very quick.
The faster runs to test only.

But I should not mind if a dyno run should show 120rwp

My normal cruising speeds around 80-100 kmh with some additional 160 kmh on fun and beautiful country roads as the photo above and below.

[PeWe]

WOT and AFR.
About my latest Dyno where it looked like to be very rich.

Just whacking the throttles wide open from low rpm can show very rich when jet(s) are not too rich.

Here seems the needles to be rich plus main jets.
I lowered needles just a little, M3 washer 0.5mm and it started to run lean.
Lowered a full notch and engine ran fuel starving lean from 4000 to 6000 rpm.

Choke or a quick acc pump squirting throttle twist fixed it.

I have earlier verified needles and  main jets in accelerations to high speed, 180-200 kmh where bike reacted slow at the top.
Reduced throttle just a little and it became happier.
Main jet one step bigger solved that behavior.

Accelaration stronger at high speed after the correction.

The power curve is strong up to 9000rpm. 200kmh just after 8000rpm.

I'm sure this can be a carb depending thing too. The Mikuni TMR 32 can deliver fuel and that quick.

I have tested the acc pump that squirt fuel thru all spigots like 4 water guns.




I found a chart that supported my thoughts about AFR at WOT so I can continue using the jets I have found work well by road tests. 

The picture below from this site.
https://help.summitracing.com/app/answers/detail/a_id/5230/~/engine-basics---air%2Ffuel-mixture



AFR 11.5 seen to be OK which match the Dyno AFR at WOT  rather well.

Main jet is now 1 step smaller (145)  so another WOT test should show a leaner curve than used 147.5 during the Dyno run.

[SOHC4 Cafe Racer Fan]

From 93 to 104 and torque at 90 ft lbs. Impressive.

[PeWe]

Hi and thanks!
The torque is in Nm.
90.7 Nm ----> 66.82 ft lb


Same numbers in ft lb would be fun. The clutch should complain when using the power

Last dyno look fine but could have been running 500 rpm further.
It does not look like it flatted out at 9000 rpm where it stopped. The curve were still rising.
Maybe another pony lurking there....

Bike feels powerful. The parts runs in synergy making power.
Before around 90whp, compression was lower. DP315 cam.
Later RC295 gave 93 whp.
Same compression as today should give some hp more.

****** EDIT: ********
Compression by my cheap tester 185-190PSI when getting 90-93whp,  4-4 exhaust. Yamiya no numbers got 93 as the later Lotus Root 4-4. The bikes original head with 5mm valve kit, ports really opened. Bronze valves survived 20.000 km.

101whp billet block 1005cc- around 200 PSI K2 head.
Last autumn 104whp 970cc a little over 205PSI. 392 head.

Last dyno with same head (K2) as the 101whp run, 215PSI compression, 104.7whp.

K2 head has F2 stock invalves 34mm, lash caps for the soft stem and get similar lenght as K valve. Stock size Kibblewhite 28mm ex valves.

392 head has Kibblewhite valves 33.5/28.5mm.
That head might be slightly better ported.
*****************
It is very relaxing to sometimes ride my almost stock brown K2, 57.85 whp DIN, 56.67 Nm (41.8 ft lb)
(K7 pistons give higher compression). Its K7 cam might not improve over a stock K2.

I had to dig deeper into the Dyno swamp and correction factors DIN, SAE, STD vs uncorrected.

This to compare different dyno runs on different days, locations with different climate...

https://www.svtperformance.com/threads/sae-or-std-when-using-a-dyno.76757/#post-907101

[PeWe]

Checked the size of pilot air jet (PAJ) today. I have concluded that it is too big when the pilot circuit can not be properly jetted by changing fuel screw, pilot jet, needle jet and needle height.

Found it to be #130. I thought it should be closer to 100 after some internet searches on other bikes using TMR carbs.

This can explain the need of relatively big pilot jets and fuel screws between 1/2-3/4 for OK idle but always wrong between 3000-4000rpm. Smaller pilot and smaller needle jet fixed that but on the expence on acceleration that sucked.

It can be the pilot air jet (PAJ) that needed to be replaced by a smaller long time ago.

This will hopefully richen up the pilot circuit where needed not making idle-3000 rpm too rich.
I'll see if it will work with existing pilot jets or if they need to go smaller.

Order of removals after pod filter:
Filter adapter had to go off first followed by next part held by 2 allen head M5.
Really thin sharp threads on the filter adapter.
It felt as it had some thread seal/locker. Hopefully no future issues with them. I might not find the proper PAJ size at first attempt.

This adapter can be replaced by velocity stacks.
They are in different lenghts. Webike japan has.

The Pilot air jet gets unfiltered air! Design flaw by Mikuni.
That explains some of my thoughts that carbs feels like they change, dirty pilot circuit has been in my mind for some years.
The hole has threads so maybe possible to place a nipple there for a hose I can route upwards a little.





An irritating seal I'm sure can cause assembly issues when fiddling with the carb 2-3 still on bike.  A little oil will hopefully hold the seal in place vertically.


Main air jet (MAJ) plugged as the part chart show.  Size 00.
Optional jetting.
Can be useful if a little lean at the higher part of main jet area
Air thru here + bigger main jet.
(If I have understood it correctly where I read about it )


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This little sucker that most likely had made jetting between idle to 4000 rpm very tricky

A nearby shop have them in stock, the hole series in steps of 5.
 I need to guess size to avoid investing in a complete serie as I have done with pilot jets and main jets for both TMR and VM29 smooth bores, needle jets included.
Brass with golden prices.

If 120 will be too big too.
Difficult to read the size, side light and  photo to be sure.

[grcamna2]

Looks good PeWe.
I appreciate the education.

[PeWe]

I write about it when doing it so others can get information.
Good for me too when look at it later.

I found a Mikuni design issue, the Pilot air jet is not filtered nor protected against dirt into the carbs!

There are full of fine dirt around that area.
I have had several issues where the pilot circuit behaves as it changes,  like dirt inside.
Now I found it.

I have to find an M4 hose fitting, 45 degrees.
Or a 4mm pipe, thread it and bend it a little. Hose  up over the carbs where it get less dirt.
Pipe 4mm OD, max 2mm ID. It must be possible to thread it M4.

Threaded hole in filter adapter/Stack holder. It is outside the filter or stack.
Tons of fine dirt to be sucked in and block.
The threads must be there to hold something. Threads are not a dirt stopper.
Same dirt seen outside must be sucked into the pilot area.

This explains why carbs work better one day, another not.


Below when that holder is removed, air jets visible. Only PAJ that is used.


There is 9mm space for a fitting. The holes have M4 threads that must be designed to hold something.

This reminds me about the vent fittings that had no hoses when carbs were delivered. Found that 5 years later. I bought the carbs early 2015.

#
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RS34 has airjet filtered.

[grcamna2]

I understand.
Mikuni must have designed the PAJ to work with only atmospheric pressure rather than filtered air being blown past them at a forced pressure.I wonder if you can find a very small 90 degree elbow with 4mm special threads ? The other option is to custom make 4) small elbows;possibly design a custom filter to hook all the 4 hoses up with so that no more dirt gets pulled into the pilot circuits of each carburetor. 

[PeWe]

Searched internet for copper tube OD 4mm inner 2mm. To cut in correct lenght, thread outer M4 and bend ca 45 degrees.
Only in China.
The other alternatives too thin gods, 3.1mm inner gives not enough for threads.


Found these 2 M4 nipples.

This 90 degree bend will probably work if threads are cut a little shorter.
Will use that model.


#


This can also work if shortened.
There is max 9mm under the filter adapter. Hose to be fitted without kink.

[grcamna2]

PeWe,you may be able to adapt those elbows in the top picture if the threads are identical..
I suppose the outside height of the hexed-area could be cut-down a bit on the threaded area and modified? to give you 9mm of clearance.

[PeWe]

I think it can be oriented correct with a flat side against the filter adapter. Fixation with loctite.
Thread look too long so I'll cut it to appr 3 mm.

Would be fine if it sit towards 9-10 o'clock.

[grcamna2]

I hope you use another sealant besides Loctite;you may need to remove it again to change the PAJ for a different size later.