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

[PeWe]

I have Loctite 222, a weak version. Perfect to not let nuts and bolts come loose on their own.
Front wheel holder nuts, front brake disc nuts, sprockets(front sprocket bolts came loose on my K6, one missing the 2nd almost off Loctite after that), rear nut holding exhaust etc.

Release later feels like a nylon lock nut that need force all the way off.

Loctite 290 is very thin and can be applied afterwards which ensure no locker into the carbs. A very small drop when nippli is mounted, capillary action.

This nipple does not need to be removed for future jet replacements.

The jet sit inside of the part where this nipple is to be mounted.
See photos in earlier posts.
The filter adapter/stack holder sit with 2 x M5. Remove it and jet is visible.

So the nipple can sit really hard. But no need when hose will stop it from rotate.
I have Loctite 638/648. The latter is thinner, better for threads. Otherwise bearings and similar stuff.

[PeWe]

Weekend job fixing the carbs pilot air jet hose connections with filter.
The hoses might restrict as smaller air jets will.
My nearby shop had jets and a small filter. All hoses need to have same lenght if they will restrict.
ID 3mm, OD 5.5, squeeze all 4 into the 3/4" filter inlet. Some tape around the 4 hoses making a square of 12mm. Maybe some oil hose material  around.

Old air jet was 130, bought 115 and 105.
Nipples will fit fine when I have shortened the thread side by 2mm. Now hit the bottom.





I need to remove the float bowls, pilot jets, main jets and fuel screws. Spray the circuits with carb cleaner and gentle compressed air to blow the pilot air jet from inside so the dirt will go back out the way it entered.

I'll try 1 size smaller pilot jets when assemble everything with the new pilot air jet filtering system with smaller pilot air jets (PAJ).
Start with 115 which can be enough step from 130.

[grcamna2]

That elbow looks like a tight fit.

[PeWe]

It was easy to adjust the nipple.
An M4 nut on before grinding a little bit shorter, ca 1mm.

The bench grinder did it like butter. A little bit too quick.
Removed nut that fixed the thread start.

I also massaged the M4 threads in the adapter with a tap that gave an extra mm.

Rounded off the nipple top.
No need to grind the filter adapter.


I have to remove tank to find a good place for the small filter, OD 35mm
Height 31mm, with spigot 42.5mm
My CB750 will soon have 5 pod filters.

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Filter adapter removing tool


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A shiny spot 6 o'clock where Pilot Air Jet take air!
So dirt into carb!

[PeWe]

I had to take off the carbs off the bike.
Remove float bowls, pilot jet, main jet, fuel screws.

Pilot air jet (PAJ) easier to remove whith carbs off since Mikuni had tightened them very hard. No meaning to overtighten. The jets can not fall out. Filter/Stack adaptor cover them.
Good idea to remove all jets on new or old carbs when getting them to document the jetting and make sure they can be replaced later without stuck jets.



Main air jet were pigs to remove too. Saw they sit in another angle.

Sprayed all holes with carb cleaner. Most carb cleaner sprayed to clean outside.

I noticed that no air flow thru the MAJ holes. Compressed air in hole for main jet, pilot jet and fuel screw. Full throttle did not help.

Compressed air thru hole for Pilot jet blew very well thru the Pilot Air Jet which is the big air supply to pilot jet.
This explains the need of big pilot jets as I thought.

Carbs almost ready to go back on bike. Only the clear ID 3mm (EDIT: changed to 4mm) hoses to be mounted onto the nipples for PAJ.

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Fuel screws with spring, washer and o-ring in correct order.

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Nipples are on. Hoses next followed by the filter adapters.

I'll change the direction for carb 3 nipple, aiming towards 4.
Nipples are grinded to fit both ways and tested with filter adapter for clearance. Same as 1-2.


I'm curious about how the pilot circuit will work now with less air with PAJ 115, 130 before.
The hose must restrict a little too.

Pilot jets 1 step smaller. From 35 to 32.5.

I might need to step down 1 size further to 30.

Good with no dirt causing issues and eventual synch problems too.

[grcamna2]

The carburetors look good Per 
I think it might be possible to drill passages for all 4) 3mm hoses from your PAJ elbows to allow them to share into one small K&N filter to:combine them ?

[PeWe]

Yes, all 4 hoses will use the small KN filter.

[grcamna2]

Yes, all 4 hoses will use the small KN filter.

Four small hoses to share the one K&N breather filter ?

[PeWe]

Yes, all 4 hoses will use the small KN filter.

Four small hoses to share the one K&N breather filter ?

I'll see if I can just strap all hoses together, rubber tape and into the filter without pinching the hoses.

[PeWe]

I bought new hose today. 3mm is what I had home but 4mm fit the nipples better so 4mm it is.
Must improve flow too. Restriction in nipples except for the air jet itself with even smaller hole.

Green hose, ID 4mm now on the carbs.
300mm long that I think should be enough for a good filter placement.


I got a 2nd thought about pilot jets so I reduced one step more to 30.
Pilot jets get much less air now. From jet size 130 to 115 plus the nipples and hoses.

Hoses stacked and sealed together with vulc tape. Outer layer of 3M electrical fiberglass tape. Both tapes a must have.
Vulc tape does not like oil and fuel but very useful.

Filter squeezed on, finalized with a black strap.


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The lower straps around green air hoses near the nipples removed when not needed. Pinch risk.


Nipples sealed with Loctite 290 and fixed after hose routing, test with filter adapters.
290 is a very thin threadlocker, thinner than water.
Made for thread lock/seal efterwards when nut/screw is tightened.

[grcamna2]

Looks good. 

[PeWe]

Thanks,
Hopefully back on bike tomorrow.

Carb test rides continue. I hope my guess on pilot jet size is correct

[Medyo Bastos]

Great job, keep us posted on jet sizing, vroom vrooom !


Sent from my iPhone using Tapatalk

[PeWe]

Yes, carb tests is a very good reason for a ride
Jets will be written here.

[PeWe]

Quick update.
The hose lenghts seems to be OK.

I hope that the warm location will not cause any problems with warm pilot air into carbs.

[PeWe]

I talked to a guy at my local shop about the non filtered TMR pilot air jet.
He had seen TMR40 carbs on Suzuki with filter adapter including the airjets.

When back home I searched and found.
https://www.japan-webike.se/products/24600898.html#specification
Yoshimura saw the problem.
.



This part above replace these assembled 2.

Of course more....! Air filter..
My adapters for 54mm filter, Yoshimura has 50mm.
Out of stock so search for other sellers.

But my ventilation system might work.
I have a feeling that the hoses restricts enough so I need to go back to the stock pilot air jets 

This adapter can not be that old. I should have seen it earlier when looking for TMR parts at Webike.
I bought my TMR's early 2015

[PeWe]

Test ride today.
Smaller pilot air jet tested to verify if filter hoses limit.

I'll test 1 step smaller Pilot jets tomorrow.
 Changed direct when home after the ride, engine still hot.

I'll see tomorrow if not raining.

[grcamna2]

I'm glad to see that you have gained success on those pilot circuit modifications Per 

[PeWe]

Maybe right jet combo finally found
Installed bigger Pilot Air Jets (PAJ) today (again).
A 130km ride adjusting fuel screws.
Found in the end of the ride that fuel screws must be less than last ride with PAJ 140. Strange....

I have found that Mikuni has another jet that fits TMR carbs as PAJ. Same size (4.0x6.5mm), only a small design difference on outside

Much more common and often cheaper.
Sub number -  size.

Low speed jet
N100.606-145
Mikuni Super Bn carburator

Pilot air jet
N211.100-145
Mikuni TMR carb

TMR  PAJ to the right.
The more common jet to the left. Installed today, 145.
Compared with TMR 140


Need a longer ride to verify last setting found late at todays ride with rain showers.
Acc pump stroke lenght to be increased again. Now shortest.

[grcamna2]

The PAJ of 145 now? seems like the largest air jet you can install ?

[PeWe]

The PAJ of 145 now? seems like the largest air jet you can install ?

There are even bigger out there
Webike had not TMR jets in stock until late September.
I found alternative when picture searched.

It is a little bit strange that I had to increase PAJ. But it still need relatively small fuel screws 3/4 for OK idle.
Probably due to relatively big pilot (35).
The only jet I have not tried to change is the needle with different taper, thinner the first few mm of lift.
If that could make smaller pilot possible. Probably smaller PAJ.

Bike runs better now from Idle to 4000 rpm. Plus up to 9000 which has worked before

Yoshimura designed their MJN system for theses carbs. I wonder how tricky they are to setup.
https://www.bikebros.co.jp/vb/sports/sfeat/yoshimura-carburetor-en/

[grcamna2]

The MJN's.. Great 'eye candy'  Lol !
Yoshimura has designed them for the racetrack ?

[PeWe]

Not only racing according to the article, tuning customers too.
Good fuel consumption which I have noticed can be lower than stock K6 carbs, stock engine or not. In legal speeds
I have not seen consumption less than 6L/100km on my almost stock K2 with K6 carbs (K7/8 pistons, cam).
This bike with 1005cc and much more power, almost down to 5L/100km in legal speeds.
5.5L/100km more often.

Same as K2 when twisting some and higher cruising speed.

I guess that the PAJ and pilot jet setup has to be found anyway. MJN does not change that.

The article mentioned that std needle deliver fuel when throttle is completely closed (idle) which I have noticed.
I raised needle 1 step (1mm) and got super rich from idle and up. This with the 1 step smaller needle jets.

All jets need to be correct, if not, the underpressure in carb will suck fuel where possible. With bigger PAJ the pilot circuit can breath better and deliver a fuel mix in its area, release underpressure affecting needle/needle jet.

More sensitive with 970/1005cc than 836.
But the idle to 4000 rpm crusing lift has been an issue all the time.

[newday777]

The MJN's.. Great 'eye candy'  Lol !
Yoshimura has designed them for the racetrack ?

Eye Candy Indeed and the technology game changer.

Yes the track but very street worthy flow too according to the writeup. But where to buy them.......

[PeWe]

Both Yamiya and Webike have had Yoshimura MJN32 for CB750. Filter option.

In UK, 1 set available
https://www.ebay.co.uk/itm/261740453643?hash=item3cf0f25f0b:g:myUAAOSwMKFcNqot

But no filter adapters that costs another $250US + KN filters 50mm fitting.
A little bit too much.

I bought my Mikuni TMR32 from Germany, stacks or filter adapters mentioned when order. Those filter adapters like mine, PAJ filtering to be done which does not cost much.

https://www.bikeside.de/MIKUNI-TMR32-flat-slide-carburetor-for-all-CB-750-Four-69-78
Here price with EU tax

Mikuni throttle kit not needed.

Stock CB750 works fine, just add an M6 nut with cut side so it can be fitted over the wire and used at the return side down the carb.

I started to think MJN during my ride today. My K6 is well worth a set! 
My other engine project i planned VM29 for could use my TMR32 I almost have found out the best jetting for!
$3000US shipped and taxed a little bit too much.
TMR 32 almost half price!!