Topic: Mikuni Needle Jet differences 0-6 vs 0-8?

[PeWe]

Anyone that had changed the Needle Jet and got a feeling of the steps between the sizes ?  Mikuni VM29

I have far too late discovered that my old VM29 were configured as Sudco describe for Kawasaki.
Needle Jet 261 series size 0-6.

Sudco say 0-8 for CB750. My engine is tuned so it might need even richer. (836, ported head, cam)

I had to use 35 pilots to achieve not too lean and overheated at 4000-5000rpm with these too small Needle Jets. Idle too rich.
EDIT: Main jets 130, needles 1 step up (clips 1 step down from middle) air jets 1.1 (1.4 did not help too rich idle)

I'm thinking about 2 steps up of needle jet to P-0  from 0-6 when it need to be so much richer at this range.
Experienced carb tuner might know...

I'll find out.... after some changes and $$ invested

[01Thomas]

Hi Pewe

I'm currently fitting and tuning a set of VM-33 smoothbores to a Bimota KB-3 with a Kawa GPz1100 (now 1170) engine and I have some data on the needle jets.  I have attached a file from SUDCO which may help you.


regards
Thomas

[PeWe]

Thanks Thomas,
Very small differencies indeed.

I'll know if I need bigger when I have tested the P-0.  The name of the sizes is verified in your attachment. O often written as 0, like 0-6 vs O-6.

All my jetting tests have given me some experience of the VM29's. Pilot jet must be correct for smooth idle without fouling plugs, black smoke. In my case 20, maybe 22.5. 17.5 give clean smooth idle
Too lean at 4000-5000 rpms, Needle jets. I might get a complete range of sizes until the carbs are correct. I'll round up with correct mains in the end. I have the complete setup from 110 to 145 in increments of 2.5.

[Gearheadgreg]

Had a mid range flat spot for years with a 45HSR flatslide , did it all richer Jet Needle, set the clip to richest setting, even went so rich on the slow Jet that i killed the sparklers to black in 1/2 mile...finally after some pro advise went one size up on the Needle Jet and Flat spot mid range was gone..only took 7 years lol,,gas millage went from 30 mpg to low 20's now 

[PeWe]

WOW! Only 7 years? That was quick!
My carbs made my rods bluish, RC 836 forged pistons got extra ventilation on top. Year 1984. I blamed the cracking Mikuni carb boots.
Used CB750 K6 carbs after that with other fresh rods and pistons. Bike for sleep fall 1990. Bike restored and started October 2013.... some test drives this year suffering very hot engine.... again......

I decided to increase pilots until it ran OK at 4000-5000 rpms. size 35 did it (needles one step up 4:th slot from top)
Pilots of size 20 give smooth and not smelling idle, needle jets must do the rest.
I hope I will finalize my VM29 carb tuning after 30+ years . They look very fresh when not much used.

[Gearheadgreg]

the whole midrange is different now and even the exhaust note has deepened around 3-4000 rpm, motor seems effortless in the midrange, could prolly go on more step richer on the needle jet, K6 carbs are the PD pumper carbs ? if so i have them on my F3 ....they are so lean from the factory, i stepped up the slow one step richer also with factory stock 4-1 headpipe and stock muffler for back pressure, midrange also has more punch now, also shimmed the Non-adjusting stock needles with washers. 

[PeWe]

I used the old type of CB750 carbs, no pumps.
http://www.cmsnl.com/honda-cb750k6-four-european-direct-sales_model14384/partslist/E19.html#results
Not perfectly jetted: pilots 40, needles 1 step up, mains 147.5 but it ran pretty well
I should have increased pilots 1-2 steps

Still waiting for the letter from Niche cycle supply. My local tuner had only 2 needle jets in stock

[PeWe]

P-0 needle jets solved too lean at 4000-5000rpm together with 22.5 pilots.
Idle still too rich, much better with 17.5 pilots.   D-D-D-D-D sounding engine, too rich.
I like the RRRRRRRRRRRRR and nice exhaust aroma

Smaller pilots, better idle ,then too lean again at 4000-5000 rpms on 5:th gear.... worse when opened the air screws 1/4 turn more when idle was to the rich and stumble when start to drive the bike on really low lift.
Mains still 130

Even larger needle jets once more.... probably up to P-4.  Pilots might be 15 as I have home.
I do not give up smooth idle and correct at higher throttle lift. I see the light in the tunnel.... Mikuni made different versions of VM29? Some with missing holes inside or too many in need of drastic jetting?

I have tried some combinations of pilots + mains... purchase a complete series of needle jets is next

[PeWe]

Update:
It is the needle that cause lean at 1/4 lift. Larger needle jets (P-4) do not do that big difference.

5DL31 is too thick (lean) at this throttle lift. It should help if the second taper started ~4 mm earlier. I have not found a needle that has that spec OR thinner from 20mm otherwise the same as 5DL31. Only the much richer needle 5F12. I'm sure that I must go down in needle jet and main then.

I have lifted the needles max. Waiting for better weather to verify if that helps. Test drive must be minimum 50km  when the too lean start to happen after 30 minutes.  I drive to friend 25km away, stop and talk some bull, then go back home. The engine usuallay run rather bad then begging for choke.
It would be better with a needle that make engine to run fine with clip at middle which make it possible to adjust in both richer or leaner if needed.

It is the standard needle 5DL31 that must have too large pilot causing too rich idle and fouled plugs... I have pilot 15 now and rather good idle with needles max lifted. Clip in 5:th groove

Below the needle chart that mostly cover leaner needles than 5DL31 except for 5F12 which is 6.5mm shorter that hopefully work well with correct needle jet and main jet.
5DL31 is a double taper type, 5F12 single.

I hope this will be my last order for these carbs. I could have invested in new CR31 carbs instead of all jets
Edit: Added the taper dimensions and where on the needle.
https://jetsrus.com/a_jets_by_carburetor_type/needle_mikuni.html

[01Thomas]

Thank you for this update.
My project is waiting for the smallest possible idle jets (#10), (ebay) to arrive and then I'll continue with my VM-33 on the Kawa GPz1100 motor.

I eventually noticed that my present idle jets had been inconsistently drilled (by some PO) by probing them with a thin needle: on one idle jet the 0.585mm diameter needle went right through the jet while on the other three jets it didn't, so I'm bringing everything back to a known base.

[PeWe]

See my last jetting update here: http://forums.sohc4.net/index.php?topic=114046.50

Very important that the needle jet is not too small and/or needles to lean. My bike had to be driven over 30 minutes to reach temperature where the jetting became too lean due to Needle jets/Needles. That tricked me the first months of testing.  Before that too early advance due to sloppy springs, flowing floats (clear tube show if OK) fouled plugs on idle regardless of pilot sizes.