Topic: VM29 Jetting Specs

[PeWe]

I'm feeding them with Shell V-Power we have here in Sweden. Yes, the old nice gas with led was different. It's only good if I can use smaller jets and engine running strong, clean and less gas consumption.
I hope I'll have time to change to 17.5 later today and report here!

[Retro Rocket]

I'm putting the VM 29's on a 900cc {888} MTC kitted engine, hopefully your work will make mine easier....

[PeWe]

This might be helpful then... Tuning Manual
http://www.generalfiles.biz/go/154097798300

Important information here (page 4 fig5) about VM carbs is that the pilot affect the main fuel system, not only low.
Page 6: Pilot Jet and the low speed fuel system.... good info...

[bwaller]

Possibly the new combination of components is creating better vacuum.

[Ace]

I'd make sure all the passageways including the pilot circuit were clean.  I wouldn't go as far as ordering pilot jets down to 110 just yet.

Black smoke from the exhaust, maybe just clearing out the crap in it.

Are you using the proper intake rubbers from Sudco or stock CB750 intake rubbers? Maybe your problem lies there as well.

It would be interesting to see what combinations of jetting you have tried and the results eg. 125 mains/22.5 pilot/airscrew 1 turn out = rich.  It may help others as well.

[PeWe]

I'd make sure all the passageways including the pilot circuit were clean.  I wouldn't go as far as ordering pilot jets down to 110 just yet.

Black smoke from the exhaust, maybe just clearing out the crap in it.

Are you using the proper intake rubbers from Sudco or stock CB750 intake rubbers? Maybe your problem lies there as well.

It would be interesting to see what combinations of jetting you have tried and the results eg. 125 mains/22.5 pilot/airscrew 1 turn out = rich.  It may help others as well.

I use OEM CB750  intake rubbers. I have modified the carb setup, carb 2 10 mm closer to 3. Cut and welded the aluminum...projected started 20 years ago when a friend cut and welded the zinc mixed aluminum..aluminum professionals. I described that here
http://forums.sohc4.net/index.php?topic=119335.msg1350058#msg1350058
Probably better vacuum now.
I'll find the pilot and then go for mains. 17.5 hopefully OK. If not order size 15.

[PeWe]

17.5 pilots installed. 120 mains as before
Still not good. Air screws affect different now. Working range < 1 turn out.
Black smoke when revving, less than before.
Next step will be smaller mains, 117.5  Order sets of jets again. 117.5, 115.

Maybe I need to switch back to 20 pilots when they affect each other according to the Sudco Mikuni tuning manual that explains the VM carbs and tuning very understandable. (link in earlier post)

[PeWe]

Status:
Waiting for main jets. Ordered sets of 4, 117.5 - 115 - 112.5 - 110. 

I'm prepared with plugs as well when the tuning eat plugs. 3 set of fouled plugs now.  1 set NGK Iridium, 2 sets D8EA.
I'll continue with last fouled plugs at next attempt with 17.5 pilots and 117.5 mains. I aim to not getting black smoke when twisting the throttle. I have fresh plugs
DR8ES-L worked really fine back in the days.

[PeWe]

I think I have found the jets.
pilots 22.5. mains 117.5
The roads are too cold for driving. I have to wait for high speed road test. 

It is possible to step up mains to 120 if too lean at +200km/h
Maybe pilots will follow in opposite direction then (20) when pilot and main work in parallel according to SUDCO tuning manual and I have verified with my jetting experiences described in this thread.


Todays test and final jetting.
Test 1
New plugs NGK DR8ES-L
smaller mains, 117.5 pilots still 17.5
Lack of gasoline, like clogged fuel lines, petcock.

Test 2
bigger pilots 22.5  mains 117.5
Bigger step up of pilots when gasoline was really lean.
Engine runs rather good now.

I think the hotter cam has an affect like more nervous idle and little bit black smoke when revving the engine  from 1500rpm to 4-5000rpm.

[PeWe]

I have tried another combination of pilot and main jets. I think I have found the combinations that can be used in road tests.
It seems that the cam used affect the idle running a little bit rough and need to be cleared by revving the engine.

All tests I have done since beginning of last month. The green combos can be possible for road test when the season is back.
I think that "pilot 20  main 117.5" is good OR "pilot 20  main 120" depending on how the engine behave in road test.

The main jet affect idle much more than I thought. Pilot and main are really parallel circuits as the Sudco Mikuni manual say.

I'm also thinking about another cam, like DP295 and get smoother idle. (I'll take out the engine and replace the HD cyl studs anyway)

Old jetting from 1984:
Pilot 25  main 127.5   
Way too rich in the new setup with CB750 much stiffer OEM carb boots than the soft and very flexible Mikuni boots delivered with the carbs back in the days.

2/11
Pilot 22.5   main 122.5
Still too rich

3/11
Pilot 22.5      main 120
New plugs NGK D8EA
Today much better, good enough for a ride.
Huge difference when revving the engine between 1000-4000 rpm.

11/11
Pilot 20 pilot   main 120
still rich on low, 1000-2000rpm.

12/11
Pilot 17.5      main 120
Still not good. Air screws affect different now. Working range < 1 turn out.
Black smoke when revving, less than before.

16/11
New plugs NGK DR8ES-L
Pilot 17.5     main 117.5
Lack of gasoline. like clogged gasoline lines, petcock.

17/11 *  setup that works rather well.
bigger pilot 22.5   main 117.5
Bigger step up when gasoline was really lean.


19/11
change when running rich on idle.
pilot 22.5 smaller main 115
Result: worse, not rich

28/12 *
pilot 20  main 117.5
Engine runs rather well on idle.
Still a little bit rich on idle.
This must be the cam (CX-7) and/or combination with ignition that might need to be slightly later.

EDIT: Next is to adjust the ignition on idle, appr 1500rpm. Turn the plate to get a more stable idle. Probably later setting (clockwise)
I read somewhere that advanced cam timing will give higher dynamic compression when the inlet valve will close earlier.

[Ace]

My high lift cam was lumpy at idle with the smoothie, tried two different cams, webcam 61B and an Ivan Tighe 369 (Australian cam grind). Idle was 1400-1600 rpm it would idle at or otherwise die.  Considering you are most likely going to ride around at a higher rpm, maybe you'll have to sacrifice idle for mid/top end performance an less stress on tuning a bike since it won't idle for long.  I like a strong midrange cam as I don't race that often  and that's why I bought a higher performance bike, the CBR1100XX.

[Medyo Bastos]

i get more consistent results with autolite plugs.   

[PeWe]

I'll try to improve the idle when I'll not race with the bike. City driving and nice country roads. It must be possible to wait for green light and engine running rather smooth. OK if idle is around 1500 rpm as long it's not fouling the plugs.

About other plugs... I checked Autolite and found Autolite 4164 (for CB750) which was cross referenced with NGK D7EA on another site. Autolite 4163 is referenced to NGK DR8ESL that I use.
I think that a warmer plug will change idle to the better, but might be too warm for the road.

- DP295 might be too big step backwards?  The numbers here seems to be very modest. http://www.dynoman.net/engine/cams/timing-cards/DP295.pdf

EDIT: Changing the ignition timing did not change the problem at idle.(moved the plate 2-5 mm in both directions of actual that is spot on F at 1200-1600rpm. It runs fair but accumulate rich so it will stop when twisting the throttle, slow or quick. Must restart with plenty of throttle, a rich cloud of smoke from the exhaust.  I can hear occasional carb backfire pops that indicate lean setting or too advanced ignition.

I cannot use smaller jets (tried that), all cylinders are the same when checking the plugs. The carbs did not have any issues with fouling plugs at idle back in the days. Floats looked OK when I checked them.

I might give warmer plugs a try and see what happen before changing cam that will confirm whats wrong or not.  A DP295 cam might be good to have anyway when the grind is the famous RC295.   Order more stuff..... $$$   

EDIT again: 4x D7EA ordered from eBay.co.uk. I need to know if these will change to the better. If it will be better the carbs must be OK...
DP295 will most likely jump in later on....
836 is too small displacement for a CX-7 cam?

[PeWe]

  FINALLY!! 

The engine is running very well with NGK D7EA. No road tests yet.
Checked the ignition again, almost spot on F at idle 1400-1600rpm (leaning to late), full adv @~2800rpm
I'll increase mains to 120 before road (pilot might be one step bigger too 22.5). Test later in spring when the climate is warmer.

I'll find biggest jets possible to compensate the warmer plugs.
The Global Warming is not enough here 

[Turbogrimace]

Sweet deal PeWe.

I'm finally getting back into tuning mine up again.  Here's my specs and performance right now:

ENGINE:
CB750K/836cc with 10.9:1 CR
Megacycle 125-75 Cam
JMR Stage II Ported head (1976 K head)
33.5 mm intake valves
Stock exhaust valves
VM29 Smoothbore Carbs with crappy Sudco adapter boots and Ramflo filters (recently added a bar to support the carbs to minimize potential boot leaks.)
4-2 exhaust, relatively unbaffled
Altitude: Sea Level

SETTINGS:
Pilot: 15
Main: 115
Needle Position: #1 (needle is in the lowest, leanest position) with #5DL31-3 0-8
Air Jet: 0.9
Float Height: 23 mm
Throttle Valve: 1.5

PERFORMANCE: 
Nearly perfect except for off-idle performance under cruising load.  I am planning on installing larger air jets to lean out the off-idle mixture and see if I can get rid of the inconsistency.  my picture above shows the condition of the plugs at these settings.  after a lot of long distance riding, the bikes hesitation starts to become more prominent.  Once I start to clear it's throat on a few stop and goes, its a little better. 

Other updates that may affect the performance: I've added a bar under the carbs to support them rather than relying on the cheezy Sudco adapters.  I did a propane leak check and didnt notice any leaks, even with some physical harassment of the carbs (jiggle jiggle).  But that isnt to say that the vibrations of the engine at speed, along with the road, arent causing the boots to leak intermittently.  However, the even color across all four spark plugs indicates that they are all burning similarly. I'd be inclined to suspect a leak if one was slightly different (potentially richer). 

This January weather is getting me down, but the bike work is adding up, so it looks like it will be an enjoyable spring.

[PeWe]

OK, your jets are very small.
I think My carbs might need very small jets too. Warmer plugs cannot be the correct thing in the long term. I have not tested the bike on the road yet. Before it was too rich and very wrong jettings. I had to wait for new jets. Now waiting for Global warming or spring.

I have ordered a clear tube tool, float bowl button plug with a clear tube.
http://www.z1enterprises.com/ItemDetails.aspx?itemDescription=Fuel+Level+Gauge+Tool+for+VM29+smoothbore+carbs&item=ZSM-VM29FT

I must see the actual fuel level inside the bowls. It must be correct, not too much or too little. The floats might be wrong adjusted, stucked or bad valves. (Or just fine)

When this is done the engine will go out when I need to replace some HD studs and double check an o-ring under the cam towers anyway. Other complete cam towers with everything on too :-)

After that new cam timing with more advance (~4degrees) if float levels are OK. I have read that late inlet valve closing can cause reversed airflow on low rpm ending up in very rich condition at idle which is rather common when using hot cams (car forums). This can be the explanation in my case.

[PeWe]

Now possible for road test, not perfect but better each time
More adjustments. Float levels were too rich. High fuel level in all 4.
All float tangs were earlier untouched. I had to bend them rather much upwards to the float valves. I had no flowing VM29 carbs 29 years ago as I recall. -Different fuel nowadays that affect the floats?

Photos after adjustments. Now a little bit low to ensure no flowing carbs at idle.

January 29 Update:
 pilot 20 17.5, main 120, air screws 1/2 turn out

Today even better! Not that rich fat smelly exhaust fumes after change of pilot jets to 17.5. Larger is definitely not actual in my engine. I have tried some combinations the last months.
My worst problems were:
- Too early ignition advance caused fuel squirting carbs.
 Solved with spring fix, cut 1/2 turn on each spring

- Floats not adjusted, too high fuel level. Fuel Level Gauge Tool from Z1 Enterprises helped to fix it. I was 100% that floats should be OK since I did not have that much problem 30 years ago with these carbs. Therefore only a quick check that all 4 had same measurements.
I might visit my local tuning company and complain about their setup of the carbs I bought new from them 30 years ago

Remaining issue:
-Clear tube also showed that carb 4 fuel support is not that good. Visible when letting the fuel flow freely out from the clear tube to remove bubbles. Fuel stopped to come after draining the float bowl (the other 3 flowed freely).  I think I heard the result today when engine started to sound lean after a few minutes on clean idle, best so far. This might have tricked me to think lean and too small pilots. I got to check/solve this.
Maybe OK when driving the bike leaning it more to the right, fuel will flow easier into 4

Next is to change the plugs to typical heat range. I'll try Denso X24ES-U next time

Other parts as written in SUDCO manual. (verified when I had carbs appart)
Air Jet BS30/97 0.9
Jet Needle  #5DL31-3
Needle Jet #261 Series 0-8
Throttle Valve VM29/15 1.5
Needles at the middle notch
Float Height: 23 mm is a starting point. Final adjustment by clear tube routine to see the real thing, fuel level.

Carb boots CB750 OEM much stiffer without curved 2-3 boots as Mikuni. Possible when carb rack adjusted, carb 2 10mm closer carb 3.

836cc NOS RC cast pistons that need wider chambers as I have.
Ported K6 head 34mm inlet (F2 valve), std EX. Stage "street" 30 years ago.
CX-7 cam timed with symmetrical lobe center in focus.
Lightened alternator.

I'll wait for road tests. Taking out the engine again is no longer actual. Next winter after road experiences.

[PeWe]

Going richer!
Too advanced ignition made me to jet too lean to avoid fouling spark plugs. Engine ran as crap on the road
This might be the explanation of dífferent jettings when comparing different bikes.

Reason: play in adv unit and not checked the real TDC relative the markings T versus case index mark. (F for ignition)
Advancer unit at max counterclockwise: T align with the case index correctly. Advancer unit as max clockwise, a new case index mark 3mm at the right of the real mark. 
Ignition now correct. At least much better.
Pilots 25, mains 120 which runs too lean. I need at least 125
I'll try 22.5, 125
Changed cam from demanding and fuel drinking CX-7 to a more moderate Action Fours SS-1 that offer better driving.
CX-7 is a race cam! I miss the scream from 7000rpm

[Retro Rocket]

Keep the updates coming PeWe, I have some VM 29 smoothbores that i'll be attacking very soon...

[Turbogrimace]

Glad you're working your way through PeWe.  I got your message, sorry it took so long to respond.  Anyhow.  here's an update on mine.

Based on a suggestion from someone in this thread, I upped the size of the air jets to get rid of a slight stumble while cruising with the throttle only slightly open.  I went from 0.9 to 1.1 and the range of the stumble significantly decreased.  At this point I could almost attribute the stumble to the large cam disagreeing with the lower rpms and the throttle position.  The engine is smooth all the way through the rpm range and pulls well without stumble even in 5th at 30 mph (really low rpm).  The throttle response is quick and doesn't bog.  I have a bit of popping at idle if the idle goes low, but if the idle is kept a bit high, it's as smooth as glass.  I can kick start it cold without the choke and it behaves nicely.  I really like the megacycle 125-75 cam for the purposes I'm using it for.  With all of the other engine mods, I have tolerable behavior at low rpms and idle, good street manners through the mid range, and an excellent power curve up top for some good fun.  The low rpm behavior is what you'd expect from a big cam, but it's never been a bother for me at any point.  it can be very lopey but just keep the rpms up.  Later on, I might bump the air jets to 1.3 just to see if it gets any better, but i'm pretty happy with the performance right now.

I'm very happy with it.  It might be a tad rich in some spots, but you cant have it all.

here are the specs as it stands:

ENGINE:
CB750K/836cc with 10.9:1 CR
Megacycle 125-75 Cam
JMR Stage II Ported head (1976 K head)
33.5 mm intake valves
Stock exhaust valves
VM29 Smoothbore Carbs with crappy Sudco adapter boots and Ramflo filters
4-2 exhaust, relatively unbaffled
Altitude: Sea Level

SETTINGS:
Pilot: 15
Main: 115
Needle Position: #1 (needle is in the lowest, leanest position) with #5DL31-3 0-8
Air Jet: 1.1
Float Height: 23 mm
Throttle Valve: 1.5


I hope this helps some other fellows out when they start playing with the vm29's.

[PeWe]

....
I upped the size of the air jets to get rid of a slight stumble while cruising with the throttle only slightly open.  I went from 0.9 to 1.1 and the range of the stumble significantly decreased.
....

Aha.... thank you very much for that
This is what I have too.  OK something new to order... (have a OEM TEC ign plate coming while they still exists)
My engine hesitate when giving the throttle from the beginning at very low throttle lift.

I have not tested last jettings. Should have done it this evening but my job interfered...
I increased mains from 120 to 125 when the bike ran faster when releasing the throttle at 170-180km/h
Pilots from 25 to 20 when plugs were black. I blamed the hesitation at low lift on too rich pilots.

But your jets are really small.... and needle too..You use the fuel better than my bike and all others out there do...

My carbs have std setup as Sudco state for CB750.
836 10 something CR
Ported head, opened chambers (same as bore 65mm) that decrease CR a little bit

Cam Action Fours SS-1 (Good street and touring cam)
At 0 lash and 0.004" (1mm) lift:  IN 21/53  EX 54/22 duration 254/256. Lobe center 106/106
Lift: 9.18/9.18 mm (0.361")

I have unfortunately got used to the CX-7 bahaviour at 6000 rpm and over...

34mm inlet valves, 28mm ex. All new with an extra radius cut for better flow at low lift.
Mikuni VM29 thar are adapted for CB750 carb boots. carb2 sit 10mm tigher to carb3. Had to shorten throttle axis...

Verified floats with Clear tube. They floaded the carbs and fouled plugs. changed until the level was 3-4mm under the float bowl gasket.
Sea level
I'll try points again later on   Maybe next week!

[PeWe]

Pilots 20, Mains 125.
Need bigger mains.... next will be 130 (bad response over 180km/h)
Bigger Air jets ordered 1.1 (now 0.9)

I'm back from a test drive without Dyna S that I removed.
New complete plate with TEC stamped on it went in yesterday evening. Set it today. Static timing finally OK on 1-4, 2-3.
http://www.hondachopper.com/garage/carb_info/timing/Static_and_Strobe_Timing.pdf
Spot on when checked it with strobo lamp, idle as well as full advance. Bike ran much better with new TEC low-tech. Hondaman box next....
I can drive with H4 headlight ON without too low charging voltage. LED that supervise voltage

[PeWe]

Update:
Got the new Airjets of size 1.1  ordered on eBay. I removed the old jets with carbs still mounted, unscrew them all then pulled them out with a sticky rubber stick.

I finally understand why it has smoked and fould plugs on idle despite smaller pilots that have made me crazy... smaller pilots until popping sound on decel did not help.
The airjets mounted were not 0.9. They were 0.6! I wanted to see std 0.9 when checking them last year.
Now 1.1, good idle without black smoke after 30 seconds and need of revving to clear and avoid stop.
Next bigger mains and pilots as I wrote earlier.  pilots 22.5 OR 25, mains 127.5 OR 130 might be step by step just for sure.
I'll update when I've found out. I hope 130 will be enough, if not I need to order new main jets again.....

[Turbogrimace]

sweet deal.  0.6 airjets?  Yikes.

[PeWe]

Time for an update of my latest Mikuni VM29 experiences
I have learned the hard way that the test drive must be minimum 50km (30-40 minutes). Good road and drive 100-150km/h. After 30 minutes of driving will eventual lean due to needles/needle jet appear.  I usually drive to a friend 25km away. Stop and chat with him for 10-20 minutes and back home again. During this return the bike had started to run too lean at 4000-5000 rpm due to needle jet, needle.
I had thoughts about richer needle at this lift around 20-25mm (Thinner needle or tapering starts 2-4 mm earlier). NO alternatives found except for 1 much richer. Many needles available are leaner than 5DL31

This evening best result ever! Global warming or warm winter make it possible to drive the bike in November, pitch black and raining... 
- air jets 0.9   (1.1 caused too lean at 4000-5000rpm with this combo. Needles at max lift did not help)
- pilots 15      (17.5 was too large with this combo)
- needle jet P-0 (richer than std O-8) I have tried larger, P-4 not better
- needles 5DL31-4 (std needles 1 step up, richer)
- throttle cut out std 1.5
- main jets 130
- air screws 1/2 turn out (more has always been worse independent of jetting. More popping sound at deceleration/throttle off)

Idle rather good, not as wet as often before sounding D-D-D-D instead of RRRRRRRR.  Engine stumble a little when driving away at very low throttle lift. Can be fixed with air jet 1.1 but that will cause too lean at 4000-5000 rpm when warm.  Carbs can be better synced.

Not too lean at 4000-5000 rpm as I so often have encountered earlier this year after 30-40 minutes.
Temp in oil tank this evening was around 80 degrees C. Ambient temp over +10C!!!! Maybe 13C
(To be compared with 100-110C when I had O-6 needle jets, needles at middle. Close to fry the pistons again)

Could not test main jets when it was too rainy and too dark. I had severe problems to see the road in 130-160km/h and thinking about all animals running around here. I think they can be larger than 130.

Next to ensure whats possible or not
I'll test to change pilots to 17.5 and needle jet to O-8. The other as above. To see if this can be an alternative when changing exhaust from 4-1 to CB750 4-4 no number K0 style without defusers.

I had too small needle jets, O-6 which is recommended here:
http://www.z1enterprises.com/Z1CarbGuide.aspx
CB750 setting here will run way too lean and risk for melting pistons. 29mm smoothbores    CB750
I know when that happened with my VM29's for 30 years ago with:
- air jet 0.6 (larger will add more air into the pilot circuit, leaner)
- pilots 25
- needle jet O-6
- needles 5DL31-3
- throttle cut out 1.5
- mains 127.5
- air screws ?

This jetting map is a help. Niche Cycle Supply has some jets as well.