Topic: VM29 Jetting Specs

[Ace]

I'd be happy if my plugs looked like that instead of the black sooty plugs I've got.

[bwaller]

What about a larger air correcter? Originally I found the opposite, couldn't get it rich enough, until I installed smaller air jets.

[Turbogrimace]

So i checked for leaks using propane around the intake boots.  No leaks at all.  I'll check it a few more times, but I feel that if the boots were leaking, it'd be shocking if they were all leaking the same.  All the plugs looks the same.  Perhaps at this point my off-idle stumble is due to imbalance.  I bench-synced the carbs with a drill bit, but I know I can get a lot better with some vacuum ports.   With such a big cam, it might be possible that the carbs need to be tightly balanced for smooth operation at lower rpms. 

Sync Sanc Sunc... Sync them allllll!!!!!!

another thing of note is that my gas mileage is only around 35.  I'm surprised it's that low.

[Turbogrimace]

What about a larger air correcter? Originally I found the opposite, couldn't get it rich enough, until I installed smaller air jets.

What do you mean air correctors and air jets?

[bwaller]

In the inlet throat of the carb you'll find the air correctors. These meter the amount of air supplied to the emulsion tube for fuel atomization. Your's may be too small. They're tiny & delicate so you may need to grind a screwdriver to fit, but remove one to see what size you have, probably 0.7 or 0.9?? Order some larger, they're cheap & available. Check with Z1 enterprises or Sudco.

[PeWe]

#
Well this is an old, but helpful thread.  Thanks for digging it up PeWe!
#

I'm just selfish I need advice from other CB750 guys with tuned engines with VM29 carbs and their jetting experiences.
Your thread is a good start since it contain facts about one case. (Now with more setups since other VM29 guys have joined the thread! )

I read Kawasaki threads and understood that the VM29 jettings differ due to the different VM29 setups from the very beginning.
CB750, Kawasaki and Suzuki have different std setups in the Sudco catalog, not only mains and pilot.
My current setup worked 1984. Not too rich, I jetted richer when the bike ran a little bit lean.
Popping sound during decelerations (pilot) and bike accelerated when release the throttle at full speed, (lean main). pilots 25, mains 127.5 was rather OK then.

After the problem with cracked boots I used original carbs. Needles one step up (clip one step lower) and mains 147.5! Worked very well, maybe a little bit rich, but not what I experienced when driving, visible when lifting the head many years after.         

2013: Hotter cam , CX-7 (close to Megacycle 125-75) , stiffer carb boots and NGK iridium plugs + PAMCO Ultimate ignition.
I think I can jet the carbs OK by using jets closer to the initial recommendation.

I hope that more people can write their VM29 setups here as this thread asked for from the beginning.
If all 4 carbs behave equally it is smaller risk for cracks or other problems, right?
I can imagine that all 4 can become dirty, oxide that block the internal passages. But too rich, float levels OK must be the jettings inside.
If the carbs are bought used, the PO might have drilled somewhere? Marks on jets say something else than the reality.
- Air jets can be blocked? It must be easy when they are placed very open.
Thread with air jet talk http://www.jdsleds.com/community/viewtopic.php?t=5206&p=37206

Here another good carb tuning links.
http://www.3cyl.com/mraxl/manuals/carbtune/carbtune.htm
http://www.dansmc.com/carbs2.htm
I have to spray my carbs with carb cleaner. They were very clean but I saw some white powder when opening them to check the setup. Oxide that can still be there and need to be flushed away.

[Retro Rocket]

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[Ace]

Could be unrelated however I had a mechanical advance unit that wasn't going to full advance (other issue with it as well that I didn't know) and that contributed to stumble issues cruising 30-40 mph. Swapped out the advance unit for another one and problem reduced, then back to changing pilot jets and all good in the end.

147.5 main jets wow.

[PeWe]

Could be unrelated however I had a mechanical advance unit that wasn't going to full advance (other issue with it as well that I didn't know) and that contributed to stumble issues cruising 30-40 mph. Swapped out the advance unit for another one and problem reduced, then back to changing pilot jets and all good in the end. ................................

I have similar "problem".  When checking with a timing strobo lamp, almost full advance all the time. I tried to change it to the F direction, engine did not like that. It ran worse. I have to check the advance mechanism when the ignition base plate is close to center position. I do not think that close to any end position of the slits is correct.

Rich setting? My new plugs look nice, right?... bike not driven only revving outside my garage, 1800-4000 rpm, still waiting for pilot jets...

EDIT: One plug has black threads too. My mistake, forgot to tighten it cyl 2.  It was tightened just a few turns so the threads were full of soot. Plug hard to tighten when threads were lightly carbonized. I lubricated the plug with copper paste, just a little to make it easier to tighten it.
Another mistake when sorting plug thread. I removed the tacho cable from engine, did not bottom it when putting it back so I created an oil leak flowing around the head to intake 1. Just a few minutes revving the engine was enough.

[Ace]

IMO rich looking at those plugs. Go for a ride, plug chop and see what the plugs look like, take photos and post back.

Carby tuning is an art with many variables and it can take a lot of time to sort out.

I initially static time then get the strobe light out later and check timing at idle 1200 rpm and then full advance and see if timing is between the advanced marks on the stock advancer. This is when I found the problems with my advance, 12 months after initially doing all this. How I came about this was to put the stock carbys back on and still had a stumble.  Checked advancer, problem fix, run with stock carby all good so back to VM29s and sort out again.

[PeWe]

The streets are cold and slippery here, the engine feels very explosive and fun to drive, risk for wheelies:-)  It will take a while when I hopefully get my pilot jets during next week. Upon that a new rivet link for my chain. Solid PL link won over the cheap riveting tool (DID copy). I need PLS link (soft hollow ends) + new  Heavy Duty riveting tool.

I checked the advance mechanism and it works fine and correctly mounted. I can easily turn the PAMCO rotor clockwise, it snaps back when release, not 180 degrees wrong either double checked the instruction and internet (again).
I have switched 1 and 4 ignition cable which can be why the timing look wrong despite engine running fine ignition wise. 
http://www.xs650.com/forum/showthread.php?t=18560
Pamcopete:
The other factor to consider is that the dual output coil produces a positive voltage on one spark plug wire and a negative voltage on the other. Most timing lights are made to work on a conventional distributor type ignition with a negative voltage going to the spark plug. The timing light will not work properly on a positive voltage wire and they produce erratic timing flashes. The answer is to switch your timing light to the other spark plug wire.

I'll update when I've installed smaller jets and hopefully got correct colored plugs, brownish grey..

[Turbogrimace]

In the inlet throat of the carb you'll find the air correctors. These meter the amount of air supplied to the emulsion tube for fuel atomization. Your's may be too small. They're tiny & delicate so you may need to grind a screwdriver to fit, but remove one to see what size you have, probably 0.7 or 0.9?? Order some larger, they're cheap & available. Check with Z1 enterprises or Sudco.

Thanks for the info.  I believe they're 0.9s  but I'll check again.  From what I read from PeWe's link, a larger air jet would lean out the mixture in the emulsion tube.  Does this air jet correspond to any specific throttle position (i.e. idle-off idle)?

Could be unrelated however I had a mechanical advance unit that wasn't going to full advance (other issue with it as well that I didn't know) and that contributed to stumble issues cruising 30-40 mph. Swapped out the advance unit for another one and problem reduced, then back to changing pilot jets and all good in the end.

147.5 main jets wow.

That has been the case on other engines I've ran, but I'm using a dyna 2000 on this one and the timing is spot on. 

[bwaller]

I breezed back to the beginning of the thread and obviously I don't remember everything I reply to, so I'm sorry for repeating myself. Must be an echo in here, huh?

If you have a .9 air corrector that probably isn't your mainjet issue but stranger things have happened. A larger air corrector will supply more air to the emulsion tube for larger throttle openings. If you continue to be rich at lower mainjet sizes, then try something larger than 0.9.

I see you had a stumble off idle, not sure if that's still an issue, usually if carburation it's a pilot jet issue, either partially blocked or sized incorrectly. Slide cutaways are 1.5? You mentioned synching with gauges, my understanding is that these carbs don't respond well to vacuum synching with the carb tops off. A good bench synch is recommended, and all I did.

[PeWe]

I'll check my new toy after I have changed my jets to smaller. Gunson carbalancer to verify the bench job with 3mm drill.
Gunson Carbalancer used to check carb balance on CBR600
I hope 20 pilot, 122.5 main will be OK in my case. These carbs need very small jets, a little bit strange....  I have more tricks written here to test.

[PeWe]

I'll got my pilots, but not all. I had planned 20 pilots, but have to try 22.5 when I miss 2 20's.
Mains 122.5. Lets see if the plugs will be less black or if I must clean them manually.

Some jettings for Kawasakis, a few tuned bikes with VM29's
http://kzrider.com/archive/viewthread.php?fid=26&tid=18321&action=printable

[PeWe]

Still too rich with 22.5 pilots and 122.5 mains.
Got the ignition correct today when it was at constant full advance earlier. Must be correct when jetting I suppose. http://forums.sohc4.net/index.php?topic=131339.0

Now possible to hear popping sound, better by less turns on air screws. Or is it the sooty plugs that misfires too?
Next: 120 mains, clean the sooty plugs (Iridium DR8EIX) and increase the gap to ~0.9mm (0.035") which I have read Pamcopete answered in some threads using the PAMCO ultimate as my bike is blessed with

[PeWe]

Update. Smaller mains, 120. Pilots still 22.5. I hope I do not have to change the pilots again and change back to 25 so I'll get the same jetting as Ace earlier in this thread Remove the float covers with carbs still mounted take time. They are attached with allen head screws so it's not that hard as it could be.

Today much better, good enough for a ride and clean the spark plugs which is unfortunately not possible yet, a few things to do first.

A huge difference even when only revving the engine between 1000-4000 rpm.

I have worked with the ignition too which make a difference. Advance curve vs pilot jetting have a synergy to find when they affect each other if I have understood the stuff correctly and what I have seen.
See my other thread http://forums.sohc4.net/index.php?topic=131339.0 were I have link to another very informative Hondaman thread. http://forums.sohc4.net/index.php?topic=7401.msg67248#msg67248

Next step back in with the NGK Iridum plugs with bigger gaps 0.035" (0.9mm)  according to Pamcopete's recommendation for the Ultimate coils I read somewhere.

[Ace]

Update. Smaller mains, 120. Pilots still 22.5. I hope I do not have to change the pilots again and change back to 25 so I'll get the same jetting as Ace earlier in this thread Remove the float covers with carbs still mounted take time. They are attached with allen head screws so it's not that hard as it could be.

Today much better, good enough for a ride and clean the spark plugs which is unfortunately not possible yet, a few things to do first.

A huge difference even when only revving the engine between 1000-4000 rpm.

I have worked with the ignition too which make a difference. Advance curve vs pilot jetting have a synergy to find when they affect each other if I have understood the stuff correctly and what I have seen.
See my other thread http://forums.sohc4.net/index.php?topic=131339.0 were I have link to another very informative Hondaman thread. http://forums.sohc4.net/index.php?topic=7401.msg67248#msg67248

Next step back in with the NGK Iridum plugs with bigger gaps 0.035" (0.9mm)  according to Pamcopete's recommendation for the Ultimate coils I read somewhere.

Fun isn't it! No one where I live wanted to tune the bike in so I did it myself.  To put it on the dyno it was $100, jet change and put back on another $100 and so on.

[PeWe]

Fun isn't it! No one where I live wanted to tune the bike in so I did it myself.  To put it on the dyno it was $100, jet change and put back on another $100 and so on.

Yes, its fun now when the engine runs rather well, just some small adjustments. Before too bad, rich, wrong ignition causing sooty plugs making the engine sound un-synched, too lean and just wrong.
I think I have found a good base jetting with 120, 22.5.

I've searched on internet about ignition advance curves which must be adjusted for the engine.
Honda recommended timing settings in the manual might not be actual when using high CR pistons, hotter cam (more duration, later closing of the inlet valve) and different fuel (now with alcohol). I believe that it must be close to F at low rpm and max advance somewhere between 2000-3000 rpm. Pilot air/fuel mix to match that. I have adjusted the advance springs so F will work fine at low rpm, full advance kick in at 2600-2700 rpm.

I'll check the timing again with the other plugs with more gap (0,9mm), adjust the timing so the engine will run smoothest on low rpm's 1000-1500 and see where it lands, but not too much advance at higher rpms. Last time set to F, I remember the engine could run smoother before or after F.

It would be nice with electronic advance system defining the curve exact what the engine setup need. I have to live with the springs or replacing them with other to achieve the exact curve needed. More advance at idle will cause more advance on higher rpm's which might not be the best choice and opposite.

Back in the days I adjusted max advance possible (by turning the base plate) without pinging sound when twisting the throttle on 5:th gear from appr 100-120km/hour (after static timing of 1-4, 2-3 with test lamp). But that was with the points. Higher CR need less advance.... If I have understood correctly. Many factors to please, some of them against each other with the simple advancer unit....

Interesting info about advance curves:
EDIT: My last test and information from Hondaman point to retarded ignition at idle as a better choice with a hotter cam, not advanced compared with the 'F' mark. Threads below interesting anyway to understand how it works.
Car engines, so what as long it's gasoline, right?
http://www.firstfives.org/faq/timing/timing_for_modified_engines.PDF
http://www.kitcarmag.com/techarticles/0809kc_tuning_carbureted_kit_car_engine/ignition_timing.html
http://www.classicinlines.com/IgnitionTiming.asp
http://www.rodandcustommagazine.com/techarticles/0604rc_ignition_timing/
http://www.circletrack.com/enginetech/ctrp_0404_jetting_carburetors/viewall.html

[PeWe]

Progress!
-New plugs from my local dealer today, NGK D8ES 0.7mm std gap. (0.0275") Not possible to continue with fouled plugs that will disturb the tuning. I have also ordered 8 extra NGK D8ES just for sure. Less than half price from England.
- Ignition OK.  My modified springs hold back the advance on lower rpms, full adv at 2600-2700rpm.
-Behaviour now is rich pilot. It did not help to exceed 1.5 turns on air screws. That gave pops when releasing the throttle when cleaning out the rich idle with black smoke.
Very close now My biker friend from 35 years back had the same feeling as I have about the jetting.

-Next step is smaller Pilots, 20 instead of 25 22.5. Keep 120 mains.

My friend covered his ears with his hands when I let my CB750 sing as before. I have also noticed the nice sound that was normal many years ago. I understand better now that the police and others were so crazy about my 4-1 back in time.

EDIT: Ordered 2 sets plugs, Nippon Denso X24ES-U just for sure when that specific plug has got so good reviews on this forum.  Still waiting for the missing pilots.

[PeWe]

Vm29:  20 pilot, 120 main
still rich on low, 1000-2000rpm.  next 17.5

[bwaller]

Keep working.  You're down to the pilot jet (17.5) they recommend for the CB750. Nothing like figuring it all out for yourself though.

[Ace]

Vm29:  20 pilot, 120 main
still rich on low, 1000-2000rpm.  next 17.5

How often are you going to ride around at 1000-2000 rpm? Do the airscrews make much difference to help lean it out? My bike idled around 1400-1500 rpm, only time it was at that rpm was on start up or at the lights.

[PeWe]

When warming the engine before ride, waiting for green light...  I hear (and smell) that the engine is not running smooth, typical sound for too rich and a fat smell that accumulates during idle that can be cleaned out by revving the engine with black smoke thru the exhaust. Sooty plugs, not nice.
I have not driven the bike yet, only standing still, in and outside of my garage.

The strange thing is that these carbs had 127.5 mains and 25 pilots 1984 when I used them in the same bike.
Russ Collins 836, ported head with 34mm inlet valves, Action Fours SS-1 cam (same numbers as Webcam 41).
-Now other pistons, old RC with hopefully similar CR and hotter cam CycleX CX-7 (like MegaCycle 125-75 + 3 degrees more)

I THINK that the carb boots affect the tuning. I changed the carbs to fit OEM carb boots when the Mikuni boots had very sloppy and flexible rubber that cracked very soon. Now very rigid new CB750 boots. Maybe 17.5 pilots will work better.

When I used the std Keihin, I used std pilots (40), throttle needle 1 step up and very rich mains 147.5. This worked fine with other pistons that gave less CR. Bosch silver, and recommended Nippon Denso plugs fouled within minutes while NGK just worked fine. The last ones were D8EA. Before them (in the middle of the 80's) I might have used NGK DR8ES-L which I have ordered a batch of 12 just for sure + a batch of 8 Denso X24ES-U + 8 D8EA.

I hope 17.5 pilots will work. One thing is that the idle and revving was very sooty when running with 122.5 mains, much better with 120.
I might get into same situation as the thread starter

Time to order smaller mains 4X117.5. 4x115, 4x112.5, 4x110?
I have not cleaned the pilot circuit yet, might be time for that, Carb cleaner is no longer sold around here. The carbs looked clean when I had them appart. They have been stored dry and warm in a shoe box since 1985 without gas inside.

[Retro Rocket]

One of the reasons your experiencing different results than the first time you used them could probably go down to the fuel quality, the fuel is completely different these days.... Keep going mate, I have some 29's as well and am following closely....