Topic: VM29 Jetting Specs

[Turbogrimace]

What are you guys running in your VM29's on CB750's and with what setup (headwork, cam, valves, intake, exhaust, location for average altitude and Temp).

I just put a set on an 836 with 10.9:1 CR, ported head, OS intake valves, Yosh Daytona cam, velo stacks, and a 4-2 exhaust.  So far I've tried 120, 115, and 110 mains.  I'm still running rich throughout the throttle with the 110's in.  I'm surprised by this because a stock CB750 with VM29's calls for 115 mains and this engine is much more than stock and has a lot more flow potential because of the stacks and exhaust (potentially, not sure of the back pressure difference created by the exhaust).  Could I actually need a smaller main that what's recommended for a stock engine?  Does that make any sense?  more efficient flow, more draw on the venturi? 

This same engine ran a bit rich on stock carbs with 120 mains in as well.

Any thoughts gents?

[bwaller]

What size air jet are in them? A larger size will provide more air to the emulsifier tube. I have a set on my 550 race bike and unless I took the mainjet out completely I couldn't get it rich enough. One size lower on the airjet made a big difference in this case.

I want to say the 750's should use a 0.9. If what you have is smaller then this should help, otherwise go smaller with the main until it's happy. An efficient engine is just that. 

[Turbogrimace]

I have a 0.9 screw, 20 pilot, 110 main right now.  The air screw is out two turns and is still a tad rich.  I've heard on occasion that even a stock cb benefits from a smaller screw down to about a 0.7.  But a lot of this is really fine tuning and most of the ranges can be achieved with the 0.9.  Since I'm 2 turns out, I could see going a little bigger.  Anything passed 3 turns and I'll bump it up.

[Xnavylfr]

TURBO,
Contact a guy named George at   http://www.wgcarbs.com/
he specializes in these carbs and can probably give you the answers you need!!


Xnavylfr(CHUCK)

[Turbogrimace]

Thanks Chuck!  I'll check that out.

My main reason for bringing this topic up was to see if we could develop a list of configurations for different engines that use these carbs.  It'd be a decent resource for people when they try and tune them up.  From what I've read around this site and others is that the standard jetting for the main is between 115 and 120.  Some drop the air jet size for a richer idle.  The engine I'm working on now is tending to need a leaner setup than what I've read.  I have to throw in some new plugs and fire it up again and we'll see what that 110's doing.  115's fouled out the plugs in about 10 miles, which was very very surprising.  I barely made it home.

[bwaller]

I agree these are usually an answer to fine tuning but in my case I couldn't believe the difference one size made. Perhaps a silly question but you're sure the orifice from that airjet to the emulsifier is completely open? Try 17.5 pilots.

[Turbogrimace]

im as sure as I can get without seeing light through it. but the air screw isn't going change anything.much other than idle right? Or am I missing something here.

[bwaller]

http://forums.sohc4.net/index.php?action=dlattach;topic=93535.0;attach=157290



Have you checked float height, and is there a chance the needle/needle jet are worn?

[Turbogrimace]

Wordddd.  I am tuning based on that chart. 

It's been a while since I checked the float height, what should it be as I have heard different values?

I've replaced all mains, pilots, and needles, the only thing I didnt replace was the air screw i believe.  I'll have to go back and look at my notes.  only the main and pilot were significantly corroded, others were fine, but I replaced them just to be sure.

If the air screw was worn out I'd feel like it would make it run leaner than it should. 

[bwaller]

I believe proper float height is 23mm but I use the tube method and set fuel level to 2.5mm from the lower edge of the carb body.

[Turbogrimace]

how do you do the tube method on these carb bowls?

[bwaller]

I've got a fitting that replaces the bowl plug and has a nipple for a hose. Actually check Z1 they might sell something.

[PeWe]

Time for continue this Mikuni VM29 smoothbore setup thread.
My carbs have also too rich setting. They did not have that 1984. Difference now like hotter cam, BUT rigid carb boots might have changed things to how it should be, before sloppy Mikuni soft rubber that flex in-out, must have had less flow.
I remember that my local tuner reacted on my increase of both pilot and main jets that ended up to 25 vs 127.5

Setup is like  Mikuni CB750 setup except for main and pilot jet.
Application Honda CB750 (SOHC) -1978
Carb Model VM29-A2
Main Jet N100.604        #115   (My jets 127.5)
Air Jet BS30/97          .9
Pilot Jet VM22/210       #17.5   (My jets 25)
Jet Needle   #5DL31-3
Needle Jet #261 Series  0-8
Float Height            23mm
Throttle Valve VM29/15  1.5

Very interesting to compare with Kawasaki and Suzuki settings that use bigger pilots 25, but they have different needle jet. Suzuki has upon this also different throttle cut out, 2.0 instead of 1.5.

I started to think in correct way after reading this thread with very interesting guidelines in pictures that I have attached below with comments from the thread: http://www.kzrider.com/forum/3-carburetor/432938-mikuni-vm29-problems?start=20

How to perform clear tube verification about float heights seems to be very good step too. This is the final verification of how the carb works.

My next step will be downsize the pilots and mains. 25-->20 (maybe 17.5) and 127.5-->125
I hope the Pilot will enter an adjustable range with air screws like 1/8-2 1/2 turns out.
I'll order 3 setups of pilot jets just for sure, 17.5, 20, 22.5.
Main jets: I have complete setups in 2.5 steps from 120-130

- Anyone that have a similar setup with perfectly working VM29's?
836cc, CX-7 cam, ported head with 34mm IN valves

[Ace]

CB750 F2 Ported head, 63b cam, stock header pipes with straight through exhaust, K&N pod filters, air 0.9, pilot 25, main 120, needle middle air screw 1/2 turn out. Smoothies did bring it alive. Altitude, close to sea level.

Took a while tuning the carbs, went through lots of pilot jet sized.  Good luck.

[PeWe]

CB750 F2 Ported head, 63b cam, stock header pipes with straight through exhaust, K&N pod filters, air 0.9, pilot 25, main 120, needle middle air screw 1/2 turn out. Smoothies did bring it alive. Altitude, close to sea level.

Took a while tuning the carbs, went through lots of pilot jet sized.  Good luck.

Good information!
- Do you have these KN pods?
http://www.knfilters.com/search/product.aspx?prod=rc-2454
(I had smaller cones earlier, the new ones have like a short rubber velocity stack)

I have a rather open 4-1, Alpha.  Like Yoshi pipe, but 60mm instead of 70mm.

My air screws were also only 1/2 turn out when I used them many years ago.

[MCRider]

SUDCO has VM29 parts and a book on tuning. Original setup as for a Z1 Kaw is what I'm using on an 888cc, mild cam ported head, 10.25 CR. I'll get specs later and post. They list set up for several bikes.

[PeWe]

SUDCO has VM29 parts and a book on tuning. Original setup as for a Z1 Kaw is what I'm using on an 888cc, mild cam ported head, 10.25 CR. I'll get specs later and post. They list set up for several bikes.

OK, that part of the Sudco catalog with the carb setups for CB750, Kawasaki and Suzuki is attached.
Kawasaki setup according to Sudco:
Main Jet N100.604        #120
Air Jet BS30/97          .9
Pilot Jet VM22/210      #25
Jet Needle   #5DL31-3
Needle Jet #261 Series  0-6  Here is the difference that affect the bigger pilot and main?
Float Height            23mm
Throttle Valve VM29/15  1.5

This might be the explanation for differences in pilot and main jettings. Some guys have the CB750 carb, other Kawasaki or Suzuki Vm29 setups?
I bought mine new for my CB750 from my local tuner.

[Xnavylfr]

You guys have to remember the 750/836cc will in NO way flow as much air as the Z. The Z I had was bored to 1075=73mm pistons and I had a helluva time getting the 29s tuned for (correct) for it. The 29s tend to work better with a little smaller jet for the topend because they are less restrictive than the Round tops.


Xnavylfr(CHUCK)

[Ace]

CB750 F2 Ported head, 63b cam, stock header pipes with straight through exhaust, K&N pod filters, air 0.9, pilot 25, main 120, needle middle air screw 1/2 turn out. Smoothies did bring it alive. Altitude, close to sea level.

Took a while tuning the carbs, went through lots of pilot jet sized.  Good luck.

Good information!
- Do you have these KN pods?
http://www.knfilters.com/search/product.aspx?prod=rc-2454
(I had smaller cones earlier, the new ones have like a short rubber velocity stack)

I have a rather open 4-1, Alpha.  Like Yoshi pipe, but 60mm instead of 70mm.

My air screws were also only 1/2 turn out when I used them many years ago.

My filter were conical.

Also slides were 2.0, just remembered that.

[lucky]

Thanks Chuck!  I'll check that out.

My main reason for bringing this topic up was to see if we could develop a list of configurations for different engines that use these carbs.  It'd be a decent resource for people when they try and tune them up.  From what I've read around this site and others is that the standard jetting for the main is between 115 and 120.  Some drop the air jet size for a richer idle.  The engine I'm working on now is tending to need a leaner setup than what I've read.  I have to throw in some new plugs and fire it up again and we'll see what that 110's doing.  115's fouled out the plugs in about 10 miles, which was very very surprising.  I barely made it home.

Main jets  do not foul out spark plugs unless you drove it home on wide open throttle all the way home.

[lucky]

What are you guys running in your VM29's on CB750's and with what setup (headwork, cam, valves, intake, exhaust, location for average altitude and Temp).

I just put a set on an 836 with 10.9:1 CR, ported head, OS intake valves, Yosh Daytona cam, velo stacks, and a 4-2 exhaust.  So far I've tried 120, 115, and 110 mains.  I'm still running rich throughout the throttle with the 110's in.  I'm surprised by this because a stock CB750 with VM29's calls for 115 mains and this engine is much more than stock and has a lot more flow potential because of the stacks and exhaust (potentially, not sure of the back pressure difference created by the exhaust).  Could I actually need a smaller main that what's recommended for a stock engine?  Does that make any sense?  more efficient flow, more draw on the venturi? 

This same engine ran a bit rich on stock carbs with 120 mains in as well.

Any thoughts gents?

Make sure the jet blocks are not cracked. Many used sets have broken jet blocks.

You need to get the slide needle set correctly because that controls 1/4-3/4 throttle which is what you will be using most of the time.
It is never even mentioned.

Motorcycle CB750.
What year?
Altitude ? Are you in Colorado?
4 into 2 exhaust.
Velocity stacks.
Slide needle clip position?
Mixture screw adjustment?


Once you give me more facts I could tell you how to jet this bike to have a chance of getting close.

BTW...
Float bowls should never be almost full because they would be higher than the overflow tube in the float bowl and be spilling out gas all the time. Think about it.

[PeWe]

CB750 F2 Ported head, 63b cam, stock header pipes with straight through exhaust, K&N pod filters, air 0.9, pilot 25, main 120, needle middle air screw 1/2 turn out. Smoothies did bring it alive. Altitude, close to sea level.

Took a while tuning the carbs, went through lots of pilot jet sized.  Good luck.

Good information!
- Do you have these KN pods?
http://www.knfilters.com/search/product.aspx?prod=rc-2454
(I had smaller cones earlier, the new ones have like a short rubber velocity stack)

I have a rather open 4-1, Alpha.  Like Yoshi pipe, but 60mm instead of 70mm.

My air screws were also only 1/2 turn out when I used them many years ago.

My filter were conical.

Also slides were 2.0, just remembered that.

AHA!
Close to the Sudco Suzuki setup then..
Main Jet N100.604        #115
Air Jet BS30/97          .9
Pilot Jet VM22/210      #25
Jet Needle   #5DL31-3
Needle Jet #261 Series  0-6 
Float Height            23mm
Throttle Valve VM29/15  2.0

Smelly business switching jets
I'll try pilot 20, 125 main, or 122.5, or 120   Pilots just ordered, so I have to wait a week and have time to decide main jet size.

This thread is going to become a VM29 jetting information database as Turbogrimace the thread starter intended

[lucky]

The Mikuni website says default main jet size would be 122.5's

[PeWe]

I'll try 122.5 mains, 20 pilots next time.  Maybe I'll soak the holes in the carbs with carb cleaner when changing jets, just for sure.
Waiting for pilots........

[Turbogrimace]

What are you guys running in your VM29's on CB750's and with what setup (headwork, cam, valves, intake, exhaust, location for average altitude and Temp).

I just put a set on an 836 with 10.9:1 CR, ported head, OS intake valves, Yosh Daytona cam, velo stacks, and a 4-2 exhaust.  So far I've tried 120, 115, and 110 mains.  I'm still running rich throughout the throttle with the 110's in.  I'm surprised by this because a stock CB750 with VM29's calls for 115 mains and this engine is much more than stock and has a lot more flow potential because of the stacks and exhaust (potentially, not sure of the back pressure difference created by the exhaust).  Could I actually need a smaller main that what's recommended for a stock engine?  Does that make any sense?  more efficient flow, more draw on the venturi? 

This same engine ran a bit rich on stock carbs with 120 mains in as well.

Any thoughts gents?

Make sure the jet blocks are not cracked. Many used sets have broken jet blocks.

You need to get the slide needle set correctly because that controls 1/4-3/4 throttle which is what you will be using most of the time.
It is never even mentioned.

Motorcycle CB750.
What year?
Altitude ? Are you in Colorado?
4 into 2 exhaust.
Velocity stacks.
Slide needle clip position?
Mixture screw adjustment?


Once you give me more facts I could tell you how to jet this bike to have a chance of getting close.

BTW...
Float bowls should never be almost full because they would be higher than the overflow tube in the float bowl and be spilling out gas all the time. Think about it.

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

I've rebuilt the engine since I first asked this question because I lost compression.  here are the current specs:

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

I've been riding the bike for a while now and it runs great, but it has a slight stumble at cruising throttle (just off idle).  It is continuously rich even when I jet down like mad.  I'm currently running 115 mains, 15 pilots, air screws out at 1.5, and the needles on the #1 clip (needle is as low as it can go).  It's still showing a little soot and the stumble off idle is still present.  This jetting seems really low in comparison to the other numbers out there.  Here is a shot of what the plugs look like with this jetting.



The fact that I keep jetting down makes me think my junk Sudco boots are leaking.  I recently built a support for the carbs, but I havent been able to ride the bike enough to see if it's made a difference.  I'm going to check the boots for leaks as soon as I get a chance.  Lucky, any thoughts on where my jetting should be based on the specs I listed?