Topic: VM29 Help Needed ASAP

[BrockSamson]

Hey guys sorry to make the title seem urgent, but it sort of is.

I have been slowly trying to get my 1977 CB750K running.  It has VM29 Smoothbores.

In my project thread you can see all of the frustration and trouble shooting I have gone through:

http://forums.sohc4.net/index.php?topic=100624.msg1160387#msg1160387

I do not get many responses to the questions I have because not many people have the VM29 smoothbores.  The guys over at kzrider.com are trying to help me out... but they have no experience in my application.

Here is the problem:
1) I am noob and tried to get the bike running after cleaning the carbs and replacing internals.
2) I had vacuum leaks so I bought new "intake manifolds" from sudco.
3) They fit great no more vacuum leaks.
4) Bike is still not running how it should and I am at the end of my patience and putting in hours trying to figure out what is wrong.
5) Took bike to mechanic.
6) He just called me and said that there was nowhere to hook up the vacuum gauge to balance the carbs.
7) KZ guys tell me their manifolds come with a nipple for this.
there is no logical way to hook up a vacuum gauge to the carbs.
9) WHAT DO I DO??!!  Haha, sorry for the caps but I just don't know what to tell the mechanic.  He says his hands are tied until he figures out how to balance them.  I am searching all over and I just don't know what to do here.

Thanks and sorry for all the urgency.  The mechanic wants me to call him back and we were going to discuss what to do next.

Any advice would be very much appreciated.

-Brock

[MCRider]

I used the VM29s on a 888cc engine for many years and 50000+ miles. What a joy. Really, not sarcasm. Started quick, very little choke, idled smooth, very linear pull and breath at speed. Routinely got 45 to 50mpg at speed. 

No they don't have balance ports. Even if they did have you ever had the caps off? Pretty hard to work with anything there. I never balanced mine. If yours are running bad, its not likely the balance which is a fine tune issue. You need to get the coarse tune out of the way first.

I can't remember where I bot mine from back in 1982 or so. They came jetted for the 888 as i described it to the sellers. I only had to change the main jet a size or 2 which is really easy with carbs on bike, float bowls on as well. There is an access cap to the jets.

I hope you get yours running. If you are putting them on a stocker, its probably too much carb. Mine was 888, with a ported head. They are originally used as upgrades for the 1000 cc Kawasaki's which came with VM28s and needed more carb. (I think that's the story or something like that) A stock CB750 may be better off with the VM28s which would be an easy trade.

These carbs with CB750 manifolds (or without) bring really good money, routinely 400+ on eBay. So you have an exit strategy.  Sorry I haven't read your other thread for more background.

[MCRider]

Take it away from the mechanic unless he's a Mikuni specialist. If the carbs weren't bot from a supplier who set them up to your displacement, you'll need to work on the slow jets and needle position I fear.

Unless he's willing to proceed without balancing them. Does he know how to change the needle position? What size are your pilots and does Z1 have a selection?

But be assured they can and do work very well.

[BrockSamson]

I do have a backup plan.  I have a rebuilt set of 77 PD carbs, airbox with new rubbers and filter.  Not sure how it mounts up because I have never had it on the bike.  It also needs the choke and idle adjusters.

Here is a copy paste of my order from sudco:

1x - #005-026 Intake Manifolds
4x - #002-524 Gasket MC Top
4x - #002-502 Gasket Float Chamber
4x - #002-531 O-Ring Float Drain
4x - #004-004 20 Pilot Jets
4x - #004-187 117.5 Main Jets
4x - #002-154 0.9 Air Jets
4x - #002-268 Jet Needles
4x - #003-535 Needle Jets
1x - #005-230 Motion Pro Push/Pull Throttle Assembly

I have pod filters on VM29s and a 4 into 1 exhaust that seems to be very unrestricted.

MCRider thanks for taking the time to pass on the advice.

[MCRider]

Your welcome. As I said mine came set up and all I changed were the mains.

Embarrassed to say I don't even know what's in mine. Hope to use them on the current project which is a nicer 888. ASking thoose questions was merely rhetorical, I have nothing of substance to add. 

As mentioned if yours is a stock displacement with a stock head, they may be simply too much carb.

[bwaller]

The stock CB750 carbs are 28mm. Granted the Mikuni nearly hold their ID through the body whereas the Keihins shape is complicated and likely helps with mid range. So are the 29's too big? Try them & see, if they are, I have a beautiful completely rebuilt set of Mikuni VM28's that will fit in your manifolds........

Those smoothbore aren't easy to synch well with vacuum gauges anyway. If you are meticulous you can use 1/16" drill bits and bench synch them very close, at least from my experience.

From Jeff's Z1 site here are specs for CB750 jetting

Slide cutaway 1.5mm
Jet needle 5DL31-3
Needle jet 0-6
Main jet 115
Air jet 0.9
Pilot jet 17.5 or 20
Air screw 1.5 turns

I use a set on my race bike and I think that jetting info should be really close. I dropped to a 0.7 airjet, but I'm using them on a smaller engine though. The only thing I'd add is to try the needle clip in the #2 or 3 position. If I remember correctly the fuel level (using the clear tube method) is 2.3 or 2.4mm below the carb body flange. Do check these levels.

Good luck and let us know.
 

[Jim F]

Get with Bill Benton (Bellcow54 is his forum name)
He just got his very close for his 836 when we where at the dyno

[MCRider]

Wow, Now I'm saving this thread for resources!

[BR]

I just got a new project running right yesterday  that sounds alot like your bike. I am going to have to go or possibly 22.5s will solve this. Mine carbs had the additional accelerator pump added on when they were originally purchased by the first owner. The engine is a heavily ported, megacycle cammed 836 that was built in the late 80s and hardly ever ridden. When I got it it was having problems with the manifolds leaking so I ordered new ones....they sucked and are being sent back. Fortunately I dug thru the box of parts I got with the bike and found a real good set. I spoke to Mike Reick (who was very helpful in just running thoughts by him thank you sir) the mains are 120. I bench synched them when I went thru them, truthfully didn't even need it though.  Let me know if I can help somehow, I know I just got lucky and it didn't require a lot of tinkering, and I think the accelerator pump helps quite a bit on these as I used to put them on most of the KZ sets we sold back in the day.

I forgot to mention that you definately need to check to make sure there are no leaks at the manifolds with starting fluid or something as this will be a big problem if these are the same manifolds I had just purchased.
BR

[MCRider]

The stock CB750 carbs are 28mm. Granted the Mikuni nearly hold their ID through the body whereas the Keihins shape is complicated and likely helps with mid range. So are the 29's too big? Try them & see, if they are, I have a beautiful completely rebuilt set of Mikuni VM28's that will fit in your manifolds........

Those smoothbore aren't easy to synch well with vacuum gauges anyway. If you are meticulous you can use 1/16" drill bits and bench synch them very close, at least from my experience.

From Jeff's Z1 site here are specs for CB750 jetting

Slide cutaway 1.5mm
Jet needle 5DL31-3
Needle jet 0-6
Main jet 115
Air jet 0.9
Pilot jet 17.5 or 20
Air screw 1.5 turns

I use a set on my race bike and I think that jetting info should be really close. I dropped to a 0.7 airjet, but I'm using them on a smaller engine though. The only thing I'd add is to try the needle clip in the #2 or 3 position. If I remember correctly the fuel level (using the clear tube method) is 2.3 or 2.4mm below the carb body flange. Do check these levels.

Good luck and let us know.
 

Yeah, I was led to believe the stockers are 28mm at their widest point. But because of their oval shape they are effectively much smaller. How much I don't know. So much so that the VM29s are effectively 30% "larger? I don't know.

But like you say, give them an honest try.

[MCRider]

I ran a pair of Webers DCOE40s on my raodrace bike which was a 750cc (true) and they took 32mm venturis, perfectly round. I did have a ported head, but little extra displacement. Ran great. So it all boils down to combinations.

[bear]

Hey guys sorry to make the title seem urgent, but it sort of is.

I have been slowly trying to get my 1977 CB750K running.  It has VM29 Smoothbores.

In my project thread you can see all of the frustration and trouble shooting I have gone through:

http://forums.sohc4.net/index.php?topic=100624.msg1160387#msg1160387

I do not get many responses to the questions I have because not many people have the VM29 smoothbores.  The guys over at kzrider.com are trying to help me out... but they have no experience in my application.

Here is the problem:
1) I am noob and tried to get the bike running after cleaning the carbs and replacing internals.
2) I had vacuum leaks so I bought new "intake manifolds" from sudco.
3) They fit great no more vacuum leaks.
4) Bike is still not running how it should and I am at the end of my patience and putting in hours trying to figure out what is wrong.
5) Took bike to mechanic.
6) He just called me and said that there was nowhere to hook up the vacuum gauge to balance the carbs.
7) KZ guys tell me their manifolds come with a nipple for this.
there is no logical way to hook up a vacuum gauge to the carbs.
9) WHAT DO I DO??!!  Haha, sorry for the caps but I just don't know what to tell the mechanic.  He says his hands are tied until he figures out how to balance them.  I am searching all over and I just don't know what to do here.

Thanks and sorry for all the urgency.  The mechanic wants me to call him back and we were going to discuss what to do next.

Any advice would be very much appreciated.

-Brock

I doubt carb sync is your main problem but you can purchase a sync. tool that does not require manifold nipples from Redline for less than $50 freight included.
We have used this type of tool for quite awhile and it does a good job plus it can be thrown in the tool box no worries.

http://www.carburetion.com/Weber/webermisc.asp

Cheers,
Brian

[scottly]

I ran a pair of Webers DCOE40s on my raodrace bike which was a 750cc (true) and they took 32mm venturis, perfectly round. I did have a ported head, but little extra displacement. Ran great. So it all boils down to combinations.

Ron, you don't happen to recall what jets you ran with the 32's, do you? I have been running 30mm venturies on my 836 with Webers with very good results, and would like to experiment with larger chokes someday...

[MCRider]

I ran a pair of Webers DCOE40s on my raodrace bike which was a 750cc (true) and they took 32mm venturis, perfectly round. I did have a ported head, but little extra displacement. Ran great. So it all boils down to combinations.

Ron, you don't happen to recall what jets you ran with the 32's, do you? I have been running 30mm venturies on my 836 with Webers with very good results, and would like to experiment with larger chokes someday...

No. That was ancient history. I had a bunch of them.

As you know, but for others sake, they are easy to change on the Webers as they pull out from the top of the carb. You don't even touch any gasoline. Probably a design favoring the automotive applications but good for bikes too.

[PeWe]

I bought VM29 smoothbores to my CB750 for many years ago. Engine with RC 836cc, ported head, Action fours SS-1 cam, 4-1 exhaust
The inlet rubbers (manifold) were really bad. Too soft so it could not work as it should when the manifold is flexing. The rubber finally cracked after max 5000 miles and 2 pistons melted.

New pistons and no more smoothbores. I used the CB750 Keihin carbs with main jet 145 (4 x KN filters) after that with no specific loss of power.  I have started a project to maybe use the smoothbores again. The plate holding all together is cut between 2 and 3 and welded together again so carbs 1-2 come closer to 2-3. I think 1cm was removed. I have to make a new throttle axis to match this. I hope with this be able to use CB750 OEM manifold rubbers that are rigid, not cracking as the crappy Mikunis do.

I syncronize the carbs visually, removed from the bike. Hold the set of the carbs in front of your eyes and aim to a lamp or sunshine, adjust the idle screw so you just see a small ray of light just under the throttle of one of the carbs. Adjust all the other 3 carbs with the adjuster screw at the top of each carb so all look the same when look thru them. This had worked for me. Smoothbores as with CB750 Keihins

[turboguzzi]

Regardless of the flawed wisdom of fitting bit finicky 29's to a stock 750, I do agree with your mechanic on wanting to sync first.

you could indeed fit vaccum nipples to your rubber boots if you wanted, again not sure its newbie stuff, you'd have to invent a one-off solution

So maybe for the time being, bench sync them with a dril like Brent proposed, it will get you into the ball park at least,

 but for proper tuning, sure, you really have to create vacum pick up points and sync with gauges.

   

[BrockSamson]

thanks for all the advice.  It is really difficult finding information on the VM29s. 

I actually took the stock PD carbs to the mechanic and he just called me to let me know the bike is running great.  I should have stuck with my plan and the advice in my build section. 

I am going to be selling the smoothies and will be posting them in the forums probably this weekend or early next week.  As soon as I get them back in my hands.

Thanks again guys, I am too excited to finally be riding this weekend!

[bwaller]

Good news, go have fun.

[MCRider]

Yup, good news for sure.

[turboguzzi]

I should have stuck with my plan and the advice in my build section. 

well, since you know realize that at least some people here know something and are trying to genuinely help you, here is another piece of advice that you shouldnt overlook IMHO.
If you are also a newbie regarding riding too, then take the MSF course ASAP, preferably BEFORE taking your first ride. A license, specially a US one doesnt mean you know how to ride or know how to handle an emergency and a well used 750 sohc is not exactly a beginners bike.
so sorry, as happy as i am to hear that you are bike is running, I am not going to tell you to go out and ride, but rather go get some real training before.

my 0.02$

[BrockSamson]

Already took the course 3 weeks ago .  I wouldn't ride the bike on public roads without it.  I also have a full face helmet, boots, jacket and gloves.  Given that I am a new rider I am trying to be as safe as possible.  I wouldn't even get on the bike without the push throttle cable installed because I had fears of the throttle getting stuck open and not having the experience to respond to the situation appropriately.

I am taking baby steps as far as riding it.  I will probably drive it the mile from the shop to my buddies place.  Then I am going to research a good stretch of paved sparsely used farm road to get familiar with the bike.  I still have yet to ride it, the only bike I have been on is the little bike they gave me in the class.  So I have no interest in speeding around high traffic roads for a little bit.

After taking the class, I can say I feel much better prepared to respond to unpredictable situations.

[turboguzzi]

good boy