Topic: Adapting carbs on Cb500 with straight manifolds question

[Dresda500]

I'm going to show my ignorance on this and just come out and ask...So On my Dresda the one big thing I'm missing is some decent carbs. When I got the bike it was missing the carbs on the race motor , the previous owner said they were special POSA or Lake style injector carbs and went on another race bike. So all I have really is a set of VM26 from a Z900 from what I can tell, 27mm ID on teh spigot and 26mm venturi,  pics below. I will run a straight manifold so of course the spacing is all wrong...so my question to you racers that have straight manifolds...how do you deal with the funky spacing? I'll be honest I've never changed the spacing on a rack of 4 carbs before. I assume it would ential a new shaft and milling slots at the respective spots and then as well making a new back plate or modifying the old one with lots of cutting and welding...seems like a **it ton of work and these carbs aren't worth the time in my book...So is there a better set of carbs to start with? I know TG said he used some VM28 quite succesfully before going to CR29...But again to buy a set of used carbs, then modify...I'd rather just get a set of CR29 and be done with it, I know I'll end up with those anyway---but even on the CR carbs...can you order to the carb spacing of your liking?
I've got a full machine shop at my house, so mods are not an issue, but I have enough custom Items to build fo rthis bike, I don't need the busy work!! 

Sorry for the newby carb questions...lots of info on making various fatory carbs work with the bent manifolds and various boot configutations, but haven't come up with much on this exact topic.

and what are you guys using for carb boots? Actual carb boots or silicon hose or?

Thanks in advance---If you ahve a set of CR29 you can't get rid of let me know...

[slikwilli420]

Respacing CR carbs is simple. Takes about 15 min and your all set.

[Dresda500]

Yes, of course....thanks! I think I'll just try to find some CR29 and not use up time fitting carbs I don't even have....

[calj737]

Why not use a set of CR26 carbs instead of the 29s?

[PeWe]

I modified my VM29 to be used with stock CB750 boots.
The Mikuni - CB750 boots were crap. Way too soft and cracked first summer 1984. I used the stock carbs.

A friend modified the rack end of the 80's. He worked at a company that welded aluminum.
That project finalized 2013 when bike was resurrected after a long sleep 1990---
 
Here some photos and description.
I recommend to buy carbs that fits

http://forums.sohc4.net/index.php/topic,119335.msg1350058.html#msg1350058

I used them 2014.
Bought Mikuni TMR32 for 2015.
I have rejetted them since then. I'm positive I have found it now!

But bike has been rebuilt several times, displacements, cams and exhausts.

[Rocketman]

I'm using CR29's on my race CB550.  Lots of top end but may not be a user friendly combination on the street.    CR26's may make a more tractable power band if you are planning to put it on the street.

[Kelly E]

We we're steered toward the Mikuni RS 36's for our GS1100 E because they are more street oriented. They have accelerator pumps for the low end performance. They came all set up for our bike. I'm not sure if they have them in 26mm or 29mm but it is worth looking into.

[Dresda500]

Why not use a set of CR26 carbs instead of the 29s?

Due to this is a full track bike and with a motor build this next winter it should make full use of the 29mm. 26mm would be better for a street/track but I would give up some HP on the full boogie motor....and I sure can't afford to buy two sets of those gold chain carbs!

[Dresda500]

I modified my VM29 to be used with stock CB750 boots.
The Mikuni - CB750 boots were crap. Way too soft and cracked first summer 1984. I used the stock carbs.

A friend modified the rack end of the 80's. He worked at a company that welded aluminum.
That project finalized 2013 when bike was resurrected after a long sleep 1990---
 
Here some photos and description.
I recommend to buy carbs that fits

http://forums.sohc4.net/index.php/topic,119335.msg1350058.html#msg1350058

I used them 2014.
Bought Mikuni TMR32 for 2015.
I have rejetted them since then. I'm positive I have found it now!

But bike has been rebuilt several times, displacements, cams and exhausts.

Lovely work on those carbs PeWe! I can machine and weld aluminum in house but I agree  , I think I'll try to find a set of CR29 that can be easily adapted and save the custom work for other parts..

[Dresda500]

I'm using CR29's on my race CB550.  Lots of top end but may not be a user friendly combination on the street.    CR26's may make a more tractable power band if you are planning to put it on the street.

I do have the possiblity to get plates on the bike, but for now it will be a dedicated track bike. Abny jetting advice? I guess if I bite the bullet and buy a new set I can choose jetting and a couple up and down.  Used sets go for 98% of a used set so not much sense in used unless a killer deal.
I've really enjoyed your thread on your build and return to racing. I'm in a similar boat, 50 years old and figured I better get back on the track now before it's too late. I figure this should be a fun build and the vintage race scene is a bunch of good guys. I'm in Norway so would mainly be here and a bit in Sweden , maybe Denmark if I'm lucky

[Dresda500]

We we're steered toward the Mikuni RS 36's for our GS1100 E because they are more street oriented. They have accelerator pumps for the low end performance. They came all set up for our bike. I'm not sure if they have them in 26mm or 29mm but it is worth looking into.

The RS are a great carb, but in our Classic class, pumper carbs aren't allowed, the CR is the most modern carb allowed....

[turboguzzi]

hei dresda, i went through the same as i also used the VM26 carbs and yes, youll need to angle each stub so you get a match between head spacing and carb spacing, its very little difference but surely not a bolt on.

manifolds are essentialy an aluminum tube turned to fit the stock rubbers to the carbs and with two weled ears to attach to head, took a while to sort out.....

http://forums.sohc4.net/index.php/topic,19554

[BomberMann650]

hei dresda, i went through the same as i also used the VM26 carbs and yes, youll need to angle each stub so you get a match between head spacing and carb spacing, its very little difference but surely not a bolt on.

manifolds are essentialy an aluminum tube turned to fit the stock rubbers to the carbs and with two weled ears to attach to head, took a while to sort out.....

http://forums.sohc4.net/index.php/topic,19554

This is relevant to my interests.

Anyone have pictures or blueprints they're willing to share?

[bwaller]

I cut the original inlets and re welded to match the (Kawasaki) spacing of a set of Mikuni 29mm smoothbores with a 9 degree incline. Like turboguzzi shows it really straightens the inlet with some port work. I use the 77/78 CB550 inlet rubbers which are shorter and the carbs are as close as possible to the valve cover.

Mike Rieck suggested I keep them as tight as possible for a good signal and it's true. With that Z1 port spacing and 9-10 degrees incline the charge doesn't enter the center of the cylinder but still toward one side so swirl is maintained.

At the time I built mine Keihin CR's were not class legal, (did become legal 2 yrs later) so I found and rebuilt the smoothbores. They are still a very good carb and far lighter than any other of the MIC VM's. Parts were still easy to find 10 yrs ago. I also have a set of VM26's and early VM28 (first year Z1) and the spacing is all the same.  I had pics of all that, but were stored on photobucket and lost them. I think there were pics on my build thread....somewhere!

[turboguzzi]

hei dresda, i went through the same as i also used the VM26 carbs and yes, youll need to angle each stub so you get a match between head spacing and carb spacing, its very little difference but surely not a bolt on.

manifolds are essentialy an aluminum tube turned to fit the stock rubbers to the carbs and with two weled ears to attach to head, took a while to sort out.....

http://forums.sohc4.net/index.php/topic,19554

This is relevant to my interests.

Anyone have pictures or blueprints they're willing to share?

i used that setup some 14 years ago before moving to the CR29s..... those manifolds must be somewhere in a box, I never throw such stuff, gotta have a look. i might have the 3D CAD file in Pro-e format , can be exported to other formats too, question is if you have any SW to open .stl, .igs, parasolid...

[BomberMann650]

Are the oe aluminum castings an easy weld up?

[Dresda500]

hei dresda, i went through the same as i also used the VM26 carbs and yes, youll need to angle each stub so you get a match between head spacing and carb spacing, its very little difference but surely not a bolt on.

manifolds are essentialy an aluminum tube turned to fit the stock rubbers to the carbs and with two weled ears to attach to head, took a while to sort out.....

http://forums.sohc4.net/index.php/topic,19554

This is relevant to my interests.

Anyone have pictures or blueprints they're willing to share?

i used that setup some 14 years ago before moving to the CR29s..... those manifolds must be somewhere in a box, I never throw such stuff, gotta have a look. i might have the 3D CAD file in Pro-e format , can be exported to other formats too, question is if you have any SW to open .stl, .igs, parasolid...

If you happen to come across the file I'd be interested. Yes, I use SW. I'll likely set up a set of VM as well as I'm sure I'm going to end up with a at least a couple motor variations

[Dresda500]

Are the oe aluminum castings an easy weld up?

You results may vary as they say...in general these are high quality castings and weld wonderfully, but you have to get them clean which can prove difficult and the porous aluminum like so suck up oil. Clean it, bake it, pre-heat it and crank up the cleaning action on your welder and once you get a good puddle going you can usually do a nice job.

[Rocketman]

My jetting is based on a series of dyno runs with an air/fuel sniffer. My first and only attempt, so there is probably more to be gained by an experienced dyno operator.

Main jet 115
Main Air jet 180
Pilot jet 65
Needle YY5
Needle Height #4
Slide 25

I don’t have the needle jet in my notes, but will check and post.

[BomberMann650]

hei dresda, i went through the same as i also used the VM26 carbs and yes, youll need to angle each stub so you get a match between head spacing and carb spacing, its very little difference but surely not a bolt on.

manifolds are essentialy an aluminum tube turned to fit the stock rubbers to the carbs and with two weled ears to attach to head, took a while to sort out.....

http://forums.sohc4.net/index.php/topic,19554

This is relevant to my interests.

Anyone have pictures or blueprints they're willing to share?

i used that setup some 14 years ago before moving to the CR29s..... those manifolds must be somewhere in a box, I never throw such stuff, gotta have a look. i might have the 3D CAD file in Pro-e format , can be exported to other formats too, question is if you have any SW to open .stl, .igs, parasolid...

Enjoyed looking through turboguzzi's thread.

Had some thoughts of my own.  Compared to the cyclex cr adapter plates. TG's modded oem intakes didn't appear to loose much of the original length.  Whereas cyclex gets those fancy carbs real close to the port with their kit.

Would the VM carbs play nice if they were affixed closer to the head?  Im thinking about high rpm intake pulses.

[Dresda500]

hei dresda, i went through the same as i also used the VM26 carbs and yes, youll need to angle each stub so you get a match between head spacing and carb spacing, its very little difference but surely not a bolt on.

manifolds are essentialy an aluminum tube turned to fit the stock rubbers to the carbs and with two weled ears to attach to head, took a while to sort out.....

http://forums.sohc4.net/index.php/topic,19554

This is relevant to my interests.

Anyone have pictures or blueprints they're willing to share?

i used that setup some 14 years ago before moving to the CR29s..... those manifolds must be somewhere in a box, I never throw such stuff, gotta have a look. i might have the 3D CAD file in Pro-e format , can be exported to other formats too, question is if you have any SW to open .stl, .igs, parasolid...

Enjoyed looking through turboguzzi's thread.

Had some thoughts of my own.  Compared to the cyclex cr adapter plates. TG's modded oem intakes didn't appear to loose much of the original length.  Whereas cyclex gets those fancy carbs real close to the port with their kit.

Would the VM carbs play nice if they were affixed closer to the head?  Im thinking about high rpm intake pulses.

I don't have much experience with using a VM carb on a tuned motor, but in general the the higher the rpm, the higher the intake pulse frequency and thus the carb doesn't have much time to react to the fluctuations. I think they have more trouble with really big cams and low rpm and fewer cylinders, but then again they work pretty well on hopped up HD motors as well. In theory having the carb as close to head does give better response, but I'm not sure one can really see a difference in 10mm intake length , but another bonus of a carb being close to the head is that it gives one more room to play with velocity stack length. It's much easier to have your carbs at a fixed position and change out stacks than moving your carb around and potentially changing more than one thing at a time.

[BomberMann650]

If the spacing/angles allow for it.  I wanted to try really stubbing the carbs right against the head.

You're right about the difference in velocity stacks.  There's a youtuber who messes around with his 4age project car on a dyno.  Even taking silly requests from the audience for odd items to use as v-stacks.

The traffic cone was a definite no go.  Cup'o'noodles was disappointing as usual.  Oddly enough there's real power and performance to be gained from a can of peaches!