Topic: Which carbs to use?

[Jinxracing]

I've got a set of carbs that came with my '70 K1, and another set from a '76. I'm going to be using the '76 motor in my '70 cafe project and I'm not worried about period correctness, just good carburation. I'm also considering the dual carb conversion here: http://www.cyclexchange.net/Carb%20System%20Comp%20Page.htm though knee clearance looks like it might be a problem on a cafe.

Both sets of carbs are complete but need cleaning, and assuming that they will both actually bolt up to my '76 motor, is there an advantage to using one over the other? Looks like the early carbs are going for quite a bit more money on eBay and rebuild kits seem to be more scarce for the early ones as well. Is this due to better power potential or just rarity?

Jim

[Geeto67]

I've got a set of carbs that came with my '70 K1, and another set from a '76. I'm going to be using the '76 motor in my '70 cafe project and I'm not worried about period correctness, just good carburation. I'm also considering the dual carb conversion here: http://www.cyclexchange.net/Carb%20System%20Comp%20Page.htm though knee clearance looks like it might be a problem on a cafe.

Both sets of carbs are complete but need cleaning, and assuming that they will both actually bolt up to my '76 motor, is there an advantage to using one over the other? Looks like the early carbs are going for quite a bit more money on eBay and rebuild kits seem to be more scarce for the early ones as well. Is this due to better power potential or just rarity?

Jim

Are you looking for all out power, ease of use, or tunability?

All out power I would go with the FCR or CR racing carbs, followed by K0, mukuni conversion anf finally the 76 in that order.

For ease of tunability the mukuni conversion, the FCR carbs, Then the K0 or K6 carbs.

For ease of use (set them and forget them), the K6 carbs, the mukuni conversion, the K0 carbs (only because parts are hard to come by), and the FCRs.

I haven't used a mukuni's on a cb750  but I use the mukuni vm series on all my kaw triples and the carb is really an easy carb to use and tune and cycle ex claims more power over stock (I have no reason to doubt them). The long manifolds work like a tunnel ram on a car would, increasing low end. The only thing I don't like is that  you do not have one carb per cylinder which I feel is optimal. For brand new carbs where you bolt them on and go, $500 is a pretty good deal. As far as frame clearance cycle ex sells a carb rubber that will allow you to adjust the angle on the carbs so they run inside the frame rails.

[8 Track]

all my kaw triples

Ahhhhh

[Geeto67]

all my kaw triples

Ahhhhh

is that an ahhhh because they came stock with mukuni VM's?  I do keep the carbs stock for the most part but when I go larger like on my KH500 drag bike I stay with mukuni VMs (I have a set of h2 carbs on my KH, I was running posa carbs but I like the early h2 VM's better). 

[kghost]

I haven't found parts hard to find for K1 carbs.....

I prefer the earlier carbs then the later ones. Easier to work on and tune.

[Jinxracing]

As far as frame clearance cycle ex sells a carb rubber that will allow you to adjust the angle on the carbs so they run inside the frame rails.

Thanks for the info. I'd like to kid myself and say that all-out power is top priority, but in reality I'll gladly give up a couple of HP to have smoother running and easier tunability. If Cycle X does in fact have a way to fit their kit between the frame rails, I think I'll end up going that route.

[8 Track]

is that an ahhhh because they came stock with mukuni VM's?

No that's ahhh because I love the smell of two strokes in the morning!  It smells like victory!

[Geeto67]

is that an ahhhh because they came stock with mukuni VM's?

No that's ahhh because I love the smell of two strokes in the morning!  It smells like victory!

Here ya go two stroke crazy - been working on it for five years now. been stalled for two because I can't find a decent welder in my area (picture is from 04). 1974 H1 500 with Fzr 400/600 suspension and rims:



http://springvinmoto.com/gallery/view_album.php?set_albumName=album261



Correct me if I am wrong but if it is a 1970 they are K0 carbs and not K1 - which means they will have the 4 into 1 throttle cable instead of the rocker style throttle.

[Jinxracing]

Correct me if I am wrong but if it is a 1970 they are K0 carbs and not K1 - which means they will have the 4 into 1 throttle cable instead of the rocker style throttle.

From the way the serial numbers read compared to the chart on the main SOHC website, the K0 to K1 shift happened sometime in mid '70. My serial numbers indicate a K1, and the build date on my VIN tag is 8/70. My throttle uses a 2 cable setup that appears to be the same as the setup on the later model carbs that I have.

[Geeto67]

Correct me if I am wrong but if it is a 1970 they are K0 carbs and not K1 - which means they will have the 4 into 1 throttle cable instead of the rocker style throttle.

From the way the serial numbers read compared to the chart on the main SOHC website, the K0 to K1 shift happened sometime in mid '70. My serial numbers indicate a K1, and the build date on my VIN tag is 8/70. My throttle uses a 2 cable setup that appears to be the same as the setup on the later model carbs that I have.

Then I take it back about the earlier arbs being harder to get parts for - the K1 and K6 are about equal in terms maintenance. I agree with kghost, if you are going to use the stock carbs use the earlier set, they just seem to run better.

[kghost]

For heavens sake avoid any with pressed in jets

[mlinder]

As far as frame clearance cycle ex sells a carb rubber that will allow you to adjust the angle on the carbs so they run inside the frame rails.

Thanks for the info. I'd like to kid myself and say that all-out power is top priority, but in reality I'll gladly give up a couple of HP to have smoother running and easier tunability. If Cycle X does in fact have a way to fit their kit between the frame rails, I think I'll end up going that route.

Im having trouble makin em fit. I've just stuck em off to the side, since there is NO way in hell that they can run along the inside.

[Jinxracing]

Now I'm really wondering. Looking at this photo, it looks as though they're running at a much straighter angle. This is going to require further investigation...

http://www.cyclexchange.net/High%20Performance%20Carb%20System%20Page.htm

[Bikebuff]

Hey Jinx,

I've got a 70 cb750 K0 (manufactured 11/69) and I'm currently cleaning and rebuilding mine but have been considering the mikuni setup from cyclexchange.  After you've investigated, let us know how feasible this swap could be. 

[Bob Wessner]

I've got a 70 cb750 K0 (manufactured 11/69)

Likewise, our K0's might have been bunk mates on the boat over.

[Geeto67]

Now I'm really wondering. Looking at this photo, it looks as though they're running at a much straighter angle. This is going to require further investigation...

http://www.cyclexchange.net/High%20Performance%20Carb%20System%20Page.htm

before they revamped their site, cycle ex had a pic of the carbs in a stock frame where they were parallel with the frame rails. They also solf an extension that you could use to level the carbs (angle cut rubber boots). I noticed that both are gone now, but you can try e-mailing them and seeing what they have or if they have pics.

[8 Track]

1974 H1 500 with Fzr 400/600 suspension and rims:

There ya go!

[Jinxracing]

I spoke to Ken from Cycle X tonight, and have a little more information on this topic for those interested...

He said it had been awhile since he'd tried fitting them to a stock motor/frame layout since he mostly deals with custom choppers and the like. The clearance issues that seem to come up on stock frames are not with the carb bodies themselves, but with the air cleaners hitting the first vertical frame rail and sidecovers/oil tank. He said that he's had a few customers who have fit the dual carb setup to their stock frame/motor combos with varying degrees of success, but there isn't really a carb "package" to fit the stock-framed bikes...it's been more of a "if you can figure it out, then it'll work" situation.

We spoke a little bit about the possibility of him selling more to the cafe/stock frame crowd and he will be checking intake boot/air cleaner combinations this week to try and find a suitable setup on a stock-framed bike that he has in his shop right now.

I'll post more info as I get it.

Jim

[kghost]

Sorry maybe I am missing the point of the two carb set up.

If you can tune two carbs you can tune four.

Is the purpose to get new carbs? Do you not have a rebuildable set of carbs?

Guess I'm lost.

[Jinxracing]

Sorry maybe I am missing the point of the two carb set up.
If you can tune two carbs you can tune four.
Is the purpose to get new carbs? Do you not have a rebuildable set of carbs?
Guess I'm lost.

I've actually got two sets of rebuildable carbs, both of which are complete but need to be rebuilt. For me, the idea of a new two-carb setup has some advantages:

1. I don't have to rebuild any carbs. I've rebuilt carbs in the past, so I'm not worried about the difficulty aspect of it; however, rebuild kits aren't free and I've never actually enjoyed the process.

2. Half as many carbs to jet and sync when setup time comes.

3. I suffer from "do things differently" disease (DTD), and the two-carb setup really scratches the demented itch I have that makes me want to take the less-traveled and more complicated path whenever possible. 

4. And as a purely selfless gesture, the two-carb conversion will allow me to sell the two complete sets of stock carbs that I have to traditionalists, keeping someone else's stock CB750 flame alive while slightly offsetting the cost of the new carbs. 

5. When sharp-eyed girls see my bike they'll notice the dual-carb setup and be so impressed by my ingenuity and devil-may-care attitude towards conventional intake systems that they'll immediately want to sleep with me. I'm not holding my breath on that one though. 

Jim

[Sam Green Racing]

Hey Jim, get yerself a set of CRs, not only will the girls want to sleep with you......you may get a little more

Plus, you'll have the best carbs money can buy.

Sam.

[Joshua650]

No one has brought up fuel distribution with the dual carb conversion so I will. Because of the firing order each carb has a cylinder drawing air/fuel right after the other then a large pause. During the pause fuel stops flowing so the 1st cylinder to draw from the carb is leaner than the 2nd. This is a well known phenomenon with two venturis feeding an I4. It bothers my inner engineer.

[Geeto67]

No one has brought up fuel distribution with the dual carb conversion so I will. Because of the firing order each carb has a cylinder drawing air/fuel right after the other then a large pause. During the pause fuel stops flowing so the 1st cylinder to draw from the carb is leaner than the 2nd. This is a well known phenomenon with two venturis feeding an I4. It bothers my inner engineer.

Sorry maybe I am missing the point of the two carb set up.

If you can tune two carbs you can tune four.

Is the purpose to get new carbs? Do you not have a rebuildable set of carbs?

Guess I'm lost.

This is the age old debate over at hondachopper.com. Seriously, it has probably been argued 100 times and with a lot of name calling and what not. The way I see it the two carb setup by itself is inefficient for a host of reasons. Anybody who has worked on carb'ed cars where the the front cylinders run rich while the rear lean out (or vice versa) will understand why. However the manifold design is a power adder similar to a tunnel ram in a car which offsets the inefficiencies of the two carb design. It is not better than stock from an efficiency standpoint but it is if you want more power. If you notice the setups are primarily for stock engines - hig perfromance engines don't respond well to running off two carbs, espically if you have a hot cam which doesn't draw a lot of vacuum at idle.

Comparativley speaking the mukuni VM carbs I find are a lot easier to physically tune than Kehin carbs. Everything is right there and you aren't disassembling the throttle linkage to make needle adjustments. Plus they really have a cool look to them. And there is something really nice about having a brand new set of carbs. The last time I checked into it FCR or CR carbs were $800 per set. I've only ridden one bike with them but what a difference it made.

[c_kyle]

Check out this link, it pretty much shows the Cycle X carbs sticking out of a stock frame.

http://www.cyclexchange.net/index%20pics/customer%20doug%201%20%20%205-27.jpg

[Jinxracing]

Check out this link, it pretty much shows the Cycle X carbs sticking out of a stock frame.
http://www.cyclexchange.net/index%20pics/customer%20doug%201%20%20%205-27.jpg

Well, that's definitely NOT the look I'm going for. 

I spoke to Ken from Cycle X on the phone some more, and he confirmed Geeto's earlier supposition that he does in fact have angled boots (from a Ski-Doo snowmobile) that allow the carbs to tuck inside the frame rails. I'm going to go for it and see what happens.