Topic: CB 500/550 Single Weber?

[Lostboy Steve]

I know that the 750's have been seen with twin webers. Was wondering if anyone has ever tried a single weber on a 500/550.

[BPellerine]

only time I have seen a single weber is on twins,hd,triumph,or 450,dont know if they even made a manifold for the 550s,you would have to make your own.bill

[754]

It would be worth it..

[Lostboy Steve]

It would be worth it..

I know... It would be cool. Looking at those 450 500 twins, it seems like the weber is "a lot of carb" for those engines. What are you thinking 754? Make a 4 into 2 manifold and start playing with jets?

[Lostboy Steve]

Looks like I need to learn how to sandcast parts. Check this out. Cylinder Balanced.

[CBGhia]

Here ya go:

Edit:  Even better stuff here:  https://www.youtube.com/channel/UCNC9gjgYrzLfyMQmp31DnjA

[754]

I would just fab a manifold, way easier than building say a header.. No sync problems, easy to get at jetting (usually)
Incredible throttle response, plus an increase in mileage that may probably even pay for it all.
 They did make a single Weber  kit for 750, MPD made it, I would have mp at the chance to try one for touring..

[TwoTired]

I've never used a Weber, but know they are in high regard.  These use a butterfly valve for the throttle, right? 

How well do the butterfly pivot shafts hold up to the strong induction pulses?  Is there a replaceable bushing there?  Just curious. 

I can see how they might help power at red line operation.  But, it would seem like operation below mid range and the low air velocities, would make the engine less responsive on the street.

Putting big carb throats on small engine is a common mistake for a happy street machine, resulting in poor low speed atomization.  Works well for drag racing, high speed runs, though.

[754]

Actually one of the few carbs using Ball bearing throttle shafts.!
Venturis change in minutes, you could have 2 28mm feeding both cylinders, or maybe smaller.
 Around 6 changeable jetting element vs around 3 in some carbs..

My choice of them is reinforced by the strong low and midrange response.

[Lostboy Steve]

I had a single Weber sidedraft on a 1989 Rx-7 which had highly modified ports. It was surprisingly street-able and reliable. I also had a single down draft on an otherwise stock 1972 Superbeetle. It too performed very well. They are great carbs. I see your point about lower rpms. Its questionable. I guess theres really only one way to find out.

So figure I should start shopping for a 40 dcoe?

[Greggo]

I have a leftover ATP intake manifold for my 500/550 I've thought about using for a single carb set up...What is appropriate for the street? 

[Lostboy Steve]

no idea but a 38mm is nearly impossible to find without breaking the bank...

[BPellerine]

I have 38s on my stock 750 with 28mm chokes and there is no downside low or high rpm,if you really want to nitpik they take a little more finesse to start when cold but if I had to choose only one thing for my bike the webers would be it,did they really make a single weber kit for the 750 Frank?would be cool!

[scottly]

I've never used a Weber, but know they are in high regard.  These use a butterfly valve for the throttle, right? 

How well do the butterfly pivot shafts hold up to the strong induction pulses?  Is there a replaceable bushing there?  Just curious. 

I can see how they might help power at red line operation.  But, it would seem like operation below mid range and the low air velocities, would make the engine less responsive on the street.

Putting big carb throats on small engine is a common mistake for a happy street machine, resulting in poor low speed atomization.  Works well for drag racing, high speed runs, though.

One of the main attributes of the Weber is that it will atomize the fuel at lower pressure differentials than most carbs, allowing a greater charge in the cylinder; this is why they are often referred to as "the next best thing to fuel injection". Cruising at 65-75 MPH is still in the low-speed jet range, and will produce 50+ MPG on my bike, while cruising at 55-60 MPH has produced in excess of 60 MPG. With the accelerator pumps, which have at least 4 different adjustments, there is no lack of low speed response. Putting a brace of DCOE 40's on a 750 is not the same thing as slapping an 850 Holley on a 283 Chevy.

[BPellerine]

you have put it very well scottly!everyone that sees them here say that I must have too much carb including an instructor from the local trade school who has solex carbs on vintage Datsun two seater,so I took the bike down and let him see ,he did not know what to say!ill

[754]

According to the baseline jetting from Magnussen..an off the shelf 750 carb, ie one of the pair..is jetted the same as the one supplied for a 1200 cc Harley..!

[754]

I bet I could get 65 mpg to a US gallon with a single Weber on a 750..if not 70..
 But then again I hit around 60 mpg. To a US gallon with stock K3 carbs wit original K+N. Filters on it. (rubber end).

.

[BPellerine]

I think we all know what happens when you start playing around town!!haha bill

[754]

Yeah, I know it goes form 58 mpg to about 25!!!!

[Lostboy Steve]

Well I still say this would be a really cool adaptation. I just need to get my hands on one.

[BPellerine]

yes very cool adaptation and not seen ,the 40 should not be hard to get,the manifold would require some thought.bill

[Lostboy Steve]

Well, the project is moving slower than expected as other things have become more important financially. Once I get the roller back together, I will purchase a carb. I have the engine on a work bench and so now might be a good time to start fabbing up a manifold.

[754]

Check out the inspiration wanted thread, by Scott S in sohc bikes.
Pic of a pattern for 4-1 intake that could easily modded for Weber

[Lostboy Steve]

Check out the inspiration wanted thread, by Scott S in sohc bikes.
Pic of a pattern for 4-1 intake that could easily modded for Weber

Thanks Brother, Will do!

[Lostboy Steve]

So the question is, who will cast a part if I make a wooden model of it?

[Tintop]

As has been pointed out it will be much easier to fab up a manifold then cast one.  You can buy the Weber end in alloy from Pierce Manifold, water jet the engine mount pieces, and then use thick wall tube for the branches.  You want to use 'thick wall' so you have some meat for port matching.

[Lostboy Steve]

As has been pointed out it will be much easier to fab up a manifold then cast one.  You can buy the Weber end in alloy from Pierce Manifold, water jet the engine mount pieces, and then use thick wall tube for the branches.  You want to use 'thick wall' so you have some meat for port matching.

Yes, I know but as 754 said that balsa manifold mold is pretty sharp. It would be nice to make something similar.

[MidnightLamp]

I'd be cautious about this...

Mainly because you'd need a mani like shown that ties 1-4 and 2-3 together due to the firing order. 1-2 together act like a 180 degree twin and will pull two large pressure pulses through the carb in quick succession (not advisable) and 3-4 do the same thing. When joining 1-4 they act like a 360 degree twin and the pulses are separated far enough to work.

The problem is that the runner length on 1 or 4 (depending on how you set up the carb) is ridiculous and will affect the fueling for that one cylinder. You would want all four to draw equally with the same pressure drop through the carb, which is not easy.

You would probably be a lot better off with a pair of twins...IMO anyways

[scottly]

I agree with Midnight: feeding 2 cylinders with one throttle bore is not an optimum situation, regardless of the cylinder pairing. The 1-4 manifold has sharp bends, which can cause fuel separation, and may be too long to physically fit. Feeding 1&2 from the same throttle would be a much straighter shot, with equal branch lengths. Tit for tat. The ideal is to have each cylinder matched with it's own throttle bore.   

[754]

I got mixed feelings this..
Its worked. For decades on cars, some motorcycles..other engines..
Not ideal for all out power, but should be simple..torque, and get great gas mileage..

[scottly]

Not ideal for all out power, but should be simple.

If I were going to do this, I would pair 1-2 and 3-4. Makes the manifold a lot simpler...

[MidnightLamp]

Pairing odd cylinders is not a wise choice because what happens is that the mixture gets pulled through the carb twice in quick succession. That's not enough time to clear out pressure pulse at higher rpm's. You won't find too many performance applications that do that, but there are countless examples of it being done. Most of the twins you do see it one where you can pull good beans through them are 360 twins where the problems becomes substantially less of an issue.

If you want some math behind it, just consider this:
- Firing order is 1-2-4-3, and because all the cylinders are identical the intake order is also the same (just lagging behind the point of firing based on the cam specs).
- Stock cam duration is something like 220 degrees @ 50 thou lift, so for 220 degrees of crank rotation the intake valve is open to some degree
- Based on this, cylinder 1 opens it's intake valve, and 180 degrees later cylinder 2 opens it's intake valve as well, but because the valve was open for 220 degrees you end up with both cylinders drawing concurrently through the carb for ~40 degrees of rotation. Also consider piston 1 is slowing down at the end of the intake stroke and the vaccum in that cylinder has dropped, while piston 2 is pulling strong vacuum.
- Tack on a big cam and this problem becomes worse. Different carbs will handle this situation differently as well.

I could go on but you get the point I'm sure. Problem disappears when you draw from 1 then 4 because the offset becomes 360 degrees which is longer than the duration. Same problem exists with a 180 twin vs. a 360 twin (as I mentioned before).

Note that most CB750 twin carb setups also use this same approach. Obviously YMMV, but there's the counter argument.

[Lostboy Steve]

ok, and as I have said before I like this BMW 2002 Manifold but would still need casting abilities, obviously.

[Lostboy Steve]

I'd be cautious about this...

Mainly because you'd need a mani like shown that ties 1-4 and 2-3 together due to the firing order. 1-2 together act like a 180 degree twin and will pull two large pressure pulses through the carb in quick succession (not advisable) and 3-4 do the same thing. When joining 1-4 they act like a 360 degree twin and the pulses are separated far enough to work.

The problem is that the runner length on 1 or 4 (depending on how you set up the carb) is ridiculous and will affect the fueling for that one cylinder. You would want all four to draw equally with the same pressure drop through the carb, which is not easy.

You would probably be a lot better off with a pair of twins...IMO anyways

Two twins like dcoe 40's are a hell of a lot of air... I could only imagine that this would cause issues within itself.

[Lostboy Steve]

How about this? Using a down draft? Just a sketch...

[MidnightLamp]

You could just run a smaller venturi on the 40's, the venturi is what controls the main airflow characteristics and on a 40 you can go from 26mm all the way up to 36mm. The 750's run ~30mm venturis, you could just run smaller ones. A hopped up 550 should be flowing not that much less than a stockish 750.

[scottly]

Mid, I agree about the problems created with the 180* manifold, but the 360* also has issues with the sharp bends and relatively long, uneven duct lengths. Like I said, tit for tat. Neither configuration is as high performance as one throttle bore per cylinder.

[MidnightLamp]

Oh I can definitely agree with that scottly, no arguments there. In fact, I think the posted manifold will have some weird issues on cylinder 4/1 being fed via the same carb when the flow restrictions on 4 will be much greater due to the runner length and bends.