Topic: CB 500/550 Single Weber?

[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.