im having an arguement with a mate, hes claiming his motorbike carbs would be better to have set up on an a15 than twin side webbers. but how so, because webbers (for arguements sake DCOEs) have twin butterflies, so both setups would be very similar right? Is there individual cylinder tuning on a webber carby such as this? Why would i even want individual cylinder tuning such as bikes have?

in my opinion i reckon that hitachis are the best... even a sped kid like me can give them a nice tune to get it running prety good.. with webbers you need to re jet them and play around with them alot..

Hmmmm, Webers V Bike carbs.

Well this discussion has been & gone before & both are winners & both are losers.

With the bike carbs you will need to fabricate a manifold but with Webers, you can buy one.

Webers are expensive, complex & almost imposible to correctly tune at home.
Bike carbs are usually cheaper, comparitively simple & easy to service, but almost impossible to correctly tune at home, so to get it right, both will benefit from a competent Dyno tune.

Neither system has ported vacuum for the distributor. This is not such a drama with Webers as you will have no vacuum anyway, but bike carbs will provide a better vacuum signal in most cases.

In a 1200, you have very limited room for air filters on a Weber setup but a thoughtfully designed bike install will leave plenty of room for a nice big oval filter that will last for longer than a week before needing cleaning or replacement.

Weber manifolds have no provision for PCV but with almost no vacuum you would get no, or poor PCV function anyway & this is not good in a street application. It's also illegal.
At least with a custom bike manifold, provision for a PCV valve can be designed in & with the better vacuum they the bike carbs produce, the system should work, .... sort of.

Depending on the bike carb design, the system should offer good driveability in traffic, something that is often missing with Webers.

Properly tuned bike carbs, because of their 'higher vacuum' design, should produce better fuel economy as spitback & fuel loss from Webers at low speed hurts fuel economy more than some people realise.

As for maximum power, .... I will stick my neck out & stay that with the correct selection of bike carb, there should be no real difference in power output. Some of those bike engines make more power than most A series engines will ever see & those carbs are feeding them the fuel.

So take your pick.
If you have the facilities, the parts & the enthusiasm, then the bike carb induction gets my vote

I am with Dodgeman if you had the money a top shelf Keihin setup is close to as good as fuel injection as you are going to get, but you could easily spend double what the Webers would cost you if you went all out.

I think the grounds for comparison are fairly similar with most series of bike carbs (and DCOE webers), but once you start comparing FCR's and MSR's to the butterflied webers, the perfromance flat-sliders win hands down.

Dodgeman and Sidedraught in the same thread, I haven't seen that in years...

I'll play devil's advocate (if only very very slightly so) - I'd go the webers - as generally there'll be people you can find who can set them up pretty well, and they would have been used more than a few times on a combination similar to yours so you could at least get some sort of ballpark initial spec/setup to start with.

In contrast not too many bike carbs would be 375cc individual cylinders spinning to under 8000rpm, on a 2 valve head at that. That doesn't mean that they are unworkable, but you would (to some extent) be treading new ground as far as the setup goes. Having said that, they were utilised on race minis more than a few times over the years, esp way back when racers on a tight budget could be competitive with a fair bit of ingenuity and guts to go out on that unbroken ground on their own, so to speak.

I have to wonder about the economy side of things however, as I'd gently suggest that not too many bikes get what would be considered fantastic economy given the fact they predominantly carry 1 person and are 1/4 the weight of even some of the smallest cars.

TO close, however, I'll say this - if you took the two options and sized matched the bike carbs (and also the weber choke size) and optimised both fully, I doubt _anyone_ would ever be able to tell you which option you were running based on the performance, untill you popped the bonnet to physically show them which carbs were fitted.

Question for Dodgeman. Can you elaborate a little on the low vacuum thing regarding twin sidedraughts? Not sure I understand. Do you mean low vac because of distance from the head? I've often wondered about flat slides...they would obviously sit closer to the port giving better vac behind the slide but is that the only reason?

DCOE Webers have no provision [vacuum port] for a vacuum signal like a typical downdraught carb. If you are chasing a vacuum signal off the manifold [not good for distributor], most manifolds will only tap into one runner and thereby only getting a signal from 1 cylinder. There is a way around it by having a balance tube that takes a signal from all 4 runners.

My preference is for Webers as I beleive they are more infinitely tunable i.e you can change chokes, Aux vebts, all jets, volume and duration of fuel delivery. Where as bike carbs [to my knowledge] have fixed choke and or bore size. I see bike carbs more suited the engines at high RPM's [typically most bike engines] when compared to car engines. Weber carbs can be tuned for down low RPM performance as much as tuned for high RPM performance as well as somewhere in between.

I understand that you dont get ported vacuum but if you're fitting twin sidedraughts I dont think you're gonna be too worried about no vac advance. Vac advance works soley on cruze/light throttle for better fuel economy. I thought Dodgeman was saying you have less manifold vac with webers than with bike carbs.

aren't most bike carbs gravity feed? when you are sitting at a set of lights don't they over fuel??