View Full Version : weber side draft carbs

01-26-2005, 03:08 AM
hey a newbie here with a newbie question. i herd that weber 45 doce have no vacume ports, manifold vacume nor port vacume. so what happens to vacume advance if a side draft carb was retrofitted to a car not originally having a weber?

thanks in advance

01-26-2005, 05:49 AM
Hey Bryan,
1st of all, welcome to BCF. The short and sweet answer is to simply plug off the vacuum advance port and run without it. You can drill and tap a port into the manifold but this isn't really the way to go. It is far better to get the dizzy recurved to run on just the centrifical advance.

01-26-2005, 12:17 PM
Hello Brian,
the vacuum advance function is purely an economy device which allows the carburettor to be set up a bit leaner at cruise. Presumably you want to fit a Weber for performance and will be sacrificing economy. Just be aware that a side draught (DCOE) Weber will only have slight power gains at the high end of the rev range and may have worse driveability than the S.U.
Ther is no need to alter the mechanical advance characteristics just because you are not using the vacuum advance (sorry Darwin). Of course if your engine is going to be highly modified then the distributor may well need modification to match.


01-26-2005, 12:31 PM
you wouldn't tap into the manifold for tha vacume advance because that would be the wrong vacume sorce you need port vacume for the vacume advance other wise the engine would be overly advanced at idle. and loose advacne as the engine rpm's go up. and really the more advance the better, atleast untill you burn through the pistons, the more advanced the engine gets the more efficient it is, and not only for emission reasons but more efficient at making power so the vacume advance isn't jus to keep the air clean.

01-26-2005, 04:19 PM
Hello Bryan,
I'm sorry but you are mistaken in your understanding of the Lucas distributor's working. It is an economy device, pure and simple. Engine vacuum depends on throttle opening with maximum vacuum at a closed throttle. This distributor is designed to assist economy by advancing the engine at part throttle cruise conditions, the extra advance being necessary as the mixture is set weaker than normal. As you know a weak mixture requires more advance, due to slower flame front travel.
Normal engine timing is carried out by the weights and springs when accelerating or travelling at wide throttle openings.


01-26-2005, 10:56 PM
sorry for my ignorance about mini's but what is the Lucas distributer? honestly i only got on this forum to find out if and why the side draft carbs have no vacume ports. although i do work on the new mini's everyday and BMW's the only thing with a carb and a dist that i work on are my 350 small and big block chevies and my big block cadillac /ubbthreads/images/graemlins/crazy.gif

01-27-2005, 05:03 AM
Hello Bryan,
Lucas is the manufacturer of the distributor. and most Lucas distributors are a combined vacuum and mechanical advance type. I was trying to explain why it is not necessary to use the vacuum side of the distributor.


01-27-2005, 10:59 AM
Granted the lucas dist. may be diffrent from an american dist but if the do the same job then i'm not quite sure how it would be okay to take it off.I guess since the old minis maybe werent and high performance as the american V-8? the vacuum advance is for part throttle ignition control. During part throttle operation you need more advance to properly burn the mixture. Otherwise you're on the gas more the get the same power, screwing over both MPG and emissions.

01-27-2005, 11:28 AM
Hello Bryan,
we seem to be losing track of this thread, you originally asked about using a Weber side draught carburettor and wondered about the lack of vacuum take off point.
What you do is set up the carburettor to the engine and forget the vacuum advance side of the distributor. I was assuming that the reason for a Weber DCOE was performance.
The standard design is set lean at part throttle, which requires more advance than the revolution related mechanical advance gives. So a vacuum system is added to give more advance in those circumstances.
If you want to fit a Weber to a standard engine, it will not run as well as the standard S.U. set up.

P.S., in general, European cars tend to have more powerful engines, pro rata, than American ones.

01-27-2005, 01:33 PM
but if you were to fit a mini with a wber chances are you are gonna race it on the road corse right? nad on a road corse you are gonna be all over the rpm band as opposed to drang racing where you tend to be in the upper rpm's. and if the car won't run as good under part throttle then why would you not want the vacume advance on becasue pert throttle is where the vacume advance is gonna help out also at higher rpm's.

i'm not really trying to stir things up i'm jus trying to understand.

by the way not much will do justice to a 1000HP big block chevy /ubbthreads/images/graemlins/smile.gif

01-27-2005, 02:38 PM
Hello Bryan,
there is no vacuum advance when you are accelerating, no matter where you are on the rev band. If you disconnect it with the original set up then it won't run as well at cruise because the mixture is a bit too weak. Adjust the carburettor to compensate then it will run well. Likewise if you put a different carburettor without a connection then you need to set the mixture up for all conditions.
Don't be misled in thinking that a lot of advance is necessary for power, particularly if running at above the 'ideal' ratio of 14:1 petrol to air. Power requires a more than ideal quantity of petrol to air, and that needs less advance. Too much causes detonation, modern engines have detectors and the computer adjusts the timing automatically. Older cars without knock sensors have no such protection, and if it starts detonating, you won't hear or feel it, until the engine is damaged.

What size is a big block Chevvy? Basically I was saying that size for size on normal road cars the American engine is not as powerful, i.e. Horse power per cubic inch.


01-27-2005, 03:00 PM
hmmm well i think i'm beginning to grasp it. even though i will continue to use vac advance on all my cars. but it just seems like a lot of unneeded work to me. if weber would just put a vac port on the carb then it would solve all these problems with mid range drivability. iv they just put a simple little hole in the carb to get a port vacum signal then it would make it a lot more easier to drive daily and what not.
but as your accelrating there is less nmanifold vacume and more carb(port) vacume they are inversly proportional. and vac advance is supposed to go offf port vacume.

a big block chevy is from a 396 cubic inches to 454 cubic inches of course that is in stock trim there have been after market blocks that yield toward 600 cubic inches. oh yeah i get what you ment HP/liter or cubic inches now. and yeah your right about euro cars having more power to size ratio. although there are some pretty efficient american engines now days

Dave Russell
01-27-2005, 06:25 PM
Hi Alec,
Another slant on vacuum advance.
Under part throttle settings, the mixture is less dense. Less total fuel air mixture is being compressed into the same combustion chamber volume. The fuel molecules are further apart & burn at a slower rate. (Flame speed/propagation is slower) More advance (vacuum) gets the burning started earlier so that the slower burning mixture has time to build pressure at the optimum piston/crankshaft position.

Under full throttle conditions, much more mixture is compressed into the same space, the molecules are closer together & burn faster. Thus no need for the extra vacuum advance.

01-27-2005, 11:59 PM
Bryan, 98% of the time while road racing, you are at full throttle. I don't even run a distributor, I use a crank triggered ignition system, and at anything over 2000 rpm its running fully advanced. About 36 degrees.
A fully centrifugal distributor has always been recommended for side draft Weber carbs.

Super 7
01-28-2005, 11:18 PM
a thousand horsepower 1500cc renault F1 motor might. and that would be from close to 20 years ago. That is 90 cubic inch, and 1000 horsepower.

The 120 cubic inch version of that motor made 600 horse and wone 24 hour races, on gasoline.

Most people own or have owned american v8's anyway, and an engine is an engine when it comes to how they work.

04-24-2005, 06:26 PM
I have a 45 DCOE Weber side draft on my 1380cc Mini. Since the Alden/Lucas distributor has a vacuum port it was necessary to attach vacuum hoses for best performance through the rev range.

I had to drill and tap a vacuum fitting on each side of the carb butterfly and 'T' them together. It was done with the advice of experts like David Anton from APT who sold me the go-fast parts.

That having been said, I'd also recommend your checking David Vizard's book (AKA "The Mini Bible"). He compares and shows the dyno results of engines with both types of carbs, and, as the gentleman said above, it is hardly worth the effort, save the visual effect the Webers have.

(Did you know that you have to cut into the firewall and move the instruments to make room for side draft Webers? One more reson to stick with the SU's IMHO)

Jim (who has done it and wishes he had not :-)