Hi List,
Some of you may be interested to read a long and very comprehensive reply
David Anton of A.P.T recently sent me after I asked a question about modifying
Auxillary venturi. I doubt that you would find better advice in a book ( and
I have several but not Pat Braden's (yet)).
Daniel1312
<< Hello Dan
>Hi David,
>
>Is there much real benefit (power) to be had by de-flashing and tidying up
the aux venturi on a 45 DCOE? I am thinking about doing this before I
finally
the car back on the rolling road.<
I have done this when desperate, but unless the engine is already making 130
HP+ it will not do much.
I have found a much bigger gain by cutting away one side of the throttle
shaft and installing button head screws.
With this mod care has to be taken that return springs are not fitted to the
opposite end of the shaft from the cable (this should never be done
anyway!),
On the engine dyno we picked up about 3 HP on a race 1312cc engine.
While on the subject of Webers, we always use a 5.0mm aux. venturi with 45mm
DCOE on race engines with good results.
A typical setup for a 1340cc might be as follows:
5.0mm auxillary venturi
38mm chokes
F2 emulsion tube
45 F9 idle
155 main jet
190 or 195 air corrector
50 pump jets
and make sure the bleed back in the bottom of the float bowl is a blank type
and doesn't have a bleed back hole in the side of it.
Very often the biggest problem is the progression from the idle circuit onto
the main circuit, especially when the throttle is squeezed slowly and
therefore doesn't get any fuel from the pump circuit.
This can sometimes be helped slightly by making sure the fuel pressure is at
least 3 lbs and that the float level is set, if anything a little on the
high side as long as there are no detrimental effects like stalling or
flooding.
A couple of thoughts:
pumps that are made to "push" don't "suck" very well, so always try to mount
them low.
use a pump with a pressure of at least 6 lbs, and then use a good regulator
with a gauge fitted to cut the pressure back and keep it steady.
mount the regulator close the carburetor
keep the fuel lines away from heat like the exhaust.
>
>.... 200 air correctors in a 40DCOE. I realise that the big air correctors
will lean out the mixture but does it also bring the mains in very early?<
No it tends to delay the point at which fuel starts to be pulled from the
aux. vent.
because a siamese port engine has smoother airflow without so much pulsing
as a single cylinder per runner type engine the fuel delivery is less for
any given amount of air, leading to the "hole" in progression mentioned
above.
The particular problem you mention above with the 40 DCOE is a tough case,
but what happens with big air correctors at the low RPM area is that the low
pull on the aux. vent. tends to just suck air in through the air corrector,
then in the top holes on the emulsion tube without pulling any fuel up with
it (this is the point where it falls flat on its face).
This carb was fitted with F16 emulsion tubes which is what all new Webers
are sent out with now.
The only thing we normally use an F16 tube in is a race MGB
For the A series a thinner tube like an F2 or an F11 is usually better.
The thing to remember here is that there is always a main jet and air
corrector that can be used to give the correct full power mixture,
irrespective of what tube is fitted.
The purpose of the emulsion tube is to control the shape of the fuel curve
through the RPM range. Different engines, and to a point different
combinations of the other variables require different emulsion tubes.
The basic problem with both A & B series engines in getting them rich enough
at low RPM particually through progression, while at the sametime not being
too rich at the top.
This requirement means that a tube with more holes in the lower part is
better because as soon as there is a little signal to the fuel well which is
around the tube it will tend to pull fuel rather than air (obviously the air
comes easier that the fuel).
The point of having the fuel level fairly high rather than low will now be
appreciated. The less height difference there is between the top of the fuel
surface (height in bowl) and the point at which it issues into the air
stream, the easier and quicker will the main circuit will start to operate
satisfactorily.
The thinner the tube (sometimes we make them thinner still in the lathe),
then the more fuel (volume) there is around the tube.
The important point here is that fuel that has already come through the main
jet and is in the well around the tube can be pulled out easier into the air
stream than fuel that has to be pulled through the main jet.
The above is a fairly small, and perhaps desperate move, but can just help
get the engine past the point where it might stumble.
My final thoughts on calibrating Webers:
All of the adjustable parts of the calibration system obviously interact
together somewhat to affect the whole, however, my advice would be to
approach each one as if it was a simple entity.
Its a bit like the daunting feeling of looking at a big math equation that
fills a whole page, it looks horrendous, but when broken down into pieces it
often isn't as bad as you first thought.
The pieces I am refering to are:
The idle circuit (idle jets only and the adjustable screw air bleeds.)
The pump circuit, just the pump jets, as the bleed back foot valve should
always be blank for A or B series.
The main circuit, mostly controlled by main and air corrector jets.
Get the idle circuit working first, so that the air bleed screws make the
engine respond when out about one and a half to two turns at most.
As long as the main jet is somewhere close, the engine should rev up when
you mash the throttle hard, if it doesn't, it might be a sign you need a
larger pump jet.
(A & B series engines are very often happier with a pump delivery that is
probably more than the engine should need, but a little puff of black smoke
on hard acceleration probably won't hurt, while a hesitation may be less
desirable! The good burning characteristics seem to allow us to get away
with this little indescretion).
Last but not least is the fine tuneing of the main circuit.
Your best bet, obviously, a rolling road dyno.
If you are really out just on your own, then the K&N air/fuel ratio monitor
kit may be your best bet.
>
Just a thought Dan, perhaps you may want to post this to your e-mail group?
If you think they may be interested please feel free to do so.
We are (at long last) almost ready to put our proper web page up on the
server, I would say within the next two weeks if not before.
At the last count there are almost 100 title pages and 1500 photographs, not
including thumbnails.
No wonder we have been working on it for 6 months!
We are trying to test and double test everything BEFORE we put it up on the
server!
All the best
David Anton Advanced Performance Technology - APT
>>
|