Why does 3W scoop point backwards? and what is the little tube from the scoop to the carb do?

By pointing backwards, it minimizes the turbulence going into the carb. Facing forward, for example, would cause pressure changes if it was exposed to in-rushing air. The tube connects the reference (or static port) of the diaphragm regulator inside the half of the carb that provides the regulation of fuel pressure to the needle valves. Without this, the cowl pressure changes can push or pull the regulator diaphragm affecting the pressure that goes to the needle valves. There is no fuel or air passing through this tube. Just minor amounts of air as the diaphragm pulses in and out.

Points backwards mostly because it won't fit in the cowl facing forwards...The 3W 157 I was testing picked up 200 rpm with the scoop facing forward...But we can't cut a big ugly hole in the cowl, can we

Facing forward, it acts as a gravel scoop. Not the best for an unfiltered intake.

Yup...But there's no gravel on the local airport taxiway we fly from here...

Ralph,

Since I run my bug collecting planes the same as your race planes, WIDE OPEN THROTTLE, Do you think that I would also pickup rpms on the 3w-200 with the scoop facing forward? I don't need a smooth throttle response. I fly at 3 settings. WOT most of the time, some mid point for landing approach and idle.

Elson

Worked for me on my test stand....The drone engines we run in the race planes were pushers with the carbs facing straight forward...the carbs are custom castings using Walbro internal parts..There is no pump function or diaphragm, crankcase pressure is used to fill the cavity under the inlet diaphragm...The cover over the diaphragm has a small brass tube going at an angle from the flare at the front of the venturi to the space over the diaphragm...There is no other hole in the cover..Worked fine in the drone...The carbs were vertical in the race plan so the tube was not needed. I put a fitting in the cover and ran a tube through the firewall to the dead air inside the fuse.....Worked great, clocked 234 mph....
One of the other racers tried using ram air but didn't use the tube in the venturi with a fitting in the cover..He never got it to work and I didn't volunteer any information

We were using 3W-100's on a UAV I was working with last year that also incorporated the scoop. In the stock configuration it was mounted in reverse like in the picture in the first post. Just for the heck of it I turned one around one day for a flight and the telemetry noted a little over a 100 rpm increase from what all the other engines would turn. I'm npt going to say it's an absolute, but there were no problems with the fuel system and there was no other possible explanation for the increase in rpm.

Conventional wisdom is a misnomer

You can't push air into a carburetor-
That ain't how it works - it is an atomizer- just like mom's old perfume sprayers.
years back when the first centrifrugal superchargers were made available for cars - It was quickly apparant that the pressure from the blower had to be equalized in the entire carb-that is the carb had to run in a sealed box- then, the engine then thought it was simply in a higher air pressure enviroment
I ran a positive displacement blower with the carb on it's inlet which resolved the carb pressure thing.
FI setups are different matter --
On hotrods from year one - guys tried scoops n stacks and all kinds of tubes to help air flow increase thru the carbs .
Most simply did not understand the issue.
Model airplane guys get caught in the same trap
I use engines with carbs facing rearward wih no seperate air inlets or ducts etc..
aside from using a big sponge filter - (which keeps mice and loose bolts out - the filter changes nothing- If one has the time to experiment - a well shaped inlet trumpet MAY (may not depending on airflow speed) help airflow but typically the only noticable effet is that spit back will be captured in the inlet .

I had a Chev 283 with a Paxton supercharger, the blowers output was fed into the carburetor inlet so the pressurized air went blower to carb. to engine. We used a special fuel pump to feed the carb. so that the blower pressure did not stop the fuel from filling the float chamber, also used a special seal around the carb accelerator pump shaft so that the higher pressure in the carb. did not force fuel out past the accelerator shaft. The vent hole in the float bowl was sealed to outside air also. THis set up worked very well and made that little 283 really run.

WRK

The carb kit - that goes back a ways
Ford offered a modified THUDERBIRD engined 1957 car with the Paxton too.
A kid from California had a convertible with the setup - Iwas really surprised at the obvious hot rod addition coming right from Ford. A 283 Chev with a blower was terrific - I ran a normally aspirated 283 - which ran 103 in 13.9 --in 1958. good for back then.
My 274" flathead Ford had three carbs and the curved inlet scoops for the Stromberg 48's - were supposed to be the good setup -
really made no difference at all -I ran aircleaners (?) with little chrome bonnets. Ditto on my 283 Chev .
In the same type car (39 coupe and 40 Ford coupes) the Chev ran 18 mph faster in the 1/4- the poor breathing of the Ford killed it - and I ran the same three carbs on both engines!

HAY Dick, What did a 283 with a blower run in those days. ? Don.