http://www.powerpage.dk/just_karb/webers.htm

 

the smaller pump jet keeps so much fuel from exhausting back into the float doesnt it? I dont think that the accelerator pump circuit has any check valve. you pretty much add spring to get a bigger shot from the pump. or you can drill the orifice from the pump to the throat of the carb.

so is the flute the hole on the jet that enters from the side?

also I am running a 65f9 and it seems a little rich. like fouling the pluggs rich. I have a 42mm choke. maybe I need to go to a different flute size. like a 65 f8 or a 65 f7. would that be in effect a smaller jet?

I used a different emulsion tube and did some more tuning too. I had some f7 tubes lying around and installed them with 180 mains, 175 air, and it ran even better. particulatly at high rpm. I tried using a 200 main with that emulsian tube too but to no avail with any air blead. it always started missing at high rpms. any haw the tuning is more managable with the smaller emulsian tube.

 

I'm back with more questions!

Ok... since my last post - I have the following :

48mm DCOE
38mm Chokes
180 mains
165 Airs
F11 Tubes
65 F9 idles
50 Accel pump jets
Closed Pump discharge valve.
18mm pump rod
Heavy pump spring

The main problems I have experienced have been with flat spots/stumbles at very small throttle openings. I always thought this was idle mixtures etc... but it turns out changing the accel pump discharge valve fixed it almost straight away. I previously had a 100 discharge valve - which I soldered completely shut.

With the 100 valve, I could not get the car moving without massive clutch slips and big revs - hill starts were impossible. The car is now almost perfect. Almost...

It seems that most people really only pay attention to the main circuit, whereas nearly all of your driveability comes from the idle/progression circuit. In saying that, the accel pump curcuit is very sensitive and something i have spent a lot of time fiddling with over the past weeks. This is probably the most tuneable part of the carby, but something that everyone appears to neglect.

* Pump Rod - they come in different sizes 10,12,14,16, 18mm. The pump rod determins the duration of squirt and also the total volume the pump circuit will hold. I modified my pump rod by inserting a skrew into the bottom of the rod to extend its length. This allowed me to squirt fuel during the progression circuit eliminating flat spots during transition. (No fuel during transition is the cause of the familiar flat spot that occurs on Nikki carbs with mechanical secondaries)

* Pump spring - Webers come with many different spring tensions. If you have the top off your carb - quickly open the throttle and you will notice that the accel pump does not drop at the same speed. This is why you cannot jump on the throttle without getting flatspots. I heavier spring will let the pump follow the movement of the throttle better (On most factory carbs, the accel circuit is purely mechanical and is physically linked to throttle shaft opening) This is something I have tried to replicate with the heavier springs. It has made a noticable difference.

* Bypass valve - Determines how much of the fuel in the pump circuit is used. The pump circuit will always hold full volume, but depending on the bypass hole size, some of this fuel will be pumped back into the bowl instead of into the engine. (ie, a smaller discharge hole means less fuel is dumped back into the bowl and more goes into the engine) My experience has revealed a closed valve works best. A neat way to test, is to slip a piece of plastic hose over the hole. You can then use pin ****** in the plastic to simulate "swapping" to different size jets.

* Pump jets - meter the volume of fuel that gets sprayed into the engine. The fuel delivered also depends on the spring weight. Again, a small jet with a light spring will spray a small amount of fuel over a longer duration. Spring weight vs jet size should be pretty clear.

So......

In saying that... what pump jets are people using? I am going to by some new jets tomorrow, so I will post back to let you know how I go. I am thinking of going from a 50 to a 55 or 60.

I am still currently experiencing a small flat spot when I try and jump on the throttle during a hill start or similar.

In terms of fuel economy, I am getting around 18mpg in the city (read: thrashing it on weekends) but have stretched to 26mpg on the highway! Very impressed

I am definately making more power and torque with the Weber than the Nikki. With a standard duty pressure plate and standard friction plate, the car will spin the cluth from 3500rpm through to 8000rpm in 1st and second.

 

Just another thing... from one of the other DCOE threads, someone listed the following for a stock port 12A :

40mm Venturies
180 Main Jet
F-11 Emulsion Tube
160-175 Air Corrector
F-9 65 Idle Jet
65 Pump Jet
0 Inlet Valve
(This was from Fast Freddie - I don't know who Freddie is, but he knows his stuff)

With my mild ported 12A (basically just a clean of the stock ports) the closer I got to the above jetting, the better my car went.

I would go so far as to say that the above jetting is "perfect" or a very very good starting point for a 12A. The only thing I would change is the choke size. I tried 40 and 42mm chokes and they were just too big. The car was a stuttering gutless mess below 4000rpm and above that did not make any more power than the 38s.