cut and paste of other posts of mine on this subject.
According to my Weber bible, the 34DATR 2/250 is standard fitment to a Lancia Beta 2000. (beta 1600 is a 34datr 1/250)
primary venturi 25mm
secondary venturi 27mm
diffuser 4.00
primary main 1.20
secondary main 1.50
primary idle 0.50
secondary idle 0.80
primary air corrector 1.70
secondary air corrector 2.40
emulsion tube F30
pump jet 0.50
needle/seat 1.75
I can scan the page from my book if you need more info, but it's in Italian.
Rules of thumb with carb setup....
A larger air corrector weakens the mixture more at higher than lower RPM.
As a rule of thumb, 3 progressions of air corrector (say 1.50 down to 1.35... so 3 x 0.5mm jumps) equals (roughly) one step up in main jet size (say from 1.50 to 1.55)
Increasing the main jet orifice size enriches the mixture uniformly at high and low engine speeds.
Both these metered parts are linear in their numbering... so a bigger suffix = a larger orifice... however when it comes to emulsion tubes, this goes out the window and you need to consult reference tables to decide what is richer/ leaner... as E tubes doesn't follow a numerical progression... just to make life interesting...
if you want a copy of the E tube table, I can scan it and post with cross reference to general applications and changes
Emulsion tubes have more influence at small throttle angles and during accelerations, significant factors that influence it's operation are the outside and inside diameters of the E-tube, which alters the amount of fuel that is displaced in the jet well, generally a thinner tube is richer as it will leave more fuel in the jet well for the engine to draw on.
The location, size and numbers of holes that it has along its length control how the fuel is emulsified as the fuel is drawn from the jet well thru the primary venturi.
idle jet size on the secondary will affect how the carb comes onto the transition phase between the idle circuit and the main....that's when the throttle blade is about 1/4 of the way opened.... and the idle jets also spill fuel at WOT.
Look in your Haynes manual on page 56 and 57, these are pics from the OE specifications manual. The three pics on the top of page 57 show idle on the idle mixture screw, primary transition and secondary transition respectively. The fuel to the secondary transition ports is metered by the secondary idle jet.
Idle theory
<So the secondary idle jet only comes into play at secondary transition? If so, it has nothing to do with my long-standing idle issue. Correct?>
Yes that's correct.
<When idling, fuel is delivered via the idle passage (item 37 on Haynes page 57). Amount is controlled by idle mixture screw.>
Again, spot on correct. Prim idle passage is also notated as 35, before the idle jet where fuel is draw from the main jet well. Not really shown on the idle mixture screw (39) is an O ring, this is VITAL for correct idle mixture, otherwise air is drawn down the idle mixture screw threads. The idle mixture screw should also have a very fine point, when they are overtightened, the fine tip can bend / be flattened out, which will affect the idle mixture control. Once the throttle blades move past the primary throat progression ports, fuel flow is controlled by the primary idle jet
<The idle speed adjustment opens/closes the primary butterfly, correct? At idle, should the primary butterfly be completely closed? (if so, what's the point of the idle speed adjustment screw?).>
Correct, the idle speed screw is shown on page 58, notated as item 19, it has a spring to hold it in tension so it can't back itself out. At idle the butterfly is almost completely closed, the purpose of the idle speed screw is actually more of a throttle stop, to prevent the butterfly bashing the sides of the carb, instead it comes to rest on this stopper. The idle stop screw should JUST be holding the prim throttle blade off its absolute stop.
<I'm beginning to convince myself that I have a blockage somewhere in the idle passage. I can set the idle to 1000 rpm using the idle speed screw. Go for a drive and it idles at 1800. Return home and I can completely close the idle mixture screw and it still idles at 1800. Reopen mixture screw 1.5 turns, back out speed screw until 1000 rpm, all seems fine, turn your back and she stumbles and slowly dies.>
Classic symptoms of blocked (or partially blocked) idle fuel cicuit, or an air leak into the idle circuit (which will not let the fuel be lifted from the main jet well into the idle circuit), or too small an idle jet on the primary (but this would be if you had a long duration cam and low manifold vacuum)
I would be stripping the carb, and really concentrating on passages 35 and 37, there are some excellent spray throttle body cleaners available these days, try some of this and then blow out with compressed air, concentrate on the prim idle circuit.
Other thing to check would be vacuum leaks at the manifold fittings. Our 78 x19's have a takeoff for the fast idle check device, this can leak air and upset the idle metering too. That's the fitting on top of the manifold, on the runner to number 4. The fast idle actuation device (not shown in the haynes manual) attaches to the "rear" of the carb just below the gasket line, and when vacuum is supplied to its diapragm, actuates a lever to raise the idle up to around 1800 revs ... I prefer to cap these off myself, all this circuit does is provide a fast idle for CO check (and who does those in Australia!) it contributes nothing to the operation of the carb.
SteveC
