Lancia Beta Forum

My car has a carb of the twin choke register type, i.e. second choke only comes into play at nearly full throttle. The odd thing is that it has a bypass screw and a mixture screw for both chokes, between the chokes i.e. at the top of the carb. Why does the second one have a bypass, and how do I adjust this type of carb? The carb does not correspond to any of the illustrations in the factory manuals I have access to.

FYI the bypass for the secondary is closed, and in general the car runs a bit rich so I intend to check the mixture screw O-ring tonight.

what is model number of the carb I can have a look and see what info I have

I did not see any numbers in the castings (but did not really search). Where should I expect to find a number?

On the edge of the flange where it meets the manifold

34 DATR 2/250

That's the standard auto choke carb for the 2.0 engines. I think it's shown in the Haynes manual

Do you not think you're looking at the two slow running jets ?                                                                                                                       

I think it is one slow running (idle) jet and one mixture screw for each barrel. The strange thing is that there is one for the second stage barrel that only comes into play near full throttle.

I cannot detect any difference opening or closing the secondary barrel bypass.

There is a slight hestitation when giving a bit of throttle but the accellerator pump does squirt fuel.

Some sites showing a similar carb talks about an O-ring on the mixture screws but there are none on mine. The mixture screw has to be fully in or false air makes the engine stop at idle. The mixture screw is a long tube with tiny holes along the side. The mixture screw sits alongside the bypass screw for each barrel (choke).

I haven't got the Haynes. Only various original manuals and none showing these adjusting screws.

Per, PM me your e-mail  - I'll try to send the section of the Haynes manual that shows the carb. I'm pretty sure you're looking at the idle jets and emulsion tubes and yes there is one per choke.

Stuart, pm sent and thanks.

I will pull both emulsion tubes tomorrow. These are fixed mixture devices it seems. I will compare the primary and secondary tubes. One thought strikes me: Are there versions of this carb where the butterflies move synchronously instead of one acting as a secondary as in this application? That would explain it and the screw in the secondary should simply be closed.

Not as far as I know.

I tried opening thr pdf on two different machines but no luck. I wonder if the file got corrupted in transit.

I'll try again

I'll try to post some official tech info tomorrow just had a look and think I have some info but need to check and photo relevant bits

Thanks the second lot and also jpg came out fine. It does give the procedure for setting slow running but does not indicate how the secondary comes into it. I will experiment a bit today and post here.

As you've kind of said further up the post the secondary barrel butterfly opens as the throttle approaches full power. This is done by a mechanical link which is not adjustable. For carb adjustments/problems I found this link really good;

http://www.betaboyz.myzen.co.uk/forum/index.php?topic=649.0 (http://www.betaboyz.myzen.co.uk/forum/index.php?topic=649.0)

The idle jet on the secondary doesn't operate at idle!

It's there to help progression when the secondary butterfly just starts to open providing a small amount of petrol as it opens before there's enough flow for the main jet to operate.

The slow running jet on the secondary throat has no effect on anything that I can detect.

The two emulsion tubes are different. Both has the code F30 near the top and the primary has V120 near the bottom while the secondary has 150 (no letters). It is a mystery to me as the secondary opens very abruptly and so cannot really experience any slow running conditions except as a very short transition frpm closed to pretty much fully open. But maybe when under heavy load it will have an effect?

It is a mystery to me  ???

had computer issues after an update blank screen on Allas laptop dont you just love microsoft! anyway no time to photo the info will try to do it tonight though (assuming the system refresh worked first time it got stuck at 82%)

The numbers on the jets and emulsion tubes relate to their size. More about this in the thread that I linked above I think.

Apologies for delay this link may explain?
https://www.dropbox.com/sh/ta5mq1ng6y09r54/AADCjMZTLnvgdZDOQjLX1qTRa?dl=0 (https://www.dropbox.com/sh/ta5mq1ng6y09r54/AADCjMZTLnvgdZDOQjLX1qTRa?dl=0)

I have more info but sorry no time to do it now

this type of carb was fitted from series 2 facelift onwards so it does not feature in the official manual (which ony covers s1 cars) and possibly not haynes (didn't check) as they never have updated manuals very well 

I wonder if the secondary idle jet on mine takes the role of the "calibrated bore" with it's spring ball valve? If so the workings imply a slightly different emulsion tube compared to what would be needed in the example in the link. Anyway I have opened the secondary idle jet one full turn and will drive it like that for a while: If I do not experience hestitation when giving it the full welly that will be how it stays.

The car ran well, eventually. I sold it due to having too many, but as written elsewhere that was a stupid mistake. Yep there are mistakes that are not downright stupid  ;)
I bought it back last year when I found out it was sitting forlornly in a garage.

In the meantime the clowns had been at the carb again. It was all over the place. The lowest CO I could achieve was 6.9%. In addition the idle jets were so loose in their bores they rattled their way down to closed in a few minutes at idle. The float ensured the engine flooded, instead of the prescribed 7mm it was several mm out.

Nearly there now, next is tuning with gas tester.