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Discussion Starter #1
Howdy,

I'm freshening up a pair of DHLA40F I got of an 80's Euro Guiletta for installation on my US 2l '74 Spider.
(this is street car which is getting a set of 10548 camshafts with 10.1 motronic pistons)

Looking at the jetting I have I'm wondering what if anything I should change outright:

Choke/ starter jet: 70
Main Jet: 132
emulsion tube: 7772.8
air corrector: 210
idle jet: 55
pump jet: 33

The oft-referenced ebay website of Alfa1750 lists:
(upto 135 BHP) 32; 150; 7772.11; 200; 60; 33-40; 150

With the exception of the choke which seems way out there would you recommend changing anything else? btw, why would a choke be so high?

Thanks!
 

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Seb I'm no expert but I think you've got the wrong number on the main venturi/choke there on your set. The biggest one I know of to fit in a 40 is like a 38 and the largest recommended is a 34 or 36. 70 is just way too high. Keep in mind that this number is the number of milimeters of the opening of the venturi.

I think a performance 2L motor is usually recommended to use a 34 venturi with appropriate changes in emulsion tube and main jet. Standard 2L motors use the 32 so I bet if your motor is in standard tune that you're all set.

Karl
 

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According to the Haynes Alfetta workshop manual, the jetting for the Dellorto DHLA40F as standard was:
Venturi 32 mm (Sometimes also called the choke, this is actually the main venturi which fits inside the barrel of the carburettor. They are ideally around three quarters the size of the carby barrel.)
Main jet 1.45 (stamped on actual jet as 145)
Main emulsion tube 7772.08
Main air corrector 2.10 (stamped as 210)
Idle jet 0.52 to 0.55 (depending on date of production)
Idle air corrector 2.20 (these are usually stamped with a special Dellorto part number, such as ‘.1’ for a 7850.1, so I don’t know how that correlates with the 2.20 that Haynes shows)
Choke jet 0.70 to 0.80 depending on date (This is what you were looking at, NOT the main venturi, which would most likely be a 32 mm. Your setting of 70 agrees with this, as a 0.70mm jet. This jet is only used when you have the 'cold start' device opened, to enrich the mixture when the engine is cold.)
Pump jet 0.33

My Alfetta 2000, which was an emission-controlled Australian example, had DHLA40G carbies, and much leaner main jetting – a 240 main air corrector and 7772.10 emulsion tubes, which had massive holes in it compared to the 7772.08. Unfortunately the Australian cars suffered badly from that jetting, with flat spots, and 6.5 kW (10 bhp) less power, so I returned mine to the European jetting, which improved driveability markedly. According to the Dellorto manual that I’ve got, the main difference between the DHLA40G and the DHLA40F fitted to European cars was that the G bodies had an enclosure tube around the idle mixture screw, so that they could be fitted with tamper-proof plugs to prevent changing the mixture setting, if required. Luckily the tamper-proof plugs weren’t used on mine, or else the dealer's mechanics had already removed them!

The ‘new’ Giulietta which was sold in Australia and Europe in the early 1980’s was basically just an Alfetta with a different body and interior, and all the running gear underneath was basically the same. So your settings would be ideal for a standard 1.8 or 2.0 twin cam, although the main jet seems very slightly on the lean side compared to the jetting that Alfa usually used on Alfettas (1.45 mm). If you are going to modify the engine for more power, then you need to change the jetting, and particularly the main venturi size. When I had an Autodelta Stage 2 kit fitted to my Alfetta, which consisted of 11mm lift cams and bigger carbies, the main differences in the carbies were:

Venturi 36 mm (These definitely needs to be bigger than 32, otherwise you won’t be getting the full benefits of your cam and compression change. The 32 size Alfa fitted was slightly down on the maximum power ideal size, which is about 33mm for a 2.0litre with max power at about 5400. Alfa tuned theirs in the interests of good driveability all through the rev range, but with hotter cams, max power will be higher, maybe 6000 rpm, so you need larger venturis to flow properly.)
Carburettor DHLA 45 (but a 40 will do, at a stretch, as a 36mm venturi is the largest that can be fitted to a 40)
Main jet 1.51
Main emulsion tube 7772.07 (these weren’t that different from a 7772.08, so they will probably do OK)
Main air corrector 1.70
Idle jet 0.55
Idle air corrector 7850.1

See http://members.aol.com/dvandrews/dellorto.htm , which gives some info on tuning Dellortos. He also has a software program, jetting.exe, which gives settings for Dellorto or Weber carbs based on engine size, revs at maximum power, and whether driveability or outright power is desired. In my experience, the most critical part of jetting Dellortos or Webers is usually the progression circuit, which is set by the idle jets. The easiest way to test if it is right is to increase, very slowly, the idle speed using the idle speed screw from idle through to about 2000 rpm. If at any point the engine runs roughly, coughs or spits, then by changing the idle mixture screws, you can determine if the idle jets are too lean or rich. If by turning the screws in, the engine runs smoother, then it means that the idle jets are too rich, and a smaller size is needed. If, however, it runs rougher, then try unscrewing the mixture screws from the base idle setting – if it then runs more smoothly, it means that the idle jets are too lean, so increase their size. Eventually, you should be able to slowly increase the idle speed from 1000 to 2000 rpm, and it will run OK all through the range, including at idle. The main jets should be starting to flow by 2000 rpm, so will have the most effect at speeds faster than that.

Regards,
Don
 

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It's number 12 on the drawing above - it's actually a combined air corrector jet, emulsion tube and idle jet holder. That diagram actually shows the full list of available idle air correctors, from weak to rich, if you go to the web link you have shown. I think that .1 is generally the most widely used, as it's right in the middle between rich and lean mixture. It's easiest to change the idle jets, as like the main jets, they are usually cheaper than the emulsion tubes, and there is a wider range.

Don
 

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Discussion Starter #6
Don, Karl,
where are my manners....I didn't take the time to thank you for your help. Much appreciated! I'm waiting for my carb tunning manual and will order the jets and other bits and pieces once I better understand the connection between the various systems. Have a great week-end.
 

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NO problem Seb. Best of luck.
 
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