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Discussion Starter #1
Does anyone have the factory specs jetting for the Weber 40s that were fitted to the 2000GT Veloce (10521)?
Can the venturis/barrels be increased to 34mm? What effect would this venture change have on the overall jetting. I believe the normal venturi/barrel size is 32mm. I'd like the engine to breathe better without going to Weber 45ers.
The engine is not tuned. I believe my ports are polished and I just have electronic ignition.
Any info would be appreciated.
Wolf
2000GT Veloce
1973
10521
 

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Discussion Starter #2
At last I had the time to fit my 40 DOCE Webers in replacement of good old 40 Solex ADDHEs.

The Weber carbs were sold new to me with the understanding that they work on the 2l engine.

Below's the jetting I have. The car runs OK but could do better. I have some hesitation when suddenly giving gas.

I thinking of changing the Main Jet to 132 or 133, the air correction jet to 190. I don't know what to do with the Idle Jet. 50F8? (I don't have access to a dyno)

Does anyone have any other suggestions??

Venturi: 32
Auxilary Venturi: 4.5
Main Jet: 135
Air Correction Jet: 210
Emulsion Tube: F-34
Idle Jet: 55F17
Starting Jet: 65F5
Starter air jet: 200
Needle Valve: 150
Accel. Pump Jet: 35

Wolf
'73 2000GT Veloce
10521
 

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Hi Wolf,

Your Carbs are presently set up with the correct jets for a 2 liter engine with European cams. First thing to do as I see it is to insure that the cams are European. Being in France, you may know they are Euro cams, but if not, the part number on the cams fir Europeans cams will be 105480320001. The 48 in the 4th and 5th positions is the clue.
If the car has Spica type cams, you have to change the Main, Air Correction, and Idle jets and the Emulsion tubes.
If the cams are correct for these carbs, the hesitation is caused by poor idle adjustment or a bad carb component or it is not a carb problem.

Jets for US cams part number 105200320000

Main 130
Air Cor 200
E. Tube F-9
Idle 55F17
 

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Discussion Starter #4
The cams are European.
The car originally came with Dellorthos. As these, after 22 years of good and loyal services gave up, I had fitted Solexes as a temporary measure--now for 8years.
Thanks for the post
Wolf
'73 2000GT Veloce
10521
 

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If your hesitation happens below 3000 rpm when slowly accelerating I would say your idle jets are to lean.
typically a 50 series idle jet is "leaner" than a 55, so try a 55 f 8, 9, or 6. 6 being pretty rich.

As to the original question, I have a 34 venturi on a 1400 alfa motor, so don't think you will have a problem with 34s. An increase to a larger venturi usually means an increase of the main jets too.
However I think each set of carbs weather a little differently, as an example in tuning my car, the books all said I needed 130 mains with my motor/carb setup. I spent many hours trying to work with 125s (after 130s looked rich)trying many emulsions.
I am now using 120s with 220 airs with the 34 venturis and f9 emulsions. While I am sure another set of carbs would work well with bigger mains.

Back to your hesitation, if you are having trouble when you hit the gas, you might try a 40 pump jet. Perhaps a 200 needle jet.
Were your carbs rebuilt ? floats checked?

If you can feel any hesitation any where in the rpm band you would go a bit richer till the hesitaion is gone, below 3000 rpm it would be the idle jets above 3000 the mains or airs. If the weakness is felt at the top of the rpm band then correct the airs.
 

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Discussion Starter #6
The carbs are brand new. They were never used. I had to change all the rubber seals and some of the paper ones. Floats are metal (brass). Float levels checked out OK. Also checked out for air leaks. None.
I have to experiment and try out a few combinations. I already thought of increasing the pump jet from 35 to 45, changing the main jet from 135 to 132, and the air correction jet from 210 to 190.
Anyhow I'll give the car another run tomorrow to check exactly when the hesitation symptom exactly occurs.
Wolf
'73 2000GT Veloce
10521
 

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Discussion Starter #8
Took the car out today for a drive to check on the hesitation problem.
The hesitation appears when you drive off in first gear just when you push on the accelerater.
It also appears throughout the rpm range when you are cruising in steady half open throttle and you then push on the accelerater to pick up speed.
If this is of any help?
I'm beginning to suspect the accelerater pump/jet. The pump system might be a little stiff for lack of use (carbs are unused old stock).
Next week will check out the distributor advance mechanism. The distributor has never been replaced or overhauled and is in use now for 30 years with help of a contactless electronic ignition.
Wolf
'73 Coupe 2000GT Veloce
10521
 

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Sounds like something to do with the accel pump circuit. Try taking off the pump jet plugs and work the throttle by hand. Does fuel come out? If so, pull the pump jets and make sure that those tiny little .35mm holes are clear. If they are clear, I'd go up a step or two on the pump jets.
If fuel doesn't come out, then I'd suspect something with the pump piston or the inlet/discharge checkvalve(s).
 

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If the hesitation occurs when you suddenly open the throttle then the problem may be the pump-jets.

But if it happens as you pull away from idle, or when you open the throttle gently then the problem lies elsewhere, in fact if it's masked by opening the throttle quickly then it's not the pump jets.

If so I'd experiment with a leaner idle jet. A rich idle jet can definitely cause a stumble off-idle.

Going to 34mm venturis will probably increase ultimate horsepower - good for going racing etc, but won't help driveability around town and certainly won't help the stumble. Alfa themselves actually reduced the inlet port diameter when they built the GTA because it increased the inlet gas velocity.

-Richard
 

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I have to reiterate my feelings regarding carburetors and carburetor problems After this I will remain quiet on the subject rather than being a pain in the butt.

OK the objective of the carb. is to present to the intake valve a mixture of fuel and air in as close to the optimum ratio of fuel to air as possible. The ratio will be effected by the amount of air pulled thru the carb by the intake stroke, the amount of raw gas that the carb parts put into to the air stream, the density of the air, the amount of fuel/air mixture that the design of the intake system allows to hang around outside the combustion chamber.
All of these factors are determined at the time of engine design except the density of the air. They do not change! Unless something is "broken".
Now if the factory people, through design and testing come up with the optimum carburetor settings and sizes for a given engine and carb setup, why would you ever want to change those settings or sizes. If you take the car to a very different altitude and thereby change the density of the air used, then it might pay to change some things. If you change the car's primary use from street to racing, it will pay to check the performance of the system. But check with something that can measure the results. This is simply not a place or time for trial and error because the trials compound the errors and you chase your tail.
If a cars behavior goes from excellent to problematic and none of the above factors has been changed, why change the carb settings and or sizes. It makes no sense to me at all. Find the problem be it bad ignition, dirty carb jet(s), bad valve timing due to adjustment or human induced changes, a leak somewhere or what ever but SOMETHING IS "BROKEN".
Trying different jets or venturis gets you nowhere. It seems to me that all you are doing is masking the problem. In the past, too many mechanics have tried to convince us that there is a black art to carburetors and this myth has become "truth" to many. So we run the risk of paying for hours of redesigning part of the system and not the other parts and never fixing the real problem.
OK that is my take on the subject. Fire away if you will.
 
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