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Alas, your description of the effect of Venturi size is completely wrong.

An engine is an air pump, not a fuel pump. It's horsepower is limited by how much air can be put into the cylinders. After that, the amount of fuel is matched via carb jetting, or fuel injection control to give the intended fuel to air ratio. Too much fuel for a given amount of air results in less power, plus other issues.

Generally, Venturi size is chosen to optimize the power band to the intended use of the engine. The higher the RPM the more power can be made, but other considerations such as driveability, fuel consumption, etc often argue for less than possible power.

Most Alfa 2L engines seem happy with 32 - 34mm venturis. I expect your car is fun to drive given its likely strong low rpm torque, but you're likely missing the full experience available.



I wish I could say I ended up with the 30 chokes after careful consideration and exhaustive testing but it's mainly because the 151's come with 30's at least mine did. However my thinking on 30' vs 32's is the 30's are narrower than 32's which means the air moves faster in the 30. With faster air there is lower pressure, lower pressure means more fuel is pulled/pushed through the jet, more gas, more power. If my theory is flawed I certainly won't mind being corrected.
 
Discussion starter · #23 ·
Yeah, I get the volumetric effencicy and the air pump thing, 101 stuff. The otto cycle engine is a series of compromises towards increasing the VE and the stars only align in a 300 rpm range and in a normally aspirated engine a 70% VE is outstanding and can be as low as 25% at idle. Where the tuner puts it is where the fun is. Your logic assumes the VE is ideal and I agree if we are at max VE more fuel/air is counterproductive but it is a big if. Then again you are certainly entitled to your assumptions but lets not make them anything more than what they are.
 
Assumptions? Nah, just responding to your "won't mind being corrected".

As noted and in response to your statement that "more gas, more power", it just seemed you didn't get the basic idea of an internal combustion engine. It might be true in a steam engine that "more steam more power", but that's not what goes on in an internal combustion engine.

Having said that...

I've spent a fair number of years encouraging customers to NOT over-carb their engines. We Americans seem to be in love with the idea of re-engineering good cars by reading product advertisements and bolting on parts. Usually, this is counter-productive.

I recall many VW customers coming in to the store, buying twin two-barrel carbs, a big cam, huge cylinders, and putting it all together on a Bug with huge rear tires. Most never drove the cars more than 100 miles as they were useless as vehicles.

In your case, the smaller-than-typical venturies probably make it a great stop-light launcher, and very well behaved around town. A great choice if that's the kind of driving you do. As other posters have noted, and the guidance provided by widely circulated Weber/Alfa literature, our engines behave extremely well with more venturi than you have. By "well", I mean with great around town driveability, good gas mileage, and a fun enough top end. If I were converting a stock 2L to Webers, I'd just copy the Euro spec and carry on. I think they used 33 or 34s, but don't trust my memory on that.

I've got 33's in my 2300 (the Brazilian version of the early cast-iron 2000), and it is a sweet heart to drive. Verrrrrrryyyyyy tractable around town, but able to hit 115 on a straight. I'm in the process of bumping the cam a little, converting to 45DCOE's from the current 40's, and will start with 34 venturis and go to 35's if they behave themselves. Maybe even more if it still works for in-town driving.

Having said that.... I recall being stopped by a highway patrol on Hwy 59 in Montgomery County, Texas about 2 in the morning back in 1978. I wasn't speeding. He just didn't like the look of an all-black Alfa Sprint GT at that time of night. It probably got him interested when he saw that it was lowered, had a 5-point harness, and clearly meant to go fast. He got really excited when he discovered that the only things I was carrying were a toothbrush, comb, change of underwear and shirt, a 12 gauge shotgun, a 218 BEE varmint rifle (with scope), and two boxes of 38-40 pistol ammunition, but not the pistol. He gave up when someone shot past doing about 90, and I pressed on with my trip to Indiana.

So - having a too-fast Alfa in East Texas might work against you.
 
Discussion starter · #25 · (Edited)
It wasn't 2 but it was after 12 on the way back from a Shreveport dinner and dancing night out. I had rebuilt a friends 70 911t and it was running so sweet in that cool night air it was begging, pleading, to run, the rev counter was all to naturally creeping upwards. Top a hill and BAM, there he was. I was stopped with my hands where he could see them as he came towards the car. I had a Superior Bar and Grill cup between my legs, started drinking water about an hour before we left, as he asked for my DL, where we were going, where we had been, etc. I was expecting the field sobriety test but he wanted to know more about the car, he wanted to see the motor so I got out and showed him. He'd never stopped a Porsche before and wanted to know all about it so we talked for several minutes. As that wound down he said he'd have to give me a ticket but he would make it for 75 so I wouldn't have to be hauled in and could take defensive driving. The only condition was I had to walk back to his car and look at the radar. He got a kick out of watching my expression when I saw the 94. As we cranked up he said to just take it easy and watch out for the drug profilers up about 15 miles.
When talking about tuning I think we can safely say there's more than one way to skin a cat, it's what one is after that counts. The one absolute I was talking about is that under some conditions one might suspect that larger venturis would lean the mixture since a greater amount of air is passing through but in reality the smaller opening speeds up the air, lower pressure, more gas lifted, pushed through, = richer mixture. sometimes

I would be open to trying some 32's though, no doubt about it. I have 135, 140, 145 mains to play with as well as 45f8 idle jets so sure why not 32.
 
All agreed. However, one never changes a single parameter in a Weber. Bigger Venturi will require a bigger main jet, and possibly air corrector.

Another time I got pulled over was testing and bedding in the brakes of my straight-pipe G-prod 1300 Spider on some back roads in central Indiana. Got a good scolding, but they are generally sympathetic to race cars around there.
 
...one might suspect that larger venturis would lean the mixture since a greater amount of air is passing through but in reality the smaller opening speeds up the air, lower pressure, more gas lifted, pushed through, = richer mixture. sometimes
Not exactly.
The amount of total fuel is governed by calibrated parts such as the emulsion tube, idle jet and main jet. Higher air speed will not result in any more fuel than the calibrated parts will allow. Basically, a larger venturi will increase the RPM at which the main circuit comes in. This can result in a flatspot during transition from idle circuit to main circuit since the main has not yet fully come in. Easy to dial in with a larger idle jet. Which means if you go with 32mm venturis, a 45F8 idle jet may result in a flatspot and/or intake backfiring on engine float.
 
Discussion starter · #28 · (Edited)
This is interesting, which is the point of it all I suppose....., air pressure, pipe size, head pressure, float level, volts and amps. The shorter hair with shades of grey seems to help with the heat too.
It's weird, I wrote 30 down when going through the carbs but I see 32 cast into the venturies in my minds eye, guess I'll have to check. I got the next leanest idle jet 45 f8 since the 45 f9 is at the rich end. Either way even though everything changes with one change I'll only change one thing at a time so as not to get even more confused than I will be just trying the 135's if that makes any sense.
I do have one question though, if the amount of total fuel is governed by calibrated parts such as the emulsion tube, idle jet and main jet why does the float level affect the amount of fuel, rich or lean?
 
venturies

Actually larger venturis (choke tubes) commonly need larger main jets and smaller air correctors to feed the larger volumn of air passing at higher rpms.
The venturies from Weber are pitiful "Venturi designs" and can be much improved by reshaping to get a better low pressure creation see the thread at:
http://www.alfabb.com/bb/forums/showthread.php?p=503983.
 
I do have one question though, if the amount of total fuel is governed by calibrated parts such as the emulsion tube, idle jet and main jet why does the float level affect the amount of fuel, rich or lean?
The fuel level doesn't affect the mixture per se like the idle mixture screws do.
A fuel level change in the main well (where the emulsion tube is) will change when the main circuit comes online. High fuel level means the main comes in early; low level, later.
 
Sometimes making a mess of carefully thought out jetting to that point! (Had to add that Jim!) Fuel levels MUST be the same in both Webers for this to work as Jim mentions. It is also a trial and error process to see if you are developing the tune you intended.
Remember with Weber set-up, keep changes simple, trying only one change at-a-time. Weber tried to make these easy to set up. Try to avoiding changing three or more things when ONE jet change will do the job.
 
fuel level

Actually the fuel level unless inordinately high, or low, has little effect on jetting, as when in running mode the jets are pulling fuel not from the top of the fuel, but the bottom of the carb bowls through the orfrices (fuel jet opening) then emulsified in the tube.

A common modification to Webers was to insert tubes in the bottom of the fuel pick up bores in the body to extend them nearly to the bottom cover of the carb to increase fuel availability in high fuel demand engines to prevent fuel starvation at hig "G" force loadings.
 
Discussion starter · #33 ·
Thanks, all these years.....
 
Richard did I do that for you? I've had lots of Weber owners request the tubes, but as you know they need to be hand fitted with Webers in hand. For readers that may want to try this, here's the routine.
From a local hobby shop buy thin wall brass tube just slightly larger in diameter than the jet pick up holes in the Weber bottom. Easy as the tubing is inch and holes metric. Cut the tubes to length flush from the hole to bottom cover. Heat the tube end that will be inserted unto the aluminum body a dull red (not cherry red) which will anneal (soften) the tube. Drive the tubes about 1/4 inch into the aluminum holes. I use a drop of loctite stud and bearing mount in the holes. The ends will deform to fit the hole and you now have long pick-ups. I have also threaded tubing and the holes, but this works no better than the glued in method, but is more labor intensive.
For street users this also allows you to pick up all the water and debris that collects on that Weber bottom plate and run it through your engine. Not a great plan. Race use it IS a good aid. I'm happy RJ mentioned it.
 
Discussion starter · #35 ·
I was just talking to the gentleman who had my clutch\flywheel balanced {(sachs) 17 grams out btw} and he mentioned fuel issues in a race motor, he probably knows about that but I'll mention it anyway. Old school cool
 
What!

Gordon,

From a local hobby shop buy thin wall brass tube just slightly larger in diameter than the jet pick up holes in the Weber bottom. Easy as the tubing is inch and holes metric. Cut the tubes to length flush from the hole to bottom cover. Heat the tube end that will be inserted unto the aluminum body a dull red (not cherry red) which will anneal (soften) the tube. Drive the tubes about 1/4 inch into the aluminum holes. I use a drop of loctite stud and bearing mount in the holes.
Huh? Drill them to 3/16 and insert a section of brake tubing using JB weld.

You sure work hard!
 
Discussion starter · #37 ·
I realize the amount is small, maybe planck time small but if I have two columns of liquid one higher than the other and two orfices of the same size at the bottom the one with the higher column will pass a higher volume over time than the shorter column so in theory the higher level will pass more liquid as will the one with a lower pressure over its orfice.
"The float level setting is absolutely critical because it effects the fuel mixture throughout the entire operating range of the carburetor; too high and the mixture is rich, too low and the mixture is lean."
Are weber carburetors that different?
 
Gigem,

Your description of the liquid height in a column is correct if it is gravity flow that draws the fuel downstream. The level of fuel in a float (in all carburetors) is generally lower than the outlet in the venturi. The fuel is pulled (sucked) by force of the vacuum created. Since the outlet is higher than the inlet, there is no siphon effect.

Purely for amusement, the float type carburetor in the yellow biplane was engineered to operate both upright and inverted, so long as inverted flight was carried out at full throttle. Gotta love the French. Or pity them.
 
Discussion starter · #39 ·
I see your point but I have to respecfully disagree. Just as the mixture is pushed into the cylinder by atmospheric pressure and not pulled by the lower pressure of the cylinder so is the fuel pushed through the jet by atmospheric pressure. If one thinks of air as having weight and a vacuum as being the absence of air pressure we can see the vacuum isn't sucking anything but rather the air is pushing to fill the lower pressure because of it's weight. Wind blows because it's pushed not sucked.
 
The fundamental difference between emission Webers and "classic" Webers is in the progression circuit. Emission Webers, like most other non-DCOE Webers draw the fuel from the well. Classic Webers draw the fuel from the float chamber. At larger air flows the pressure in the well is lower than in the float chamber and the progression circuit rolls off in Emission Webers. At WOT and high revs there can be reverse flow in the progression circuit as the pressure in the well is lower than the pressure at the progression holes. Then air is drawn into the well through the progression holes. I think this is the reason for emissions Webers using smaller air corrector jets.

The progression circuit contribution is additive to the main circuit in classic Webers and it is not additive in emissions Webers. This is the reason that emission Webers need larger main jets than classics. The F34 emulsion tube that is commonly used in emission Webers is similar to the F4 that is used for alcohol fuels in classic Webers. This tube occupies less volume in the well and the fuel level is less affected by pressure transients in the well. Emissions Webers also use idle jets with much larger air orifices. It is important to have the appropriate emulsion tubes and idle jets in emission Webers.

Although John Passini's books do not cover emissions DCOE's this interaction of the main and progression circuits in other types of Webers is explained.

The main circuits of emission and classic Webers are AFAIK identical and I do not see any reason why emissions Webers should produce less power than classics of the same size.

One thing that puzzles me is the classification of 40DCOE150/151's as emission Webers as they use the same emulsion tubes and idle jets as classics. It is my understanding that they have progression circuits fed from the float chamber. Maybe they are a hybrid design?
 
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