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Discussion Starter · #1 · (Edited)
Problem: violent back fire and spitting through both webers when engine is cold. Moreover, on occasion I can even see a small ignited flame (very small and instantaneous) at the back of each carb near the carburettor mounts and the choke. When it back fires I can see the nuts that hold the choke cable move - I think from air movement (the choke cable is not currently installed). I have not warmed up the engine to know if the problem goes away however the backfiring problem is too severe to do this anyways.

The head was rebuilt and replaced some months ago and the car was running ok at this time although there may have been some very minor problems with ticking or popping which I could feel through the accelerator - I'm not sure what the reason for this was and it was rare and very intermittent. I garaged the car and removed the Webers.

I have now re-installed my 40 DCEO 76/77 Webers on my GTV 2000. These were rebuilt by an expert carburettor restorer. He described these as emission Webers.

Checked: Float level ok;
Checked: Pump jets checked (number 35) and are squirting fuel out ok although one does squirt further than the others;
Checked: Main jets correct (number 135);
Checked: Throttle plates synchronized using the inspection holes;

Replaced: New plugs;
Replaced: New Filter King fuel filter;
Replaced: New mechanical fuel pump, new fuel lines, new petrol tank;

Replaced: Inlet manifold was checked for flatness by machine shop. New manifold gasket (with metal inlay) coated both sides with hylomar;
Replaced: New aluminium carburettor mounts with rubber 'O' rings instead of gaskets. Hylomar on gaskets between aluminium mounts and inlet manifold. Carburettor aluminium mounts used rubber buffers with metal cup washers and I was advised by Alfaholics to tighten up the rubber buffers on the carburettor until there was a 1-2mm gap between the metal cups. This I have done and indeed I have gone even tighter to no avail. On my second re-installation I even put a very thin film of hylomar on the rear of the carburettor where the rubber 'O' rings sit. Again no improvement.

Initially I used Permatex Ultra Grey on some of the gaskets which is when I initially experienced the problem. At the time I thought it was an air leak so I removed everything and started again using Hylomar instead or Permatex Grey but this has made no difference to the problem.

The car has 123ignition (electronic) which was installed prior to the head being rebuilt.

I would very much appreciate people's suggestions for how to solve this problem.
 

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What idle jets are you using? I have found that 55F17 or 55F21 work best with emissions Webers. I usually run the leaner 55F21's in the summer and the richer 55F17's in the winter.
 

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Discussion Starter · #4 ·
Um I'm not sure, but I'll check this tomorrow.

I wondered if all the backfiring may have damaged the seal between the carburettors and the mounts OR is there an outlet at the back of emission Webers that permits backfiring to escape and would that then explain the small ignited flame and puffs of air near the choke?

In addition I can't find any book which identfies what these screws are (as shown in the red arrow below). Are they air intake screws used to balance the air flow in the carburettors after the throttle plates have been aligned?

IMG_1516.jpeg
 

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Hi WA-Alfa

I am having the exact same problem. My backfiring is just a small puff but it consistant. I have done almost the exact same things you did with replacements and fixes. My Alfa is a 62 spider with 40DCOE2 Webers I changed my idle jets to 60F11's but I can't get it to idle smoothly below 1500 rpm. It seems to want to run rich. I synced my carbs with a Sync meter they are right on. I would appreciate any help you can supply.I am ready to give up a go to an expert if there is such a thing. (costly to say the least). I don't the plugs you show in your picture on my carbs,but they look they something involving progression holes? Thanx again

Meatbird29
Al Conforti
 

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The screws you have indicated are just plugs. Remove one and take a look. In some applications with this Weber they can be either, for attachment of a manometer (air flow), or a fixed idle air correction (I've never seen this).
I would start by checking static ignition timing and move to cam timing and valve clearance. If all is OK there, then I would look again at low speed mixture.
I ran a 1300 101 race engine (many years ago) that use a pair of custom ISKY cams. At "idle", below 1500 rpm, these cams caused popping and flames out the velocity stacks. This was regardless of mixture. The trick was to just get it to "idle" or even run! Once into the main Weber circuit, it REALLY WOULD RUN! This was a cam design problem, the engine having essentially no torque at low speeds but ALL power from 4000 rpm to as high as you dared to go. Todays cam designs do not do this, unless timing or clearance is incorrect.
Your worst-case situation would be to contact a Weber jetting professional like Gron Parry (cell 408-781-3770) in California, and ask for jetting information from his records for a similar engine and Weber combination. As this seems isolated to just the idle circuit thus far, you might start by just really enriching that circuit and see if you can stop the popping.
Another cause might be air leaks at the aluminum manifold connectors. These connectors can cause excessive Weber shaking on street engines. This is particularly true at low engine speeds. This may be why many report the solid connectors "work better" in racing applications. Air leaks can be checked by squirting brakeclean or WD-40 on the joint (o-ring) areas. If the spray gets sucked away, or engine speed increases, you have a leak.
Next, also important, is Weber support. There needs to be a rod supporting the Webers from the intake side to the block near the engine mount (crankshaft center) to minimize Weber shaking. Without this support, Webers shake independent of one another ruining balance and the connecting linkage between the two carburetors. See the black support bar to the engine mount in picture below. Let me know if any of these ideas help!
 

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Hi Gordon

WE must have crossed each other with replys. I am using new rubber supports with support rods. I was wondering if it is the cams. They are Euro canms I purchased from Steve at Centerline.I was trying to avoid taking the valve covers off again the check clearances. But i might as well bite the bullet. My set up looks similar to your picture but I have shorter horns to fit the Guilietta. I'll keep trying Thanx again.

Al Conforti
 

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60F11's are awfully big for this appication. There is something funny going on if you need idle jets of that size. I would expect to use 50F8 or 50F11
 

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Discussion Starter · #9 · (Edited)
I checked the synchronziation in air flow between the carbs with an air flow meter and I noticed that while the throttle valve plates were lined up using the progression holes that I was still getting a different air flow reading across the two webers. Carburettor (1,2) 10 Kg/h vs Carburettor (3,4) 12.5 Kg/h.

If the throttles valve plates are in alignment then there must be something different about the cylinders 1,2,3,4 to create this air flow difference, correct?

Is it therefore necessary to take the throttle valve plates out of alignment using the throttle balance screw in order to balance the air flow difference caused by any compression differences etc that may exist across the cylinders? Otherwise how else could air flow be balanced?
 

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Well, yes and no to the balance issue. I reset butterflies with the use of a bright light shining down the Weber bore with no chokes or anything in the way. Any light leaking through is an air leak. The idea, with butterfly plate up and down adjustment, as well as rotation, is to get them light (air) tight in the closed position. BOTH OF THEM at the same time. Not always easy. Then I spray some Brakeclean or light solvent in there like lacquer thinner and see how it leaks past the closed butterflies. It will leak past. This means that there is no real total seal, and this worsens as they are cracked open. It would be very high tech machining to get them both to open the same amount with the same degree of shaft rotation. Ah-Ha! Thus all the adjustments! It really doesn't matter where they are, as long as adjustment can compensate! Visual positioning, is just the first step. The balance of air flow at idle is what you need to achieve. That is why there are air correction screws AND the balance link between he two Webers. It is not often that all four air correction screws will be in exactly the same position with "balanced" Webers!

This issue can be complicated by incorrect valve clearances, bad cam timing, bad cam lobes, poor compression, funky ignition or any number of other mechanical issues. the Weber is just stuck there in the middle, and many hope to correct engine faults with Weber adjustment. Sometimes it will work for a while, until that bad plug wire (for example) gets worse, or the valve clearance vanishes.

Years ago (45 or 50) when I was working on a 6, 2bbl Weber Ferrari V12, the best advice I got was "DON'T MESS WITH THE WEBERS!!!" Yelled LOUDLY at the teenage (me) mechanic. Mechanical tune of the engine is almost always the issue. That said, this is NOT always the case with a NEW set up. If the Webers were correct before head work, ignition work or a complete engine rebuild, they will be correct again. Jetting may require changes for fuel changes, compression or displacement changes, but these changes are predictable, and known to Weber tuners. These are the guys that go to the track and make race engines run right. I know them and respect their knowledge. The exact same thing is true for street applications.

Those of you that know me and my Weber work know I REALLY like:D Webers. I like them because they can do tricks other carburetors cannot, and can cover faults that even good electronic FI may struggle with.

The above is just my opinion, from my experience.
 

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The most forward pair of adjustable screws. The rear ones you marked on the emission Webers are for gross adjustment, and usually remain closed on Alfa applications. The forward screws are a finer adjustment.
 

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Discussion Starter · #13 · (Edited)
Update....I managed to imporve the functioning of my Webers compared to when I originally installed them however it would still backfire. It would backfire sometimes at idle and then in the progression from 3rd to 4th gear and would never backfire above 65km/hr. After a long time I had given up trying to resolve my back firing issue.

I had taken the car to three mechanics over a period of 12 months and none of them were able to fix the problem. They adjusted the mixture, timing, balanced the carbs etc. I even tried different idle jets all to no avail.

So I gave up on the problem and drove the car like this for 18 months. I had even considered selling the car in desperation at times.

Well I can tell you that yesterday I put in all new HT leads the problem has gone. Just goes to show that all the underlying assumptions about the source of the problem were wrong (and that includes my alfa mechanics who reassured me that they had checked these things). I can only assume the problem was high resistance on one of the HT leads. Frankly I don't care which lead, I only care that it's finally fixed.

So advice. Weber backfire problem turned out to be a poor ignition problem. If you have a backfiring Weber check your distributor cap and rotor and the resistance of your HT leads! (amongst other things)
 

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An American Weber rule, taught me by an Australian born in Belgium, Horst Kwech at AUSCA, is " DON'T MESS ABOUT WITH THE WEBERS!" His correct reasoning is that if at any time they functioned correctly, and were properly set up, they will remain in "tune" for a long, long time. If an issue develops with an Alfa engine over time as it is run, it is most often NOT the Webers.
 

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You mentioned a small flame from the carb near the rubber mounts. If youi look at the underside of the choke mechanism, you will see a small brass screen, which is the air intake for the choke. This is almost certainly the place the carbs backfired.

In fact, I'd check this mechanism carefully for mis-assembly. there are two small brass pistons inside the carb that this lever moves. If the pistons don't engage the lever, the choke operation can be causing your backfire.

Robert
 
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