Weber carb adjust question

[bcoats]

Hello Pelicans -

Seeking some advice on tuning some Weber's on my '79 3.0 motor.

It has Elgin 964 grind cams for what it's worth, carbs are all rebuilt, approx 2K on rebuilt motor, SSI's.

I am getting some popping on the passenger side, believe #6 carb bank. I read the Weber manual, so for each carb, I slowly turned the mixture screw in (clockwise) until the RPM's dropped. Next, slowly backed the adjuster screw out 1/8 turn, let idle, then another 1/4 turn. I did this for each of the carbs. Took a test drive, had a few pops and backfires, pulled over, backed adjuster screws out another 1/4 turn to further richen.
Car seems to idle smoother and not getting any popping but mild backfire on decel which the car always does.

Does this seem reasonable or is there another way others do this?

Thanks for any suggestions, don't want to be running lean with fear of detonating the motor as I'm hitting the track in a few weeks. Thanks

Bryce

[Jack Stands]

I seem to remember that you're doing it properly. Did you check and match the airflow through all the throats before starting? Unless you have a exhaust analyzer or start reading plugs, you're probably in the ballpark on your setup.

[bcoats]

Thanks, no did not match air flow, figure get them professionally tuned next Spring but feel like I'm in the ballpark

[VFR750]

What size jets and Venturi do you have?

[bcoats]

Hi, 36 Venturi's not sure on the jet size will have to check in the file, I haven't owned to car that long

[VFR750]

You can get a simple airflow meter from our host. That way you can check/balance the 6 individual throttle.

Cheaper than paying the shop $100/hr to do the same thing.

[dicklague]

the synchronizers are about $35-50.

synch the carbs.....don't wait months.

Then install a good AFR instrument to tune it right. Can't live with carbs without AFR gauge.

[bcoats]

Thanks guys, great advice I'll get on that air flow meter and dial them in correctly vs guessing

[911pcars]

You should use an airflow meter (Synchrometer or Unisyn) to ensure each bank receives the same airflow from the carbs. You will want to each carb to flow the same volume of air to each bank. Otherwise, idle and off-idle mixture will be different and power may be affected.

When adjusting mixture screws, make sure you start from a known reference point. Begin by rotating the screw clockwise until they gently seat, then rotate CC about 1 to 1-1/2 turns. Repeat for the other 5 mixture screws. All 6 mixture screws should be adjusted the same using this method. Each idle speed screw should be the same as well.

Airflow will be primarily affected by the idle speed screws (assuming all throttle plates on their respective shafts open and close together. There are further throttle shaft adjustments, but that's another story).
Disconnect the drop link from the cross shaft to each carb.
Engine ON. Adjust both idle speed screws equally so engine idles at about 1000 rpm and airflow is the same.
Back off each mixture screw 1/4 turn at a time and until the airflow and/or idle speed is highest. This indicates a more efficient A/F mixture. If the idle is too high or low, adjust the idle speed screw on each carb accordingly.

Engine OFF. Adjust and reconnect the throttle linkage to each carb for zero preload.
Engine ON. Confirm equal air flow at 900 rpm, then have a helper press on the gas pedal at 1500 rpm, then check the airflow on each bank. Airflow should be the same. Repeat for 2000 rpm. Adjust the linkage accordingly, then repeat the above so everything is in sync. as best you can.
Road test.

At some point, you should identify the idle, main and air bleed jet and venturi sizes on your carbs., then compare with recommendations for your engine. It makes a difference in engine operation, throttle response and fuel mileage.

Sherwood

[bcoats]

That is very helpful Sherwood thanks a lot. Is there an optimal number setting from the Synchrometer, is 10 good? 6? Or is it about having all the same at 900RPM's or so?

[911pcars]

Airflow is relative. You should be able to adjust for a convenient reading. The objective is to have equal airflow in each throat.

[911s55]

If one will only reach a reading of 6 and the rest will go higher balance to the 6, as Sherwood stated, equality is the key.

BTW do not rev the motor with the synchrometer sitting on the carb or you might be buying another.

David

[1QuickS]

Typically when "Lean Best" idle mixture is achieved the STE air flow meter will read something like 4 3/4 to 5 for a small displacement engine so 6 is probably good for you. Too high a reading indicates idle mixture is rich, more air needed to burn the rich mixture. Also, a high STE reading indicates that the first progression hole may be exposed which will add fuel from both the mixture screw AND the partially exposed first progression hole. If you set Lean Best at the high STE reading then you will have effectively diminished total fuel available from the mixture screw as RPMs rise...

Each mixture screw starts at the 1 1/2 turns out but each is individually optimized for the cylinder it is associated with.

Backfire through exhaust sounds like air leaking into the header/muffler.

The balancing of the six barrels for both air flow parity and mixture strength is surprisingly inter-related, one barrel not happy affects to some degree the other five.

[Quickstep192]

Jeesh, Paul is a goldmine of information. I never thought of the STE readings as an indicator of rich/lean; I only ever used it to just make sure I had even draw on all six throats. This opens a whole new arena of problem determination tools. I have a popping at 2800rpm and this may help me uncover the cause. Thanks Paul, for your generosity of knowledge and information.

[1QuickS]

I kinda stumbled on the secondary use for STEs when tuning...

I've used Unysyns, mercury sticks and this British tool with bouncing rods and tossed them all out the window once I started using the STE. The Unysyn allows for balancing but there is no baseline number for air flow. This is why the STE or mercury sticks are good, they always reference to zero at zero air flow.

[bcoats]

Thanks Paul, now my head is really spinning!

[bcoats]

Hello Pelicans,

I finally got around to adjusting my Weber's. I used a Unisyn, confirmed consistent airflow across all carb banks. As a baseline, I backed out the idle mixture screws 1.5X, ran super rich, so turned in a 1/4 turn, really smoothed out but need a solid road test up to operating temperature and fine tune from there but now I have a solid base line vs. before, but air was consistent across all banks. One question, on #6 cylinder, that particular carb bank had an almost hissing sound to it, still getting same airflow reading but definitely a hissing type sound. Also, ever since I bought the car, I assume that is the carb bank that has a sniffing sound on throttle, 2K-3K RPM's.

Any thoughts on that carb bank?

Thanks again,

Bryce

[1QuickS]

Spitting up through carbs indicates lean mixture, typically a lower RPM type of observation. Each mixture screw must be adjusted for "Lean Best" idle mixture, this is performed after adjusting all air flows to be the same, throttle linkage disconnected during the idle adjustment process. You need to adjust air flows at an elevated RPM such as 3000 RPM which is achieved by adjusting throttle linkage. Double check that idle jets are marked to be the same size but that does not preclude someone has drilled them larger, I'd guess 55s or possibly 60's but since your mixture screws seem to be closed down a bit I'd guess 60s or modified sizes.

Hissing sound...I assume you are using OEM intake manifolds on your SC heads. There is a very narrow area for sealing between the two and it is quite possible you have an air leak at that interface. Get some starter fluid and spray around the various seals and at throttle shaft bearings while engine is running. Sudden RPM rise indicates air leak.

[bcoats]

Thanks again Paul, appreciate all the information. I checked my receipts last night, they are 60's. Regarding OEM intake manifolds, yes they are stock, I'll get some starter fluid and spray around the various seals to see if I get a rise in RPM's.

Bryce