Seeking opinions - Air fuel ratios with 32/36 Weber DGV carbs

JFENG

JFENG

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I have an new engine and intake system replacement ready for a next-spring project. However, I wanted to check the existing setup so I can be an educated seller.

I setup my Innovate wide band AFR with the sensor up the tail pipe (no bung I’m down pipes).

The numbers I got were a bit of a shock, and I wanted feedback from others on this:

Setup: stock 197430CS with a newer head from the mid 1980’s, and Weber DGVA carbs and OE air cleaner. Emissions equipment removed but everything else stock. Bottom end has 138k miles. Doesn’t seem to smoke on engine braking or accel, and no blue when started up after sitting overnight. I would say no excessive blow by either based on inspecting the inside of the air cleaner housing.


Here are my numbers
Idle, 12.2 to 1

Steady 50mph cruise in 4th gear on level ground, 11.7 to 1

WOT in second gear, 9.3 to 1

WOT in 4th starting at 2000rpm, 9.0 to 1 (NINE TO ONE).


I’ve never had a car running that rich before!
My worst is an old Jag that goes down to 11.5 to one at WOT at redline.

i will pull the plugs when I get home but expect them to be nasty. Tail pipe is pretty black.

Car drives ok, but misfires a bit above 4000rpm and greater than 2/3rds throttle.
MPGwas around 17 on a recent 250 mile trip (avg speed in the high 70’s). Car starts on first turn of the starter motor and idles fine.

don’t bother telling me the car is running rich enough for a 50 HP nitrous setup.

John
 
bfeng said:
I have an new engine and intake system replacement ready for a next-spring project. However, I wanted to check the existing setup so I can be an educated seller.

I setup my Innovate wide band AFR with the sensor up the tail pipe (no bung I’m down pipes).

The numbers I got were a bit of a shock, and I wanted feedback from others on this:

Setup: stock 197430CS with a newer head from the mid 1980’s, and Weber DGVA carbs and OE air cleaner. Emissions equipment removed but everything else stock. Bottom end has 138k miles. Doesn’t seem to smoke on engine braking or accel, and no blue when started up after sitting overnight. I would say no excessive blow by either based on inspecting the inside of the air cleaner housing.


Here are my numbers
Idle, 12.2 to 1

Steady 50mph cruise in 4th gear on level ground, 11.7 to 1

WOT in second gear, 9.3 to 1

WOT in 4th starting at 2000rpm, 9.0 to 1 (NINE TO ONE).


I’ve never had a car running that rich before!
My worst is an old Jag that goes down to 11.5 to one at WOT at redline.

i will pull the plugs when I get home but expect them to be nasty. Tail pipe is pretty black.

Car drives ok, but misfires a bit above 4000rpm and greater than 2/3rds throttle.
MPGwas around 17 on a recent 250 mile trip (avg speed in the high 70’s). Car starts on first turn of the starter motor and idles fine.

don’t bother telling me the car is running rich enough for a 50 HP nitrous setup.

John
Click to expand...
Sounds about right to me.
Carbed cars usually run around 12-12.5:1
No carbed car is going to run the 14.7 Stoich that a FI car can run. Just can't happen.
Cruising it may get to 13.
WOT usually around 10 so maybe a little rich there and cruising.
Do you know what your timing is?
Can have a big effect and may be the source of the miss.
Also not sure how accurate a tailpipe sensor may be.
Mine is in the pipe behind where the collectors come together. I actually moved it back a few inches from where is was before to make sure I was getting a good mix of all six cylinders.
I'm running Weber 45 DCOE's.
Steve will probably chime in. He's the expert and has helped me immensely.
 
Just read a bunch of stuff on EFI tuner sites where newbies figure if rich is good mo-richer is mo-better. Consensus seems to be when you go below 9:5 AFR the mixture burns so slowly than the car will stumble, mimicking a misfire ( misfires however will show lean). Indeed I get more power at 2/3 throttle (10.5 to 1) than WOT, and stumbling above 4K rpm when the secondaries open.

I drove a bit more and saw 9:1 to1 at one point. Wow, if I get this fixed my mileage should increase by at least 25%!!

I can lean out the idle easily but need new jet parts to fix the WOT problem. One chart suggested I’m losing as much as 20% torque due to the ridiculously rich condition.

I wonder how many people dislike the Weber 32/36 because of the stock jetting being so rich as to result in significant mis -tuning.
 
Yes your WOT numbers are bad, if you improve those your cruise will likely improve as well. Idle is ok. I don't know how to rejet DGAV's, possibly put in a leaner main jet.
 
And ... I pull the air cleaner off and guess what.. I see the interlocking geared pawls: these are DGAS 38’s!

Anyway, I turn in the idle
Mixture until the engine barely idles and did a test drive. AFR at WOT is about 0.6 to 0.7 higher than before. IOW lowest was 9.6 to 1 and at 3/4 throttle it stays above 10:1.

Unlike before the car pulls cleanly up to 5000rpm and then starts to feel like it’s come off the cam. No stubbling anymore but still clearly not running optimally above 5k. I think adding a bunch of ignition advance would help but that’s just masking the root cause.

At all throttle openings the responsiveness is now significantly better, and it feels like I’ve gained >25HP.

I’m going to order new AC’s and main jets to see if I can pull WOT above 11:1 with the idle richened up to stability.

AFAIK, ideal would be high 13’s under light cruise and around 12:1 at WOT. I’m sure there is another 15-20HP to be had.

Lesson learned: check the PO’s work before jumping to conclusions.

John

Note the sooty O2 sensor adapter

PS tail pipe adapters like this will suffer from inaccurate readings at low rpm due to fresh air getting sucked back into the tail pipe. I always raise rpm to 1000. Before taking an idle AFR reading.

6E2A090E-9FAD-4768-982A-953546E11757.jpeg
 
John,
I'm interested in where you end up on this. I have DGAS 38/38s in my (new to me) Bav. What you find and change will help me immensely to dial mine in.
 
Dick, I’m doing a very simple thing, that is totally dependent on being able to measure AFR. As Gary said, best idle will be richer that 14.7 to 1. In my limited experience 13.5:1 is a good starting point. Depending on the carbs, engine setup and ignition different cars will idle best at different AFR’s.

Under light cruise, 40mph in 4th or 5th on level ground I’d like to still be in that 13.5 to 14:1 range. At WOT I want to be around 12:1.

The various instructions on the internet for setting up a DGAS seem ok to me. But few really go into changing jetting to achieve specific AFR’s. In my case, I’m rich at all RPM and the richness increases above a modest throttle opening. This I interpret as meaning the main circuit is too rich. The only thing that affects the main circuit at all RPM is the main jet. But I’ll buy one size smaller/bigger air correctors just in case.
 
John,
I have not put an A/F meter on mine yet, nor have I got into the carbs to see which idle jets, main jets, and air correction jets are in there. I have also not set up what I have per the online instructions, nor made sure they are synchronized and that I'm getting WOT. I just finished a restoration and have driven it maybe 100 miles so far. It seems to run OK, but I don't have anything to compare that with. I've got a long way to go. :)
 
For your reference, this is an original 3 liter (not a 3.2 or 3.4 or the Alpina M90 that’s waiting patiently on an engine stand).

My main jet is a 180
  • stock kits use 140 or 145
  • The jet is marked 145 but that number is scratched out. I measured an ID about 1.75mm so it’s been drilled out to make it a “180.”
  • This is root cause for why it is so damn rich

My air correctors are 190 (stock kit is 185)

My emulsion tubes are F57,
  • Again, I think it’s been custom drilled as Weber only lists an F5.
  • I may need to swap these back to stock, but at $50 a set I’ll try changing main jets first.

Here is the data tag from the place that sold the kit. I don’t know if this is JAM of Pierce or ...

BF299176-C3CC-4B0A-ADEE-2C3041A855DF.jpeg
 
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One of these days I will post my 32/36 tuning journey, but recently my AFR setup broke so I'll need to fix that first. And I see your setup is 38/38 but I'll post anyway as my setup was equally rich.

After setting the timing and syncing airflow, next step is to adjust the idle mixture screws. I had to turn mine 5 (!) turns out to get the highest idle so that told me despite my 13 to 1 idle AFR, my idle jets were too small. One size up was 3 turns out and two sizes up in my idle jet was 1.75 turns out-perfect. (idle mixture screws should be 1.5 to 2 turns out)

Idle AFR didn't really change with the bigger jets but low speed driveability got WAY better as these carbs run on the idle jets at more than just idle. Interesting to note that the number of screw turns is how you tune idle jets on the 32/36, not AFR.

Next, I went lean at high RPM on the primaries, to 17 to 1 or so. That called for a change in the air corrector. Now I have 13.5 to 1 or so at low rpm and high RPM when running on the primaries.

Next was 9.5 to 1 when cracking open the secondaries, that called for 2-3 steps smaller on the secondary jet. Now I'm closer to 12 to 1 there.

Final thing I'll say is I also replaced my power valves...I've read when those fail, and they do, they allow a lot more fuel in.
 
Below is a collection of stuff regarding 32/36 carbs that I collected over the years. I have not verified any of this but all together, the stuff might represent a consensus of opinions. I also have a bunch of similar stuff for the 38/38 carbs

32/36: f7 emulsion tubes, 140 to 145 primary jets, 185 air correctors, 50 accel. jet, 4.5 venturis. if you need parts, call autobahn at 310-821-5050

32/36: The primary venturi is only 26mm, whereas the minimum recommended by Weber would be 28mm. However, you can make them work OK with 140-145 main jets and 160/170 air correctors.

3.0 liter M30 (9:1) Weber 32/36 jet settings: Primary air correctors = 170 and 140 mains, secondary air correctors 160 and 140 mains.

3.5 liter M30 (8:1, 3430cc): Primary air correctors 185, 145 mains, Secondary air correctors 190, 150 mains. This 3.5 jetting had previously produced a light-throttle stumble, but this is currently tweaked out with the idle mixture screw, and the motor pulls cleanly at all RPMs/ throttle positions. If it returns irrevocably, I will try slightly smaller primary air correctors.

Rule of thumb for jetting: Choose a main jet number four times the venturi diameter; e.g. a 30mm venturi equates to 120 main jet (=30*4) or 1.2. Then you can adjust the main jet size by increments of five, both smaller and lager..... 115, 125, 130, 135

Larger air-correction jet number means more air, whereas a larger main jet number means more fuel. Start an air-correction jet with a value of 60 larger than the main jet, e.g., if using a 120 main install a 180 air-correction. Go for a drive and pay attention to 3k-5k RPM range, tailpipe and spark plug deposits. Use a smaller air-correction jet to establish a richer mixture or bigger air-correction jet for leaner mixture.

If the engine coughs and stumbles until it reaches 2k RPM, then you need to look at the idle jet.
 
John, I have 38s on my low compression, 3.3l with schrick 284 running the following carb set up:

idles: 60s
Mains: 135
Air correctors: 200
Emulsion tube: F50

seems about right but I had to fiddle with both of the fuel mixture screws on EACH carb, 4 total. Get the idle worked out on the warmed up motor first. This will help with your cruise also. After that, you can fiddle with jets for under load. If I can change jets, anyone can.

note: I strongly suggest a valve adjustment also. I had one done that was too loose. Once I put them back to a tighter spec measured at the cam, all was well.

the top end performance site has some great suggestions on how to jet and tune these carbs. Master steve was most helpful to me also.
 
'69 2800cs said:
One of these days I will post my 32/36 tuning journey, but recently my AFR setup broke so I'll need to fix that first. And I see your setup is 38/38 but I'll post anyway as my setup was equally rich.

After setting the timing and syncing airflow, next step is to adjust the idle mixture screws. I had to turn mine 5 (!) turns out to get the highest idle so that told me despite my 13 to 1 idle AFR, my idle jets were too small. One size up was 3 turns out and two sizes up in my idle jet was 1.75 turns out-perfect. (idle mixture screws should be 1.5 to 2 turns out)

Idle AFR didn't really change with the bigger jets but low speed driveability got WAY better as these carbs run on the idle jets at more than just idle. Interesting to note that the number of screw turns is how you tune idle jets on the 32/36, not AFR.

Next, I went lean at high RPM on the primaries, to 17 to 1 or so. That called for a change in the air corrector. Now I have 13.5 to 1 or so at low rpm and high RPM when running on the primaries.

Next was 9.5 to 1 when cracking open the secondaries, that called for 2-3 steps smaller on the secondary jet. Now I'm closer to 12 to 1 there.

Final thing I'll say is I also replaced my power valves...I've read when those fail, and they do, they allow a lot more fuel in.
Click to expand...

IMHO, AFR is legitimate for setting idle as long as you’re sure the AFR reading is accurate. If your sensor is on a bung ahead of the rear muffler, idle AFR reading should be accurate. Tail pipe ... usually not accurate.

I’d better check my power valves per your experience. Thanks for the tip.

I chatted with the guys at Pierce Manifolds and they hinted that my main jet would be ok for a 4.2liter Jag motor. Maybe these carbs were takeoff from s Jag? The suggested I drop from 180 to 140 main jets (8 steps leaner).

I couldn’t wait for my order of jets to come in from pierce manifolds so I soldered up my mains and drilled them out to about a 170 equivalent. I richened up the idle mixture to lean stumble plus an additional quarter turn open. Now my WOT is around 10.2, ignoring what the acceleration pump does. I can use almost full throttle now before the engine bogs.

I also backed off the cold idle to 1800 rpm, so I don’t get so much oil starvation rattle on cold start.

I’m pretty surprised how far out of whack the carb tuning was. Multiple shops had looked at this car for the PO (I have his receipts), and none of the caught either the over rich or ridiculous cold start adjustment (3000rpm! ).

Anyway, thanks for your story as I learned something from it.
 
TodB said:
Mains: 135
Air correctors: 200
Click to expand...

Tod,
Interesting! So you are running even leaner than Pierce recommends for a 3 liter stock cam motor. I wonder how Mario setup Stans 38’s?

I should’ve ordered 130 mains as I also bought a jet drill set so I can open up idle/main jets and even air correctors. Well, a 140 is only one step up. I hope it works ok.

4 idle screws isn’t that bad. Steve has six. In theory, the idle mixture screws are adjusted in tandem so its really only two... the rear carb, passenger side idle mixture screw is a PITA to get to. Do you have one of those semi-flexible carb adjusting screwdrivers?

www.homedepot.com

Lisle Carburetor Adjusting Tool LIS55250 - The Home Depot

Lisle LIS55250 is a carburetor adjusting tool. It is compatible with GM and Chrysler LIS-55250. It includes a carburetor adjusting tool, double D adapter and hex adapters.
www.homedepot.com
 
Just checked the power valves. They are like new and hold “up” under even a light vacuum. All things continue to point to just the main jets being too big. This is the cheapest power boost ever.

And I’m starting to be less tense dealing with that damn choke c-clip. We should all stock a few extras when on inevitably disappears between the starter and block or down the venturies:oops:
 
stphers said:
Put a magnet on it as you are taking it off, it helps when it does fall
Click to expand...
That is exactly what I do. A 1” disc magnet that both holds the choke plate flat and captures the clip no matter if it pops off or comes of slowly.

I’m still going to put some spares in my hardware box.
 
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