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

[TodB]

C clip - I use locking Hemostats but even then it’s dicey.

John, I can get to each of my mixture screws with regular screw drivers. It’s a little more challenging on the 02 with the single screw on the 32/36.

I’ve got a set of 140 jets I can drop in the mail to you if needed.

Stan runs 10.5:1 compression so that’s another variable.

When Mario first set up my car, we fought some stumbling issues on transitions. Curiously, adding the 123 dizzy helped a ton and made it sewing machine smooth.

 

[JFENG]

I’ve got a set of 140 jets I can drop in the mail to you if needed.

Tod

Thanks but I already ordered from Pierce yesterday.

123 dizzy: I’m sure it is helping you ignite lean/rich mixture points better than a stock or Pertronix due to optimized dwell and advance

 

[JFENG]

I can get to each of my mixture screws

You must have a different linkage. On mine, the cross bar blocks access to the rear P-side mixture screw. I have to use a shorty 4”, and go under the bar, which pretty much means touching the VC (I wear a glove). The other three are ok.

The idle speed screws are also not accessible with a straight screwdriver so I use a mini style ignition wrench and come in from the side. DCOE’s are so much easier.

 

['69 2800cs]

IMHO, AFR is legitimate for setting idle as long as you’re sure the AFR reading is accurate.

Well, no, and that's why I brought it up. The 32/36 and the 38/38 Weber instructions say if you are past two turns on the idle mixture screws than your idle jet is too small.

What I found was, with too small idle jets I could still get a nice 13 to 1 idle with the screws 5 turns out, but when I put any load on it, let out the clutch, moving slowly in traffic, it surged and by 2500 RPM was at 17 to 1. Two sizes larger on the idle, two turns out on the screws and I could have the 13 to 1 idle AND 13 to 1 at low RPM and light throttle applications. Driveability was hugely improved.

 

[JFENG]

IMHO, AFR is legitimate for setting idle as long as you’re sure the AFR reading is accurate.

Well, no, and that's why I brought it up. .

You are saying your off-idle performance was bad despite having a good mixture at idle. I’ll try to be polite here, but duh!! I simply said you can set idle mixture with a n accurate AFR reading. I didn’t say a good idle mixture means good low speed or transition mixture. You, sir, are arguing against something I haven’t said.

Perhaps what you are trying to say is a good idle doesn’t ensure good low speed and transition behavior. Yes, which is why the manual clearly states you must change idle jets if you can’t get good idle AFR within a narrow range of the adjustment screw.

 

[Stevehose]

Don’t obsess over idle afr, inefficient cylinder scavenging and inaccurate instruments (the cheap afr meter is a glorified oxygen sensor) can throw you off. Set it to where the engine idles smoothest within reason - use a quality digital tach and observe the corner of the hood for vibrations as you make adjustments - good old fashioned mojo! Each particular engine will tell you what it likes, as it also does with timing advance. It may like to idle at 11.5 or in the 12’s. Temperature and humidity significantly affect the reading. So in hot, humid weather my afr at idle is 11.9. On a cool dry day it’s 12.5. Nothing mechanically changed. Spark plugs are clean and brown. To truly get an accurate afr you need an exhaust gas analyzer but the ones we use with the O2 sensor will get you close, the rest is how the engine behaves. Applies to cruise and WOT as well, but less so.

 

[JFENG]

Next round:

I got my new jets from Pierce and their jet drill kit.

I immediately took out my home made 160main jets and proceeded to check the hole diameter with the drills. Indicated diam was 150. I then put a mic in the drill bits and some were within 0.01 mm and some wet off by a huge amount. The 150 and 160 drill measures 1.58 and 1.59mm.

The $45 drill bit set is going back unless they can replace the inaccurate ones (fully 1/3 of the set)

I put in the 140 mains and it was even better than before
Idle =13.5
WOT = 11.2
PT = 12.5 to 13
Pulled very nicely to 6000rpm

But, in the progression phase AFR’s were getting up to 16.5 to one. Too lean and on the verge of stumbling.

I hate these carbs as there are ten screws and that tiny c clip every damn time you want to change something. Give me DCOE’s any day of the week over these.

Anyway, I jumped up to 150 mains and now the progression phase is 13ish to 1, no stumble or hesitation. Perfect.

WOT is unfortunately too rich again, low 10’s. My highly biased butt-o-meter says it’s just a tad less eager to rev than when it was 11:1.

Highway cruising is at 13:0. Could be leaner but not bad.

Some of you will laugh at the fact that I didn’t try the recommended jetting from the start. Well, I’m interacting my way to exactly that: the recommended 145 mains, which would lean out WOT.

Btw, the idle set to 13.0 with the air cleaner off dropped to the low twelves with the cleaner on.

I also checked synchronization not only between the carbs but between the two throats of each 38DGAV’s. One carb is unbalanced between the two throats. I doubt I’ll oull the carb off to adjust this imbalance.


Lessons learned:
(1) start with the recommended jetting. And go from there
(2) check for modified jets that are sized different than their markings
(3) get an AFR meter and preferably weld a bung into the exhaust just forward of the center resonator (for more accurate readings).
(4) don’t trust carb jet drill sets. Check for accuracy before using
(5) please use the proper sized screwdrivers when working in carbs (PO’s shop mangled the heads of some jets - it’s just cosmetic but it shows lack of attention to detail).

If any one needs feedback on tuning their 38DGAV’s, don’t hesitate to ask me for opinions.

John

 

[Thomas76]

IMHO, AFR is legitimate for setting idle as long as you’re sure the AFR reading is accurate.

Well, no, and that's why I brought it up. The 32/36 and the 38/38 Weber instructions say if you are past two turns on the idle mixture screws than your idle jet is too small.

What I found was, with too small idle jets I could still get a nice 13 to 1 idle with the screws 5 turns out, but when I put any load on it, let out the clutch, moving slowly in traffic, it surged and by 2500 RPM was at 17 to 1. Two sizes larger on the idle, two turns out on the screws and I could have the 13 to 1 idle AND 13 to 1 at low RPM and light throttle applications. Driveability was hugely improved.

What did you settle in to for idle jet sizes?

 

[JFENG]

I checked in with a guy who builds both street and race Motorsports classic Alfa’s about what my AFR targets should be. He built the motor in my Alfa (which makes about 100hp per liter and is super reliable). He is one of the top builders for vintage Alfa motors in the country.

He said for a street motor, AFR’s should not go below 12:1 except for a very brief moment when the accel pump squirts. He said the 12-14:1 range is appropriate for all on-throttle driving condition. He said I’m likely losing at least 5-8% power at WOT because I’m below 11:1, and he urged me
to continue till I get it right.

If it weren’t for the work to access the jets on these downdraft carbs, I’d be much more willing to screw around with them.

What did you settle in to for idle jet sizes?

I’m using a “50” right now but am considering going one step richer to allow me to reduce the main jet down from a 150.

The combination of a 50 idle and a 140 main resulted in unacceptably lean transition/progression AFR’s.

Maybe a 55/145 plus 200 air correctors is the sweet spot?

 

['69 2800cs]

Hey John...I intended no offense with my post. I simply thought it was good for the thread to note how important the idle jet instructions are in the Weber manual. I get you don't need that schooling but whoever installed my carbs certainly did.

I got a stack of receipts when I bought my car including a full engine rebuild with weber conversion and no one took the time to set the idle mixture and count the screws out? It's one of the first few steps! Add to that how much low speed drivability changed for me with just an idle jet swap and I wanted to share that.

Where I landed on idle jets for my B34 was 60. 50 idle jets were in there and I got a nice idle at 5 turns out, with meh drivability. 55 idle jets and I had to go 3 turns out, low speed drivability a lot better and a nice smooth idle but manual says no more than 2 turns out.

I went to 60 idle jets and needed only 1.75 or so turns out on the mixture screws and got another nice bump in drivability.

 

[JFENG]

...I intended no offense with my post.

No no, I read my reply and I overreacted. Sorry about that.

I’m sure plenty of folk mistake a good idle as meaning the transition will be good. You and Steve are both right that the idle mixture is relatively unimportant.

I’m lucky that my transition/progression is good so now I just need to scale then idle and main together to lean out the main circuit.

I called Pierce about the messed up drill set and they said it’s always a crap shoot if the factory workers put the right bits into each slot and sometimes they don’t. Shame.

 

[JFENG]

And thanks for letting know how sensitive it was to just a single step change in the idle jets. Plan was to progressively drill out my 50’s, but the drill set didn’t have an accurate 55 bit. So, another $40 in jets to buy.