Weekend of Tuning (Weber 32/36)

[paulyg]

Hi All,

 

I figured I'd collect my thoughts and hopefully get some insight from you folks. I spent the past couple weekends experimenting with variations on my Weber jetting that's carried me for the past several months. The starting point was CD Diesel's prescription (and no, I did not plug the secondary enrichment hole). Aside from lowering the idle jet sizes by a half size early on, it's been pretty much unchanged:

 

Float level 40mm

 

Primary

Idle 57

Main 140

Air 145

Emulsion Tube F50

Secondary

Idle 52.5

Main 170

Air 175

Emulsion Tube F50

 

After a valve adjustment, multiple tuning sessions (123tune+), and lots and lots of driving, I was generally happy with the above setup, with a couple exceptions.

 

1) The car generally ran rich. It seemed very insensitive to mixture screw adjustment, even with the downsized idle jets. 

2) I noticed a flat spot coming off the primary idle circuit. Made starting from a stop more of a pain than it should be.

 

Last weekend I tried increasing the size of the primary air corrector to 155, mostly to address the richness I was noticing. This did little more than to extend the flat spot from the idle/progression transition to the entire primary sequence :). I drove the car all week to work and the whole time was bothered by how asthmatic it had become. This finally convinced me to perform a dive into the world of emulsion tubes, leading me to several helpful explanations and this video:

 

http://www.performanceoriented.com/performance-tuning-2

https://www.youtube.com/watch?v=1pkFSA_rRFI

 

The above two sources were more helpful descriptions of emulsion tube applications than the weber tuning manual or the books that are out there. The big takeaway: more holes up high, leaner across the rev range, more holes down low: leans out as revs increase. Overall number of holes affects the way the relationship of the main jet size to the air corrector jet size set the main circuit mixture ratio. 

 

I charged into this weekend with at least a butterfingered grasp of the concepts behind emulsion tubes and how to select a tube to get a desired result. My quest was to eliminate the flat spot that had bogged down my driving for the past several months. I focused entirely on the primary barrel, since I rarely engage the secondary throttle and thus that entire side of the carb mostly remains on the idle circuit. I arrayed the tubes at my disposal : two F6s, an F8, and an F25. Notably absent was an F66, and the two F50s in my possession were currently in the carb. A recently acquired 150 air jet rounded out the arsenal. 

 

I made the decision to swap the F50 out of the primary for an F6. The following image from Pegasus racing has those two tubes compared (with an F66):

 

 

This is a pretty drastic change, since the F50 strikes me as one of the more extreme emulsion tubes out there in terms of biasing the progression circuit towards the air corrector (lean at all RPMs). My hope, which has been borne out by some test drives, is that the hole placement of the F6 would give a richer/torquier bottom end to the progression circuit while leaning out a bit at the top end of the primary main circuit, where the secondary barrel kicks in a bit. After some driving and tuning (idle and mixture adjustment) the richness remained, so I stepped down the primary idle jet as well. The setup is now as follows:

 

Primary

Idle 55

Main 140

Air 155

Emulsion Tube F6

Secondary

Idle 52.5

Main 170

Air 175

Emulsion Tube F50

 

The results? Well, it did take care of the flat spot. The primary progression circuit is still pretty darn rich, but the gas smell with the windows down has abated at idle. It was quite a bit easier to find the right idle/mixture screw settings as well with this setup. 

 

I think the next move might be to decrease the size of the primary main jet and go back to a 145 air corrector, with a 150 in reserve. Reason being: like I mentioned earlier, with my driving style I rarely find myself engaging the secondary barrel, and my general impression is that the CD prescription makes the primary side very touchy. The effect is that the advantages of a progressive carb are pretty much squandered during around-town driving. I'd honestly be happy getting to a point where my right foot is a bit more active while driving, if it means that the engine is happy and I don't have as many stumbles/variability with ambient conditions. I believe this is pulling me in the direction of a smaller primary main jet, but time will tell. Maybe the answer will be an F66 with the 140 main jet and a larger air corrector, or maybe I will have to continue the search! 

['76mintgrün'02]

24 minutes ago, paulyg said:

The primary progression circuit is still pretty darn rich, but the gas smell with the windows down has abated at idle.

 

Are you using your nose as an AFR gauge?  If so, I'll recommend indulging in an electronic version.  I love my dial-style Innova AFR gauge.  I just bought a single gauge "pod" that sits on a swiveling base and mounted that beside the instrument cluster on the dash.  Initially, I thought I would remove it once the jetting was done, but it's there to stay.  It teaches me a lot about what to do with my right foot.

 

I'm running c.d.eisel's prescription too, aside from adding a larger air corrector on the primary side.  I think it's a 160.  That cured my running-rich problems.  There are no flat spots and the progression into the secondary is smooth as can be.  I had plugged the enrichment hole, but then found my plug swimming in the float bowl last time I was in there.  I didn't bother putting it back.  (c.d. and I both have '76 cars and I wonder if/how the #5 camshaft affects the jetting results, compared to everybody else's #2 camshafts).

 

38 minutes ago, paulyg said:

with my driving style I rarely find myself engaging the secondary barrel

 

I was dissatisfied with how deep into the pedal the secondary transition was on the 32-36, so I added a little adjustment tab to the 'loose link' to allow me to bring it in sooner.  I started a thread on the topic and made a run of ten adjusters that have all been sold, as of two days ago.  It took a little longer than expected to sell them.  I thought there were more weirdos like me out there.

 

 

Tom

 

[paulyg]

Thanks for the thoughtful response Tom. I agree that the progression on these carbs is not ideal for our application, and if you do another run of your adjustable link count me in. As it stands, the progression and main circuit on the primary barrel are about 80% of what is being used by the average 2002 driver. 

 

Even with a more rapid cut-in of the secondary barrel with a modification like yours, I still am pretty surprised at the use of the F50 emulsion tube in the primary by several tuning prescriptions and by Redline/Weber for the CARB certified conversion kit. Then again, the stock (carb) setup seems to just lean everything out the more the pedal is depressed. That, and the climbing temp gauge that resulted, is what made me look around for alternate jet setups in the first place. 

 

I'll also throw in that my manifold goes to 24KPa of absolute manifold pressure at idle (from the 123 unit). I tend to think this is a bit on the high side for these M10s, and could explain why I needed smaller idle jets than CD's prescription in the first place.

[TobyB]

So, since your air corrector logic was backwards, I do submit-

 

a larger air corrector makes for a leaner mixture as the flow in a particular 

circuit increases.  It's metering the amount of air let into the top of the emulsion

to balance the fuel that enters at the bottom, and the emulsion itself affects 

how the 2 interact when the air corrector is active 

throughout the usable flow rate of that carb circuit.

 

So:  lean flat spot?  Bigger fuel jet, smaller air corrector.  If the 'curve' of the

behavior of the circuit's not right, but the minimum effective and maximum

flow of the circuit's in the right ballpark, then the emulsion is next.

The fuel jet will change the entire curve, the air will change the top part

(1/3 to 2/3 of it, depending) of the curve, and the emulsion will affect slopes 

and bumps in the progression through the emulsified part.

 

look at the DCOE sections of 

https://www.lainefamily.com/images/WeberTuningManual.pdf

for some pretty good pictures...

 

hope this helps, 

t

does not do videos, himself.

[paulyg]

Thanks Toby. The addition of the larger air corrector was intended originally to lean out the primary mixture during transition and main circuit operation. Following a week of driving on that, it was apparent that the "flat spot" behavior was made worse and I'd been robbed of some power. That's when I turned my attention to the emulsion tube. 

 

The response with the F6 on the primary seems very even, so I'm thinking that the next step is to go down a size on the main jet and adjust air corrector as necessary. 

[Daily02]

+1 on Tom's recommendation of an AFR meter. A vacuum gauge is handy as well. The original calibrabration could be for a setup with a large cam with a low vacuum signal. Open intake and free flowing exhaust will change the calibration as well. Calibrations are unique to the particular setup.

Something that has worked very well for me in the past is to focus on and get the primary side perfect before moving to the secondaries (main circuits). The fact that you are gettting response from the idle mixture screws is a good sign. 

Another thing is overly rich can cover up transition errors (i.e. a power valve openning too early). This murders fuel economy, but is okay for a race car where cruise fuel economy is irrelevant. The AFR and the vacuum gauge make it stick out like a sore thumb. You will no longer being doing the calibration blind by the seat of the pants and the nasal passages.

 

Regards

 

Dono

[Tom Daley]

Goods stuff. Love the tech talk.

 

'76 Polaris '02 w/93k original miles.

 

I added the synchronization parts to my 32/36. Essentially its now an imbalanced two barrel

 

 

Current set up is;

Altitude ~370ft above sea level, Ambient as of this writing, 50deg.

Flat tops w/1 step overbore

274 cam with 2mm added lift (turbo or M30 grind)

E12 flowed head, stainless valves

Exh manifold air injection tubes removed and plugged

Stainless Irenald exhaust, fully heat wrapped header pipe, AF wideband just after the header

Fully ovaled out the manifold opening at the carb, ported also

Petronix, Timing 40deg at 3500. No vacuum

Float 36MM

Primary

Main 140, Air 155, F50 Idle 60

Secondary

Main 135 (was 145 to start with), Air 170, F50, Idle 55

 

Runs at 12.5:1 AF at idle and at all rpms up to 6k rpm in a cruise mode

WOT is good on 140/140 mains but has a bit of a stutter on the 140/135. Hits up to 16.8-17:1 while at WOT with the 140/135

 

I'm so in love with how it pulls like a mule from idle to 6k and at 12.5:1 and no fuel smell or burn-your-nose oil smell that I wanted do something else beside going back to 140/140 or 140/145 to eliminate the slight stutter.

 

I discovered the secondary enrichment port plugged as is recommended in this blog many times but without explanation other than "...you get more fuel control with the jets..." 

 

Any thoughts on that? Dare I unplug it and see what happens to my stutter??

 

  

 

 

[Tom Daley]

FINAL SET UP

1976 2002 M10 original block.

Altitude ~370ft above sea level, Ambient as of this writing, 50deg.

Flat tops w/1 step overbore

274 cam with 2mm added lift (turbo or M30 grind)

E12 flowed head, stainless valves

Exh manifold air injection tubes removed and plugged

Stainless Irenald exhaust, fully heat wrapped header pipe, AF wideband just after the header

Fully ovaled out the manifold opening at the carb, ported also

Petronix, Timing 40deg at 3500. No vacuum

Float 38MM

Primary

Main 140, Air 155, F50 Idle 60

Secondary

Main 135 (was 145 to start with), Air 170, F50, Idle 55

 

UNPLUGGED the Secondary Enrichment Circuit. (Fixed my lean WOT AF 16-17:1)

 

AFs now,  Idle 13:1. 12-14:1 through all throttle position and rpms including WOT with one exception, I noticed a blip at just before WOT where it hit high 14s:1 to and sometimes 15:1 for an instant. I'll check spark plug temperature/color. I.e. want it to be a tan marshmallow. and should be done.

 

Pulls like a mule, no throttle lag at any RPM, no burn your nose drain oil, no exhaust fumes.