Ramblings on what I've learned tuning a Weber 32/36 Progressive DGEV Carburetor

[Sharps40]

The Weber 32/36 DGEV (electric choke) progressive two barrel I’ve installed on my 250 I6 C10 has taught me a low about carb tuning.

For instance, in driving it reminds me of fuel injection so far as responsiveness is concerned; call its function crisp and immediate. For starting, hot or cold, it’s instantaneous and no cycling of the gas pedal is ever required. I usually short cycle the pedal the instant the engine catches to set choke at fast idle when cold, and do the same when the engine catches to smooth it out when hot. Other than that, it settles into a perfect smooth idle every time. It wants more timing, especially with the electronic ignition and I’ve learned, mine seems best timed all in by 3000 or 3500 rpm and let the initial fall where it may. (I run ported vacuum to the advance from the S-port of the carb….no vacuum to the advance at idle when the butterfly is set correctly – that is, as close to closed as possible.) Points dizzy or different vacuum can, different HEI mechanical curves may dictate different timeing than what I rely on.

I’ve tuned it to idle hot and smooth as low as 350 rpm but set it for 500 to 550 rpm as this is the tipping point where the 12SI alternator provides enough juice to power the lights with no flare up on acceleration.

When adjusted well, it idles impossibly quiet and smooth. Response on the main is crisp and immediate. The secondary comes in when the pedal is smoothly pushed with noticable power building smoothly to WOT. Slam the pedal to the floor in any of the lower gears and the secondary literally roars into action with no hesitation. Slam in the secondary in OD and the power builds firmly and smoothly, not quite a passing gear but ya get from 55 to 70 faster than ya'd think ya could: Passing ain't hard to do.

I love the electric choke, set properly, there is no warm up period. Start the truck down to 15 F and go drive, just like fuel injection. I love it so much that the cable is likely to go soon and hopefully the knob gets reused for some form of wiper delay.

Adjustment of the linkage is crucial, the only accurate way I’ve found to make sure the secondary is full open at Wide Open Throttle is to look into the throat while someone else cycles the pedal.

Spend a few minutes doing a thorough vacuum leak check. It’s imperative….I found the Weber can be jetted to compensate for and run superbly with an upside down (large vacuum leak) PCV valve. Even with that fine performance, it runs much better jetted leaner on the idle/transition with no leak!

As for jetting, there are 6 jets to play with. Two outside, 4 inside but I’ve only needed to work with the fuel jets…..i.e. I find no need to change out the 2 air jets or their respective emulsion tubes. Naturally, the number and location of the jets allow tuning of all four performance circuits, idle/transition, main, secondary and WOT, not really possible with a single barrel Rochester or Monojet.

What we think of as the screw for the idle mix jet is simply a flow adjustment…..changing the amount of emulsified fuel from the main jet and air corrector/emulsion tube up or down. The ideal range for this screw/jet on a stock 250 appears to be a 60 jet adjusted for 2 to 2.5 turns out (rich) from closed. Adjustment of this screw/jet combination has a dual function. It rapidly changes hot idle speed (main butterfly nearly closed) and the quality of idle. The setting is called lean best. i.e. fastest and smoothest. I do this adjustment by ear listening to flow thru the carb and by feel, arm on the valve cover. When right there is NO lean idle putt putting every few seconds. Just perfect smooth idling of the engine and the sound of the flow in the carb is equally smooth, no occasional slurping.

What I think of as the idle speed screw is simply an adjustment of the lower main barrel butterfly. Ideal range of adjustment is between 1 and two turns from first contact with the linkage….tiny changes of this screws position are used to fine tune the hot idle speed AFTER the idle mix jet is properly tuned. The goal is to keep this screw turned in the least amount to achieve idle speed thus the lower main barrel butterfly is kept as close to closed as possible. The idle then draws NO air thru the main barrel, taking all of its fuel/air mix thru the idle port below the butterfly. Screw this in more than two turns and the transition port is uncovered and the carb/engine does not run as responsively or efficiently.

Another screw is hidden on the bottom of the weber, adjacent to the secondary throttle butterfly linkage. Typically this butterfly is set fully closed, so as not to incur a continuous vacuum leak while driving on the main. (The secondary begins to open at about 2/3 throttle). However, even this can be slipped open a smidge if you have a hard to cure lag when bringing in the secondary quickly. But, I’m finding that leaving this screw alone and hence the secondary butterfly fully closed prior to 2/3 throttle has produced very satisfactory transition into the secondary with no “dead air” related bog.

The old Rochester B and Monojets on these engines were about 300 cfm. The DGEV is a bit larger at about 325ish, depending on which arguments ya listen to. The feel is defantly different. Again, perhaps a bit less power on the main, (smaller main barrel/less CFM for 90% of the driving) but considerably more responsive and better fuel mileage than a large throated one barrel carb. The difference being, if I understand it rightly, a better signal to the weber main barrel since it’s effectively a smaller straw. (i.e. the large single barrel of the Rochester/Monos getting best signal at high speed but sacrificing performance in the normal driving range, almost like the carb is too big for lower rpm performance.) The one barrels seemingly full power available all the time but with a compromise on drivability at anything like normal off highway rides.

From the factory mine came jetted as follows:

Idle Mix Jet: Large Body #60 (Larger # = Richer)
Idle mix screw: 2 turns out from closed
Idle speed/butterfly screw: 1 turn in from first contact with the linkage
Secondary Mix Jet, Small Body #55 (there is NO adjustment screw for this bottom of the secondary jet) (Larger # = Richer)
Primary Barrel Main Jet: #140 (Larger # = Richer)
Secondary Barrel Main Jet: #140 (Larger # = Richer)
Primary Barrel Air Corrector Jet: #170 (Larger # = Leaner)
Secondary Barrel Air Corrector Jet: #160 (Larger # = Leaner)

With minor tweaking of your timing and carb (primarily the idle mix and butterfly screws), the 250 runs very well on the main as set up from the factory.

As myself and another member found out, the secondary will need some tuning in the form of jet changes to run well on the 250.