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Different Progression Holes Weber Dcoe 45


mickem02
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Hard to tell from the photos whether you have different sized progression holes.  However, it appears from the upper photo of your 15 carb that the throttle plate is exposing a bit of the first progression hole.  If the throttle plate at rest is exposing part of the first progression hole on one throttle body, but not exposing the same portion of the first progression hole on the other throttle body of the same carb, then you'd definitely have some issues--either a twisted throttle shaft, or bent/improperly seated throttle plate.  You'd be hard pressed to get the throttle bodies balanced, especially at low revs.

 

I also note that the lower progression hole of your 16 carb appears to be star-shaped, indicating damage during cleaning, or some gunk that should be removed.  The roughness of the progression hole might be contributing to your low rpm issues.

 

I put a 15/16 pair on my 2.23L stroker (Schrick 292, Ireland exhaust--a rather mild setup), and the only way to avoid the progression stumble is to run 55 idle jets, which makes idle and cruise way too rich (AFR was in the 10s.)  It doesn't matter what jetting I use, I either get a decent AFR at idle matched with a stumble, or I get an overly rich idle and progression with no stumble.  I tried over 100 jetting combinations over an 18-month period, including:

 

Idle: 45F6, 45F8, 45F9, 50F6, 50F8, 50F9, 55F6, 55F8, 55F9, 60F8, 60F9, 65F8, 65F9

Main: 115, 120, 125, 130, 135, 140, 145, 150, 155, 160

Air corrector: 140, 150, 160, 170, 180, 190, 200, 210, 220

Main venturi: 34, 36, 38

Emulsion tube: F11, F2, F9, F16

Pump jet: 35, 40, 45, 50

Pump bleed: 35, 40, 45, 50

Needle: 200, 225, 250

 

My final street jetting is:

 

Idle: 55F9

Main: 130

Air corrector: 170

Main venturi: 34

Emulsion tube: F2

Pump jet: 45

Pump bleed: 40

Needle: 225

 

Idle is at 875 rpm.  AFR is high-10s at idle and cruise, 12.5 at WOT.  There's no stumble, but throttle response during progression isn't as crisp as I'd like due to the rich mixture.  Due to the richness, mileage is crap, plugs get fouled quickly, and the exhaust tip is black with soot.

 

I measured and adjusted the float levels using various methods to no avail, changed out the fuel pump, added a fuel regulator, changed spark plugs, changed ignition timing, and performed a full suite of provocative fuel burning dances both in the moonlight and under cloudy skies.  The 15/16 pair simply will not perform ideally on my stroker motor, at least not as a street driver.  As a track car, they're excellent.  For the track, I use this jetting:

 

Idle: 60F8

Main: 135

Air corrector: 180

Main venturi: 38

Emulsion tube: F2

Pump jet: 50

Pump bleed: 40

Needle: 225

 

Note, however, that the track jetting is total crap on the street.  Idle is 1050 rpm.  AFR is low-10s at idle and cruise, 12.5 at WOT.  The motor bogs below 2500 rpm, but on the track the car doesn't see 2500 rpm except in the paddock.  Throttle response is crisp and raspy above 3000 rpms.

 

The 15/16 carb bodies have only 2 progression holes, which make for a rather coarse, clumsy step function to get from idle to main.  The newer 152G throttle bodies have 4 progression holes, allowing for a smoother, more gradual step function.  I have a pair of brand new 152Gs which I'll put on my car this week.  I'll report back regarding my results.  I assume the 152Gs will resolve the stumble issue, and I plan to sell my pair of shiny and beautiful remanufactured Italian 15/16s, likely to someone with a smaller displacement motor.  BMW matched them with the 1800 TiSA motor, but didn't offer them on any 2 liter M10.

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Thanks William for your response! It looks like the no 16 carb progression holes have been plugged and redrilled some time in the past. That is why the hole is star-shaped. I will have a engine workshop measure the holes and redrill the holes and hope it will work out. 

The throttle plate exposing issue is just a photograffic error. I couldn't get better photos at the time, but I will check this carefully when the carbs is removed.

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It is the same way with my car . weber sidedraft carbs need a rich idle mixture not best lean . The rich  idle jet is also needed because of the 292 cam . Sport cams don't scavenge the chambers as well at idle and just off idle. with the reversion  and exhaust diluted mixture a richer idle jet and idle mixture is needed for good progression .

It is best to get off of the idle and progression circuit as soon as possible and get the main jets supplying the fuel .

New research has shown by running a very small air correction jet a higher fuel level and a emulsion tube with all submerged holes you can get the main jets to tip in very early .

Set the throttle plates so they are centered over the first holes and the slightest touch of accelerator exposes the holes to engine vacuum .

This is your best idle setting tune the engine idle around this setting and try not to change it . Try to get the best idle speed with the mixture and timing .

 

jan2011053.jpg

My car needs a 60 f8 idle jet too for good clean off idle performance with a sport cam and 34 choke .

Edited by 75duce
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