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Discussion Starter #1
Does anyone have specifications for the following on a Weber 40 DCOE 28?

1) Minimum diameter of accelerator pump plunger (piston)
2) Maximum diameter of carb body bore for said plunger
(or maximum clearance between the two)
3) Some kind of spec for the accelerator pump spring, such as free length

Thanks,

Logan
 

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All the DCOE's use the same piston. All the DCOE bodies have the same piston cylinder bore. There are basically 2 different type springs, a fine wire type used on Alfa engines, and a heaver wire type for other and special applications.
Check part numbers in the Pierce Manifold blow-up drawings and you can cross reference.
 

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Discussion Starter #3
Thanks for the reply Gordon. What' I'm trying to understand is whether the piston, bore, and spring are things that can wear, and could affect pump fuel delivery? Or do they generally not wear and don't need to be checked? I could go ahead and replace the pistons and springs as a shotgun approach, but what about the bore -- is there ever an issue with bore wear? The bore looks like it has a thin brass sleeve inside, but that doesn't show up in any of the parts drawings.

In the same vein, another suspect is the pump inlet/exhaust valve at the bottom of the fuel bowl -- it seems like it may not be closing sufficiently during the pump stroke. I took one of them apart and was somewhat surprised to find that the check ball is some sort of plastic rather then metal. Is that the norm? I will go ahead and replace the check valves and see if that helps.
 

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Generally the pump itself is self cleaning and never wears out. It function is activated by opening the butterfly plates (WOT) which allows the piston spring to squirt fuel out of the pump jets. The jet at the bowl bottom allows both refill of the pump and exhaust of excess fuel from the pump when a full piston throw is unnecessary. This is helped by the ball and weight check valves higher in the body that prevent air being sucked back through the pump jets to refill the pump cylinder.
Faults are most commonly plugged pump jets themselves as the #35 jet opening is the smallest jet on a DCOE. After sitting for extended periods, the ball and weight check valve can get gummed up, or even the pump piston stuck in the bore so spring pressure squirts no fuel. The pump exhaust jet in the bowl bottom can get plugged with debris in the bottom of the float bowl.
Generally Weber DCOE's that get used with good fuel in the tank and Webers function trouble free for extremely long periods of time. Debris in the tank (rust, bad fuel) will plug passages and jets and gum up function. Below is a drawing of the Weber pump system. If the system fails, start by checking the pump jets. The the ball check, and while the Weber top is off, see if the pump rods (pistons) move freely.
Nothing wrong with blowing out the pump exhaust jets removed from the bottom of the float bowl, and in there you can see if there is crud in the bottom of the bowl from the tank.
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The pump stroke is different by model number. Stroke is anywhere from 10 mm to 18mm. The 40 dcoe 2, 4 24, 27, 28, 32, 33 have a stroke of 14 mm.
 

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Discussion Starter #6
Some success after doing the following: Squirted out accel pump passages with carb cleaner (I love the "catacombs" term!), replaced the accel pump inlet/exhaust valves with new ones from Pierce Manifolds, and squirted out the idle circuit passages for cylinder #2 and adjusted its idle screw slightly (it previously wasn't responding to adjustment at all.)

Now the car starts like a Weber car should -- two or three pumps of the pedal, crank with the throttle full open, and it starts right up first thing on a SoCal winter morning. I'm pretty convinced that the main solution was replacing the inlet/exhaust valves, and thus am guessing that the previous plastic check balls had become deformed after 50 years, which included exposure to modern fuels. The new valves have metal balls, though not sure if brass or steel; doubt I can take one apart non-destructively. Feels pretty good to fix what seems like a long-standing problem -- see starting instructions below, which I think came from the PPO.

I still suspect the accelerator pumps are not operating at full capacity, however. Compared to memory of previous Weber cars, the squirt is not as audible, and I don't get as much of a gas fountain when pumping the throttle with the accel jet cover removed. And there still seems to be a bit of a stumble/bog when nailing the gas at idle, though can't do a road test right now because the front suspension is disassembled. Assuming that the catacombs are indeed clean, that would leave two possibilities: Weak pump springs and/or worn pump plunger/bore.

So regarding the latter, I have a request of the readers: Does someone have a new accel pump plunger (piston) lying around that they could measure the OD of (with mic or calibers) and post it? That would be much appreciated.

Logan

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Sorry, but they do NOT wear even though you seem like that solution. I have 11 at this moment in my Weber shop, so measured them all. The largest one (used) was 11.96mm measured with my Starett caliper. The rest, new and used, were all 11.95mm.
1611987

All Italian built Webers used the same check ball (steel) and weight, brass or nickel/brass, in the same weight. Did you replace the plastic (incorrect) ball checks with steel and NEW weights? The correct weight is important to correct function. Webers other than DCOE's used different weights. If your weights are incorrect you may not have correct check-valve function.
All Alfa Webers normally use the same lightweight piston spring for a timed, correct fuel squirt. Some DIY rebuilders substitute the stiffer spring (for V8's) and get a shorter duration squirt. Your pump jets should be 35, and the pump exhaust jet in the bowl bottom will be 50 to 70 depending on the specific DCOE. Any of these changed, will alter pump function.
Finally "pumping" the gas pedal does NOT directly pump fuel if jet covers are removed. The squirt is caused by a cam moving AWAY from under a rod, that then allows the spring to gently move the piston down. Squirt will be seen with the pedal on the floor, not "pumped". Unless you have an electric pump with a full float bowl, pumping the pedal without cranking does nothing as there is not enough fuel in the bowl to handle TWENTY full squirts. A mechanical fuel pump fills the float bowls gradually, so cranking with low fuel in the bowls will allow minimal fuel to wet the manifold.
In short, throw away that instruction sheet and READ THE OWNERS MANUAL. If the pump passages have never been cleaned in 50 years (float bowl to pump jet) it is very possible you have debris in those hidden passages that will restrict feel available to the pump.
 

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Discussion Starter #8
Hi Gordon, thanks very much for doing that research. Yes, you're right, my plungers are in spec. Regarding the plastic check balls, those were not in the ball/weight pairs at the output side of the pump, but rather, in the inlet check valve at the bottom of the fuel bowl (I destructively took one apart). It seems odd to me that the balls would be plastic, but it is what it is, and the new valves have metal balls by all indications. All the jet sizes are in spec, and the springs appear to be the correct "weak" type.

Do the springs ever fatigue?

And you're right, there could still be dirt in the passageways that has been resistant to my cleaning efforts so far. The Starting Instructions will go in the archive as a curiosity and testament that something was not right for a long time.
 

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Yes, the thin wire Alfa pump springs do fatigue and shorten. There are 2 easy solutions. New ones from Pierce Manifolds are stainless steel and a bit more fatigue resistant. Most seen shortened can be easily stretched back to as manufactured length if you have a new on for comparison. Most important is that both Webers springs are the same length.
The bottom of the bowl valve is the "Pump-exhaust" jet or sometimes called the "Pump-bypass" jet. It has 2 functions. 1) to refill the pump cylinder, foot off throttle. 2) to bleed off excess fuel from the pump cylinder with repeated on-off throttle. These were made by several companies over the years. I only use the steel ball versions. The old flat topped exhaust jets, marked Weber are well over 50 years old now, but most all work fine unless damaged by moisture corrosion.
It's possible the "bog" you mention is standard Weber / Alfa transition bog that is corrected by richening up the idle jets. With restored Webers and modern fuel, I usually mention to customers that the idle jetting may need some adjustment for smooth transition with todays "fuel" replacing gasoline. It's usually in a richer direction.
 

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Discussion Starter #10
That's good info. I'll order a couple of new springs from Pierce, since I don't have a known good reference to compare to. Further analysis will likely have to wait until I finish the suspension and car back on the road. Thanks again for your help.
 
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