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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.
1610331
 

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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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