Weber 40 DCOE accelerator pumps

[Bill]

I am setting up the Webers on my s-2 Cosworth and noted that the accelerator pump travel is vastly different between carburetors. I think that I have traced problem to a difference in the pump cams located on the throttle shafts. The pump cams look the same but the roll pin holes are slightly rotated relative to each other. The pump shafts are the same length and the roll pin holes on the throttle shafts are in the same place relative to the throttle plates, The pump stroke on one carburetor is 1.3mm and 4.6 mm on the other. They both work well but have to assume they are not supplying the same amount of fuel. 

Has any one else come across this issue? 

 

 

[Origin7]

One looks newer than the other. Perhaps there was a rebuild at one point, and a cam was used from a different DCOE variant. There are a number of different 40 DCOEs....18, 151, etc. 

 

If all other things remain the same, you would get different amounts of fuel. You might be able to observe this by looking down the air horns and observing the spray from the pump jets. They should spray at the same time and for the same duration. 

[Bill]

I have several extra pump cams and can possibly make both the carburetor pump strokes the same. But which stroke to select? 1.3mm or 4.6mm? Or perhaps there are pump cams available that would give a stroke some where in-between. 

 

[Origin7]

This would be a good question for the folks over at Keith Franck's forum: 

 

https://vintagetechnologygarage.groups.io/g/sidedraft

 

 

[IamScotticus]

I would ask Pierce Manifold. or buy two matching replacements.

https://www.piercemanifolds.com/category_s/245.htm

[SENC]

I would be hesitant to think this a major contributor to any issue, given how the system works.  That cam doesn't push the pump rod, rather it drops, allowing the pump rod to drop (according to its spring pressure).  That spring pressure, along with rod length, accelerator pump jet orifice size, and condition of the (valve) bearings and weights (and their chambers) determines how much fuel is delivered and for how long (and whether there is any leakage).  Also remember that if everything is in good condition and working correctly, this should be a 1-shot fuel "injection" and should only occur with a rapid push on the accelerator.  Though the cam rotates and allows the pump rod to drop under slow acceleration (or anytime the throttle is open), the weights and ball bearings should prevent fuel flow to the AP jet at all times EXCEPT during rapid accel.  In other words, any issues you may be trying to solve are, in my opinion, much more likely to be a result of issues with jet size, bearing seating, rod length or spring strength, or weights (I think some have replaced these with small springs).  Thinking about how our engines work, could it also be that the tiny fraction of time difference in how each cam would allow the rod to drop is intentional - that one cylinder optimally needs that fuel fractionally later?

 

You mention a difference in pump travel - I'm assuming you mean by that pump rod travel.  Is that actually the case, or are you assuming based on the cam?  With the throttle shafts held full open, are both pump rods fully seated (they should be)?  If one is being held slightly open, indeed you might have an issue.  But if the difference is only that one starts its travel slightly before the other - but the full travel is possible - I'd tend to discount it as a major issue.  Again, remembering that the system, as a whole, should only operate on response to a quick throttle opening.

 

You might also check the rods for each to see if their lengths are different (perhaps the carbs were already rebuilt previously from parts carbs and you really do have a mismatch).

 

 

Edited July 9 by SENC

[Bill]

Thanks guys!

I am always amazed at the help available on this website!