Re: [alfa] Weber vs. Dellorto

[bearbvd@xxxxxxxxxxxxxx (Greg Hermann)]

At 10:17 AM 5/4/04, Bill Sinclair wrote:
>  I'm
>definitely not a carb expert but the biggest physical difference I see
>between the two types of carbs is the accelerator pump.  On Webers, it's a
>piston type and on the Dellortos it's a diaphragm.  The diaphragm seems to
>give a much longer squirt of fuel (in duration) than the piston.  As to
>whether that's an advantage or a waste, I couldn't say.  Perhaps Greg or
>someone knowledgeable about the inner workings could comment.
>
>Bill Sinclair
>67 Duetto
>73 Berlina
>--
The length of the 'squirt', or pump shot, on a Weber DCOE is completely
adjustable. The size of the accel. jets is part of the main circuit
metering--since they DO act as a far top end enrichment device. Once the
main circuit tuning, including the sizing of the accel. jets is correct,
the RATE of pump squirt may be adjusted by changing to stiffer or softer
pump springs (with stiffer giving a higher rate). Once the rate of squirt
is where it needs to be (instant throttle response with no rich stumble
either) , the duration (total volume) of the squirt may be adjusted by
changing the pump return jets (a larger pump return jet shortens the length
of the squirt). If you get down to a closed pump return jet, and the squirt
STILL isn't long enough to suit you, then you can change the length of the
pump rods, and thus the length and volume of the pump stroke (stock pump
stroke is 10 mm, rods which will yield a 14 mm stroke are available. Too
long a squirt can usually be detected via instant throttle response
followed by a bit of a rich stumble and perhaps a puff of soot in the
exhaust, while too short a squirt manifests itself as instant throttle
response followed (very quickly) by a brief lean hesitation.

I don't know the Dellorto's all that well. Diaphram type accel pumps in
general have a reputation of initiating the squirt marginally quicker than
piston pumps do. I am a bit skeptical of this claim--but AM aware that
Weber used diaphram pumps on their IDA type carbys (the downdraft brothers
of the DCOE's).

You absolutely MUST have the main and idle circuits properly tuned (no
over-rich messes need apply) before any hope that tuning of the pump
circuit can be done properly can exist.

Greg
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