Accel enrichment

[Clare Snyder]

Clive Apps Techno-Logicals 416 510 0020 wrote:
> 
> >
> >
> >  Theoretically (2) max power is to the rich side of stoich; the extra
> > fuel lets you make more power for acceleration.
> >
> >  In practice it doesn't seem to matter all that much.  Millions of
> > British cars used SU side drafts with no accelerator pumps, most
> > motorcycles have no pumps, and I've disconnected the accelerator pump on
> > various carburetors with no noticeable change in driveability.
> 
> most bikes use slide bore carbs (like SUs)
> these have a self enrichening feature because of the design and son't need a
> pump
> 
> Clive
The simple low-down on acceleration enrichment is as follows:
It is a double edged sword - looks after two problems at once.
#1 - fuel is heavier than air. On a carbureted engine, there is a lag
between the increase in air flow and the resultant increase in fuel flow
caused by the resulting pressure drop. This causes a lean bog on
acceleration. The calibrated squirt of the accelerator pump overcomes
this lean spot.
On EFI this is not a problem, or at least not as much of a problem.
#2 - a rich mixture produces more power - to a point. On a lean/late
emmissions engine,this little bit of extra fuel (in many cases, like
Bosch L-Jetronic, tip-in causes an extra pulse of the fuel injector)
eliminates the minor stumble that would otherwise be expected. If the
amount of fuel discharged by the average accelerator pump circuit was
added to a EFI engine on acceleration, a rich stumble could well be
expected.
On a central , or TB Injected engine, the same situation occurs as in a
carb, but to a much lesser degree, as the air still accelerates faster
than the fuel in the manifold, but the fuel "injection" is not delayed
as in a carb.

On a CV carb, mixture adjusts itself very nicely, as a lean bog lowers
engine vacuum, causing the plunger (and metering needle) to drop until
vacuum, fuel flow, and power output, retutn to equilibrium. There are,
however, examples of CV carbs that DO use an acceleration enrichment
device.
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