>Matthew Walker <walkerm@prl.philips.co.uk> writes:
> Does anybody know if you should worry about calibrating the
> accelerator pump jets in a 45 DCOE ? I know the pump jets
> allow some fuel to re-enter the float chamber, preventing
> too much fuel going into the engine when accelerating.
> . I think calibrating the pump jets
> should give better driveability - right ?
>>Matt (matt@raptor.sccs.swarthmore.edu) replies:
>>I would look into other things first. The choke (the narrowest point in the
>>carbuerator throat) is adjustable, with smaller sizes=better low-end
>>drivability
>>and larger sizes=high-end power. Sizes are visible from the intake side of
>>the
>>carb, stamped on the bottom of the choke. Also worth messing with are the
>>air corrector jets (larger=lean out mixture at top end) main fuel jets
>>(larger=
>>richer, esp. at bottom end) and so on. The best advice I can give is to find
>>yourself a copy of the Haynes Weber book, it's a damn fine guide. It seems
>>to
>>me that the pump jets will only affect the acceleration phase, with little or
>>no effect elsewhere. And I believe that the pump jet itself is seperate from
>>the pump return circuit (dumps fuel back into the float bowl).
I say:
I'm no expert with Weber carbs but I have a book by John Passini, who is
some kind of Weber guru from England. The book is "Weber Carburettors,
2.Tuning and Maintenance", ISBN 0 85113 060 7. I don't have book 1,
presumably the book on theory of operation. This seems to be an excellent
book, published 1973, which is a good time frame for SOLs.
His points are that adjustment may be required, it depends on how the
engines handles application of throttle at low speeds. He offers a number
of examples, including Mini and MGB applications. He states that the pump
jet acts as a high speed fuel bleed, so it does have a considerable effect
on running at the top end.
The fuel bleed (return circuit) is not separate on the DCOE, but integrated
with the suction valve in the bottom of the float bowl. It comes in various
sizes, from none to quite large. The purpose is to vary the amount of fuel
passed to the pump jet based on the speed of throttle application, i.e.
open throttle slowly, less (or no) fuel into motor, snap it open and almost
all of the pump delivery goes into the motor.
I haven't seen the Haynes book, but I believe this Passini guy was similar
to Vizard in expertise (he be dead now), so it's probably worth trying to
find a copy. Drop me a line if you have some specific questions, and I
could photocopy pages and send by local snail.
Yers,
Vern
_____________________________________________________________________
Vern Klukas I'm a little teapot
vernk@carver.dataflux.bc.ca or Short and...
ug141@freenet.victoria.bc.ca or
inkspot@carver.dataflux.bc.ca
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