From: ulix @ u.washington.edu (Ulix Goettsch) @ USDESMTP
Date: 10/10/96 03:44:14 PM
Subject: Re: [MGB] fuel mixture with K&N filters
I think the vacumn would stay constant because the piston is supposed
to rise and fall to maintain a constant vacumn. So a blocked
air filter would cause the piston to rise which also raises the
needle which makes the mixture richer. Does anybody know in fact
if the SU carburetor is senitive to the air flow resistance of the
filter? If so it sounds like one should make sure the air filter is
clean before adjusting the carb!
>Todd,
>Maybe it is like this: If the carb inlet is restricted, such as by
>an inefficient air filter, the vacuum in the inlet region (between
>throttle plate and filter) would increase. This vacuum would pull in more
>fuel past the needle -> richer mixture.
>OTOH, maybe this effect is in fact negligable, I don't know.
>
>Ulix
>
>
>On Thu, 10 Oct 1996, Todd Mullins wrote:
>
>> Can somebody explain this to me? I thought the whole purpose of the
>> Constant Velocity carb design was to maintain the mixture at a certain
>> velocity (to improve low-rpm power), irrespective of the volume of air
>> being sucked in. At a given altitude, the percentage of oxygen in the
>> air is (assumed) constant, right? So as RPMs rise, the engine demands
>> more air, which raises the piston and needle, which enlarges the jet
>> orifice, which permits proportionately more gasoline to flow,
>> maintaining stoichiometry.
>>
>> they cannot increase the oxygen content of that air (can they?). If the
>> amount of oxygen is the same as for stock filters, then why do we need a
>> richer needle? In fact, why manufacture more than one needle per carb
>> size, since the needle's only purpose is to maintain a stoichiometric
>> ratio of air & fuel, which ONLY depends upon the sizes of the respective
>> orifices? At high altitudes, you may raise the jet a bit to compensate,
>> but the needle profile shouldn't change, should it? Isn't stoichiometry
>> sacred?
>
> Ulix __/__,__
>.......................................................... (_o____o_).....
>
>
>
> '67 Sprite
Bob Donahue, Still Stuck in the '50s
53 MG-TD, under DIY restoration
71 MGB, still in the shop 11 months and counting
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