Very interesting. I have always thought of the float bowl as containing a
reserve of unpressurized fuel, which flowed into the carburetor as allowed
by the needle jet opening. In this view, as long as the level is not so low
as to run dry, or so full as to flood, it shouldn't matter where the level
is at precisely at any moment (it being generally in a state of transition
-- either filling or being depleted). The float level serves as a trigger to
regulate the influx of pressurized fuel from the pump -- when the level gets
too low, the valve is opened allowing more fuel to flow in; and when the
level is sufficiently high, fuel flow is cut off (momentarily).
Now this is basically a static view of the situation. I realize that in the
real world, instantaneous demands for fuel based on throttle position,
mixture setting and needle profile may result in fuel starvation if the
level is set too low, even if that setting works at idle or at part
throttle. But once you have a level setting that is "high enough" without
resulting in flooding at low throttle settings, how does any further, more
precise adjustment of the float level make any operational difference?
It seems to me that for your hypothesis to be correct, you would have to say
that, for a given mixture adjustment, throttle position, and needle, that
the instantaneous state of the float valve and fuel level (open and filling,
closed and depleting, or in-between) makes a difference in the fuel mixture
at the carb throat. I don't see how that can be so (barring starvation, of
course). Given any fuel at all in the float chamber, the pipe going to the
jet will be filled, and the only thing that controls the flow rate to the
carb throat is the needle/jet annulus (I love that word!). Am I missing
something?
I don't have an ax to grind, here, I'm just trying to understand it...
on 3/7/01 11:18 AM, Dave Munroe at dave@munroe.ca wrote:
> Hey Max;
>
> Sorry to bust in here, but I have to tell you this: I had the opportunity a
> number of years ago to spend a day with noted tuner and engine builder,
> Jerry Branch, who, amongst other things, rents out his dyno room to
> motorcycle factory distributors to test after-market motor accessories.
> This involves, for example, installing, tuning for max. performance, and
> removing as many as a dozen or more exhaust systems for each model they want
> to test. With most of these bike having 4 carbs. and all that entails, he
> estimated that they spent 80% of their time installing, removing, and
> changing jets and needles, and 20% on the dyno.
> He came up with a neat device to cut down on the number of times they had to
> go into the carbs to change jetting: he hooked up the float bowl drain tubes
> to a small header that led into the control room where a vacuum/pressure
> pump was controlled by a sensitive valve/switch through a VERY large meter.
> Over time he was able to develop a scale that very accurately related small
> elevation changes in the float bowl fuel level to % change up or down in
> jetting. This was so accurate that he could predict the jet change required
> almost without fail and limit the required adjustment to a one time only
> operation.
>
> So, it appears that, in the case of some carbs. at least, float level does
> affect mixture.
>
> So from my experience Rick Lindsay is not too far off the point.
>
> Dave Munroe
>
> ----- Original Message -----
> From: "Max Heim" <mvheim@studiolimage.com>
> To: "MG List" <mgs@autox.team.net>
> Sent: Wednesday, March 07, 2001 2:10 PM
> Subject: Re: SU Adjustments (LONG!) was: Float Height Issue
>
>
>> That was a good write-up, and I think your proposed experiment would be
> very
>> valuable, but it still seems to me that the float bowl fuel level does not
>> directly affect mixture richness, in that it is modulated by the needle. I
>> mean, in extreme cases, float bowl fuel level can cause starvation, or
>> flooding, but in between those extremes, the amount of fuel supplied to
> the
>> carburetors is modulated by jet/needle annulus, as you said. Possibly the
>> "mean" between the two extremes is a very small range, however, and any
>> insight your experiment gives into that would be useful.
>>
--
Max Heim
'66 MGB GHN3L76149
If you're near Mountain View, CA,
it's the red one with the silver bootlid.
|