Thomas,
Don's complaint was at 6K RPM ... and that is why IMHO the needle was
still a factor. I though his original post indicated that he was only
on the second clip position from the flat end and that he was OK if he
made it to the top. Also, there is a lot of confusion in regard to
what circuit is doing the most at different throttle positions and RPM
ranges from some of the tuning "charts" out there ... and it is easy
to conclude that if you are WOT you must be on the main jet only. IMHO
that is not true ... so let me see if I can convince you .. (but I
have to put on my asbestos suit).
If these carbs were flatslides ... throttle position would be the most
significant thing to consider since on a flatslide .. if you go WOT
... the slides as well as the throttle plates are wide open ...... On
a flatslide you need to watch what you are doing because if you give
it too much throttle too soon you will fall on your face ..... If you
remember the old quadrajets and other vacuum secondary carbs on autos
... you probably remember that there was always a "door" of some type
that was engine load dependent, (vacuum). A high vacuum was supposed
to indicate low engine load ... sooooo ... you could go WOT and not
have to worry about the "big air door" flying open until the engine
could handle it, (if a certain vacuum level was attained and the
blocking leg was dislodged because you had the throttle plates open
past a certain point .. whammo ... the big whoomp sound that everyone
loved would occur ... and draw traffic officers from several blocks
away ... ).
On a CV ... this is not the case ... you can go WOT and the slide will
open as the velocity of the air increases ... going past the lift hole
in the end of the slide, (the slide is really nothing more than an
"air speed" indicator). Soooooooo ... WOT might get you a quick
opening blip but the slide will settle back down to reflect the actual
"air" being passed and will continue to open back up depending on your
RPM, (assuming the slide is calibrated to be open near the top of your
RPM range ... spring tension etc).
On a CV ... the throttle plate position is most important for the
pilot circuit .... ie: When the plates are closed ... pilot mixture is
drawn through the mixture screw hole .... extra air is also pulled
into the mixture from three bypass holes located just above the
throttle plate. As the plate opens and each of these bypass holes are
uncovered and exposed to the high vacuum below the plate ... "that"
particular hole starts pulling pilot mixture into the carb as well ...
until all three bypass holes and the pilot mixture screw hole are
seeping pilot mixture, (another reason why carb sync is important ...
not just for idle .. and this is why Tyler talks about crazy things
like checking "sync" while he is going down the road .. with his
"electronic" mercury tubes ... ). At this point there is also some
main mixture starting to seep around the needle setting in the needle
jet, (tube), even though the slides haven't moved.
It is in this area where throttle position is more important than ever
.... this is also the area where pilot jet tuning has such a
significant impact. As Bob pointed out ... these "holes" continue to
contribute to the overall mixture ingested by the engine for entire
RPM range but the amount they contribute will decline significantly as
you continue to open the throttle plates, (and they aren't getting an
unfair share of the vacuum anymore). As the throttle plates open wider
... the needle jet now "sees" more of the high vacuum from the engine
side of the throttle plate and the fuel comes out of the hole pretty
easily ... sooooooo ....... you are now transitioning into mid-range.
Opening the throttle even more ... allows the velocity of the air to
increase past the slide and the slides begin to open, (or shimmy), a
significant amount. Anywhere past this point you are "in" mid-range
until you have enough air passing the slide to keep it open completely
........ now you are running on the main mixture ... the needle is out
of the way as much as it is going to be and you are at the "top end"
.......... the engine will pull what it needs.
If Don hadn't reduced the size of his PAJ2 in advance I think he would
have experienced more typical "lean" type indications. Generally,
these would be ...... sneaking up on mid-range 4-4.5K and hitting a
big flat spot ... but being able to "jump" over it by going WOT early,
(main big enough but needle set too lean) ...... Don made his pilot
mixture richer and kept it helping out by staying with a modest
throttle position ..... and making it all the way to 6K ... then he
just had to have more fuel and it wasn't there ... not enough needle
and possibly not enough main or not enough signal to pull mixture out
of the needle jet tube.
Another typical indication that is important is to reduce the size of
the main jet .... until you "just do" start getting too lean ........
This will show as a flat spot or slight hesitation right at or
slightly below 7.5K ..... This is in an area where you are completely
dependent on pulling the mixture out of the needle jet tube ..... and
you aren't getting enough ...... if you have gotten this far your
slides should be completely retracted. I have done this test with DJ
springs and open filters so I feel confident that this is an
indication of the actual RPM that the "top end" is being achieved,
(top end defined as fully open slides).
Soooooooooooo ........ if you are buying all this so far ... (I have
to put the bullet proof vest on now .. sigh ...), you are probably
wondering what the reduced intake resistance had to do with anything
???!?? Welllllllll .... it has to do with the actual vacuum or
"signal" present in front of the needle jet to pull main mixture out
of the hole ..... The reduced intake resistance has reduced this
signal. To make matters worse ... it has a profound effect on the
relative fuel bowl pressure, (referenced though the main air jet),
that acts against this "signal". This explains the purpose of the
"correctors" or "restrictors" ... by making the main air jet "smaller"
with this device you are eliminating a big "air leak" and making it
easier for fuel to be drawn out of the fuel bowl and the needle jet
tube, (raising the fuel level in the bowl helps also). This is also
the basis for DJ tuning troubleshooting that suggests things can be
improved by removing fuel bowl vent tubes etc ... on normally
aspirated installations, (of course ram air setups etc must have the
fuel bowls referenced to the positive intake pressure or nothing
useful would come out the hole ... ). I agree with that concept,
(pulling bowl vent hoses), but it depends what you want to do ... it
is different for every setup. If you are trying to equalize your fuel
curve etc ... fine ... but if you are having a hard time getting fuel
out of the fuel bowl in the first place I think I would strive to see
that the most positive pressure possible appear at the end of these
hoses.
IMHO ... the Vmax reveals a lot of neat utilization of technology ...
intentional or otherwise ....... Like it or not ... the fake gas tank
or "air cover" on top of the airbox represents an area that is going
to achieve a more positive pressure than say ... down in the oil
galley valley ...... I have some greatly disputed views about how
neat this air box is ... (or at least the body of the airbox) .... I
feel that it confines the entry of air to the "higher pressure" area
only. It also provides a mounting position to keep the fuel bowl vents
in the "higher pressure" area. This of course is only my opinion and
even if you are buying it ... you must remember that where you orient
your air intake or fuel bowl vents may or may not be advantageous
depending on you setup and what you are trying to accomplish.
My apologies for another "book" ........ feel free to get out the
flame thrower ..... that is the purpose of the list anyhow ...... but
currently ... unless someone beats me over the head to convince me
otherwise .. I think Don .. just delayed the appearance of a lean main
mixture higher into "mid-range" because he cheated and enrichened his
pilot mixture ...... He can add correctors/restrictors, reduce the
size of the garage door he opened in the top of his airbox or see if
he can't get by with increased main jets and a richer needle setting
......
campbell
Thomas wrote:
> TC, I was under the impression that at full throttle, high
> rpm that the mains were the only consideration and the
> needles had nothing to do with it.
> ----- Original Message -----
> From: TC <[EMAIL PROTECTED]>
> Subject: Re: Stage 1 with trimmed airbox help needed.
>
> > Don,
> > You are absolutely right .... you have lost significant suck
> > power to pull fuel out of the needle jet, (tube the needle lives
> > in). The
> > snip <
> > >
> > More needle ... more needle ... more needle .....
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