http://mk1-performance-conversions.co.uk/mk1_index.htm (Technical Section).
The "Updated & Improved" date for Release 023.MkI is 19/02/2016...
What's new?
"Drop Jet" has been modified to go both ways and is now called "Adjust Jet",
you can now shuffle needles up as well as down and assess the results of doing this...
Why would you want to do this?
Well, in "the real world", the first thing that gets done when changing a needle is to reset
the idle mixture and idle speed. By implication, with no other changes, the engine will
require the same A/F ratio to idle nicely in both the before and after states. Also by implication,
the annular area in the jet at idle will be the same for both the before and after states, but this
will not necessarily be produced at the exact same index position on the needle because the
second needle will have a different profile to the first. This is the reason why we end up
winding the jet nut up or down and as soon as we start "tuning it in" and why our nicely aligned
index data from the charts suddenly becomes irrelevant...
We can now set a real-world datum point from which to compare the before and after needles -
- To find where this datum point would be, one just needs to "look" down the carb mouth with
the engine idling and make a reasonably acccurate measurement of the gap between the
bottom of the piston assembly and the bridge area. To this, we must ADD the depth that the
jet is set down from the bridge. This will be the "base-line" index point which we should be
synchronising when comparing one needle against another and will mostly always end up
somewhere between "official" indexes 2 and 3. Only once we've established this can we
realisticly start to compare the two needles.
It's easy to look at the charts and sagely say,
"Yes, needle two is obviously richer than needle one".
...But this will only be the case if the needles are fitted in exactly the same position,
and then actually run with exactly the same alignment compared to each other.
If we find in reality that "needle one" idles at index point 2, but when we install "needle two"
we need to adjust the mixture and end up with the idle at index point 2.5, we are suddenly
finding that all our numbers from the charts will be half an index point out of sync...
Let's have a look at another example, say we have two identical needles, except that
indexes 2 and 3 on the second needle are just a tad leaner.
We can scratch our heads and call this a "no-brainer" -
- the second needle is obviously the leaner of the two.
Well, it is, isn't it?
Enter "reality"... Because our sample engine idled at index 2.5, say, we will need to
richen up this "lean" needle in order to get our A/F ratio correct again at idle. And what
happens when we do this? The whole profile will be shuffled RICHER to get the idle correct,
so the "leaner" needle, is actually "richer" when fitted and configured correctly - Go figure...
Use of the "Adjust Jet" feature on the "Advanced" tab allows us to compensate for these
eventualities. The age old tradition of directly comparing perfectly aligned index data from
a chart has been consigned to history by "Needles.ods"!
The second fairly major change is the addition of a feature to directly import any of the
needles analysed on the "Advanced" tab into any of the "Data" tabs in order to locate
similar profiles from the charts. This is even possible for needles which have been modified,
or needles which have been "converted" from one jet size to another. The existing "test"
needle facility remains, but use of it will be much reduced as a result of the above enhancement.
There is still a bug in post v3.4 versions of "LibreOffice" which does not allow some of the
context help to be displayed correctly. Anyone running pre-Windows 8 operating systems,
or Linux or Mac is recommended to use no higher than v3.4.x
For the benefit of anyone who is forced to use the later software and is suffering from
not being able to read the "help", I am now including an HTML help file which contains a
set of "Context Help" messages which may be truncated when viewed. I have also attempted
to make some of the columns wide enough that the amount of truncation is reduced.
As always, I will be happy to help with any queries that anyone might have regarding this facility,
which I hope will appear here:
http://mk1-performance-conversions.co.u ... l_carb.htm
at some point fairly soon...
Ian