Modifying Twin HS4s

[dhenry]

We shape needles when we tune engines on our rolling road.

[su_mark]

have access to a facility to make one off needles pretty easily (not cheaply!) which you will need to do.

so do I

its called a bit of wet and dry paper

Ha ha,
reminds me of last years Spa 24hr Mini race. I was supposed to bring a needle for one of the other cars but left it on my desk. They set about copying the needle from our car using the 'wet n dry' method. Once installed and running they managed nearly 20 minutes more running time than anyone else on the same amount of fuel! They, of course, claimed it was pure calibration skill!

[mk1]

[ianh1968]

Shameless plug:
http://mk1-performance-conversions.co.u ... l_carb.htm
... the latest brand spanking new version of "Needles.ods", uploaded today.
(Thanks Mark F.).

With all the talk about having to "make" needles, it might be possible to just "buy"
one from a different size range...

The Needles.ods spreadsheet has a "Jet Converter" facility which works out what
size a needle would need to be when going from say, 90 to 100 thou on the jet.

Here's an example: The original needle is BB4, a fixed 90 thou needle, and this has
been entered as "Needle 1" on the "Advanced" tab.

The table at the bottom left has converted the sizes into a 100 thou needle.

Click on the images a couple of times to make them bigger,
Click "Back" on your browser to return...

Needles.Advanced.png

On the "F100" data tab, we can search the fixed 100 thou needles, looking for
a match for the converted needle. The results show "SM" as the favourite.

Needles.F100Data.png

Back to "Advanced", with "SM" entered as "Needle 2", the BB4 and SM are quite
similar, despite being from different jet size ranges. It was not quite right, so I've
leaned off "SM" by 3 flats to make the idle indexes match a bit better. The results
are plain to see...

For this example, I've "cloned" an existing profile, but it would be equally valid
to type some values directly into the "test" needle to get some different results.

You can mess about with "test" needles and various mods to existing needles,
but the one you are looking for may end up being from a different range. By way
of example, some of the early 997cc racers used a "UVP" needle in an HS6...
... this was a 125 thou needle!

If you can't get enough "richness" at the fat end of a needle, a larger jet may be
the way to go.

I am sure that "Minty Lamb" would have been able to work out something similar...

<END OF SHAMELESS PLUG>

Ian