In a message dated 11/23/2008 4:57:52 PM Central Standard Time,
mmarr@notwires.com writes:
> I've used this method for almost forty years. Never felt the need for the
> new-fangled gauges and such. And I set the mixture by the "lifting the carb
>
> piston" method.
>
One caveat: This works well on pre-1968 US model cars. Post 1967 cars do not
tune up using this method. Why? The reason this works is because the
designers and tuners profiled the needle to work this way. It is much more
than
just he nature of the beast. It shows just how clever these blokes were as it
took work, attention to detail and ingenuity to design cars that were easy to
work on. The 50's and early 60's model cars were designed to be maintained by
their owners using only rudimentary tools. No need for expensive gas analyzers
ore even timing lights. Just adjust the timing to suppress preignition and
you're good to go.
Why does lifting the piston work? First off, it only works if you lift the
piston the prescribed amount. Lifting too much or too little will give you a
false indication of mixture. But this works because the needles are profiled
to run a bit rich at idle. Lifting the piston will lean out the mixture by a
certain amount. If the jet is set properly the extra lean mixture will mix
with the rich mixture already in the manifold and provide a more optimum
mixture
and the idle speed will pick up. After the rich mixture's effect is
dissipated the engine will now be running lean and the idle will drop. If the
adjustments are off you will get a different response.
When the US implemented emission controls the needle profile had to be
changed because you can't meet the emissions standard with a rich idle. Post
1967
US models actually run a bit lean at idle and lifting the piston to set these
will not only direct you in the wrong direction but lead to a frustrating
experience since Strombergs simply cannot be adjusted enough to behave in the
manner described above.
Also note that carbs that can be tuned using the lift the piston method come
with a piston lifting pin on the side of the carburetor. If you carb does not
have this pin it is likely you cannot tune it this way.
So, hats off to those clever blokes in the British motor industry but the
system did have its limitations.
Cheers
Dave
_______________________________________________
Support Team.Net http://www.team.net/donate.html
This list supported in part by the Vintage Triumph Register
http://www.vtr.org
Triumphs@autox.team.net
http://autox.team.net/mailman/listinfo/triumphs
http://www.team.net/archive
|