[TR] synching carbs

[Dave1massey at cs.com]

In a message dated 11/23/2008 4:57:52 PM Central Standard Time, 
mmarr at 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