<VV> Re: LM carbs

[Rad Davis]

I've been through this exact scenario with my van.  It took me a year or so 
to figure out why the off-idle behavior of the engine was weak and 
unpredictable.

It turned out that I had a '65 carb on one side, and a '66-67 non-smog carb 
on the other.  This oddball has the lump where the idle bleed screw would 
be installed, but the hole is undrilled in the casting.  The idle 
transition circuits are different between these.  The newer model 
transition circuit cuts out sooner.  I suspect that it goes with a venturi 
cluster with a smaller bypass orifice to move the cut-in point of the main 
system down in the operating range of the engine.

The van ran OK with this arrangement, but it was difficult to feather the 
clutch in for soft starts with a step function in power output off idle.

As for smog carbs running leaner, there's an idle air bleed adjustment on 
all true smog carbs.  you can adjust them to be as lean or rich before main 
jet cut-in as you like.  From the factory, theyr'e adjusted to run lean 
because that was what CARB and EPA wanted.  It is useful to remember that 
the brass screw inside the air horn is an air bleed, not a fuel adjustment, 
so out is a leaner mixture.



At 06:50 PM 5/1/2006 -0400, NicolCS at aol.com wrote:
>I've heard and read over the years that "Smog carburetors run lean".  I don't
>believe this is true at mid-range cruise and it's certaily untrue at idle.
>While the AIR system controls HC partly through limiting carburetor 
>excursions
>into the excessively rich range, engineers were also concerned about 
>excessive
>NOx emissions - which go ballistic at lean mixtures and high combustion
>temperatures. To manage the two-headed beast, so called "Smog" carburetors 
>have a
>two-stage emulsion system that is more accurate than the earlier carburetors
>and this limits unwanted mixture excursions both on the rich and lean side of
>"correct".  They are more accurate, not "lean".  Of course smog carburetors
>don't have the power enrichment circuit but remember, power enrichment, 
>uh, makes
>more power and thus more heat.  I don't think I'd blame overheating on a more
>accurate mixture and lower total power production.  Of course, a matched pair
>of carburetors is best.
>Craig Nicol
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__________________________________________________________________________
Rad Davis:                                        rad.davis at comcast.net
Corvairs--65, 66 Corsa coupes, '65 'brier Deluxe   http://www.corvair.org/
Keeper of the Forward Control Corvair Primer: 
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"We did Nebraska in seven minutes today. I think that's probably the best 
way to do Nebraska."                            --Brian Shul, _Sled Driver_