<VV> Surging at Cruise

[Jim Davis]

If you are speaking of the "high speed enrichment" circuits in the 
primary carbs, it shouldn't effect the operation engine operation on a 
140 four carb engine.  When Chevy enlarged the engine to 164 cuin in 
1964, they found the carbs leaned out in the 4,000 to 4,400 rpm range at 
full throttle.  To combat his a high speed enrichment valve and metering 
circuit was added to the carbs in 1965.  Working properly the valve 
opens (the needle unseats) when velocity of the air flowing through the 
venturi reaches a certain value.  With a 166 cuin engine this occurs at 
about 3,700 rpm and is shown on a wide band oxygen sensor.  In a 140 
engine with all four carbs wide open, the air velocity in the primary 
carbs never reaches sufficient velocity to unseat the high speed 
enrichment needle.

Most engine surging is caused by a lean condition.  A vacuum leak is the 
most probable condition although a carb float level set low will also 
cause this problem.   A lean condition can also be aggravated by too 
much total advance (static, centrifugal, vacuum).  At slow cruise (13 
inHg or more),  the idle and idle transition circuit supply all of the 
fuel to the engine.  Some where around 2,400 rpm cruise the main fuel 
circuit "tips-in" supplying extra fuel through the venturi cluster.  The 
idle circuit is always supplying the fuel/air emulsion to the engine 
even at full throttle.  On two carb 164 cuin engines, the high speed 
enrichment circuit tips-in at about 3,700 rpm.
Jim Davis



On 11/12/2015 8:24 AM, edward szuch via VirtualVairs wrote:
> My stock 66 Corsa 140 also suffers from this surging condition and I've
> tried just about all of the good suggestions that have been offered without
> real success.  I've replaced plugs, wires, cap, points and condenser - no
> help. I've checked dwell variation and it's OK. It is sensitive to timing
> as Sethracer suggested.  Backing the timing off a few degrees helps but I
> didn't want to take the performance/fuel economy hit (yet).  Pulling and
> plugging the vacuum advance immediately helps but I'm not going there.
> Because of the age of the car, I replaced the damper thinking the ring may
> have slipped and was giving the wrong timing.  I've replaced the 140
> advance can with a Clark's can trying to limit advance.  This only gives a
> more lazy advance curve.  So, I'm thinking more about the power enrichment
> circuit that was mentioned in one of the responses.  The carbs have been
> rebuilt and I think all is OK but I'm not sure if all circuits are
> completely clean given that the car sits most of the time.
> Thanks - good luck!
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