“Understanding your Holley”
Let me start by stating I'm not a carburetor guru in any way. I have always found carburetors interesting and
have never let them intimidate me. It’s much easier to understand the workings of this magic mechanical
“thing” that feed our internal combustion engines if you break the whole into smaller independent systems, or
as they are often referred to as 'circuits'. Since my youth I have found Holley to be my carb of choice, don’t
take this as other brands are lessor products. In my opinion they all work well and usually it’s not being familiar
with the product that leads to comments like “too hard to tune”. That said, I’ll be making reference from time to
time to a Holley 780-cfm vacuum secondary 4bbl carb (List # 4346) found on high horsepower big blocks of the
late 1960’s. This is very much like the carb found on the 1st gen. Z/28 (List # 4053) and the very popular List
# 3310 as well as the new “Avenger” series that Holley has introduced in recent years. Let’s start with the various
systems and circuits that most carbs are comprised of.
Fuel Inlet:
The fuel inlet system stores and feeds the other systems within the carb. Fuel comes into the float bowl through the
fuel inlet and needle and seat. A float contained within the bowl maintains a constant fuel level.
Idle Circuit:
The idle circuit provides a richer mixture at idle and has adjustment screws to allow the carb to be adjusted to meet
different engine configurations. These adjustments are accomplished via the idle mixture screws and the idle screw
or throttle bore adjustment screw.
Main Circuit:
The main circuit is used when the engine is at cruise speeds. Fuel is delivered from the float bowl through the main
jets into the main well where it is mixed with air from the main air bleeds. The fuel/air mixture is then routed to the main
venturi through the discharge nozzle and booster venturi.
Power Circuit:
The power circuit comes into play during wide-open throttle (WOT) and more fuel is made available to the main well than
the main jets are capable of. This is done with the help of a vacuum operated power valve. At WOT manifold vacuum
drops and the spring in the power valve over comes the vacuum keeping the valve closed allowing more fuel into the
equation.
Accelerator-Pump:
The accelerator pump system adds a squirt of fuel directly into the main venturi. When the throttle is opened air moves
much faster than fuel does through the carb and the squirt of fuel sustain the proper mixture until the rest of the fuel
catches up. This system is made up of the accelerator pump, the pump cam and a nozzle.
Secondary System:
The secondary system is almost like having a second carb and plays a large roll in the fun factor when you step on the
gas. There are 2 types of secondary systems, mechanical and vacuum. Mechanical is as it sounds, linkage attached
to the primary throttle linkage opens the secondary throttle bores. This is beneficial at the track but doesn’t lend itself to
fuel economy and for the street a vacuum secondary system may be better. Vacuum operated secondary carbs only
open the secondary throttle bores when the engine asks. Another benefit is it makes it harder to over carb your engine
which can effect performance as much as too small of a carb.
Choke System:
The choke system provides a richer mixture and allows for a faster idle for cold starts. There are 3 types of chokes,
manual, divorced and electric.
When tuning and trouble shooting always isolate the problem and try to tie it to the proper circuit. The results will
be much better and the frustration level will be lessened. Check back for more carb info in future. Next time “Getting
Back to the Basics”.
References:
http://www.holley.com/TechService/Library.asp
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