Carburetors for Dummies
                           by George Brown {georgeb}
There appear to be a lot of questions about carburetor problems, and a lot of 
people seem to be afraid to rebuild them. If you get the correct kit and are 
careful, it's no big deal. A pail of carburetor cleaner, some brushes, and 
compressed air are helpful. Even kerosene or Gunk and a couple of compressed 
air cans will usually work.

A reciprocating engine is a pump, the carburetor and intake manifold are on the 
inlet side of the pump. A piston goes down as intake valve opens; pressure drop 
in cylinder is now "equalized" with atmospheric pressure by air flowing through 
the carburetor from the high pressure atmosphere side. Intake valve closes as 
compression stroke begins, sealing off the cylinder from the carburetor and 
intake manifold.

The velocity of the air through the smaller carburetor venturi causes a pressure 
drop (basic physics PVT = P'V'T') while the float bowl is still influenced by 
higher atmospheric pressure. So, fuel flows from the high pressure side (float 
bowl) to the low pressure side (carburetor bore). The rate of flow is controlled 
by the air pressure differential, jet sizes, float level, and passage sizes in 
the carburetor.

The carburetor meters fuel to be mixed with air that will obtain the best stochiometric 
mixure(means it will BURN efficiently) of about 14:1 by weight (14 parts air to 
1 part fuel). Gasoline doen't burn; gasoline vapor in air does, if the mix is 
between (about) 12:1 to 16:1. 

Because the engine requirements at different speeds and load conditions vary, 
there are different circuits to help maintain the stochiometric mix 
(idle circuit, transition circuit, main jets, accellerator pump, power valve, 
and perhaps high speed enrichment circuit).

A carburetor is designed and tuned for a specific engine application, and normally 
is set up just a little on the rich side (maybe 13:1). The pressure drop in the 
venturi and carb throat effectively signals different carb circuits; if you have 
a vacuum leak (bad vacuum hose to distributor advance, etc.) these signals will 
be diminished and the stochiometric mixture may be compromised to the point that 
the car runs poorly.

Setting the float level too high, or running with a worn or dirty gasoline inlet 
valve in the carb body (which is closed by the float) will cause over richness and 
poor running. A brass float that has a leak, or a phenolic float that is saturated 
(too heavy in both cases) will also cause over richness.

Jets and passages can become restricted or clogged by dirt, rust particles, etc., 
and cause the mixture to be too lean.

Bear in mind that there are also air passages in the carburetor (most notably idle 
air correction passages) that may become restricted or clogged and cause a too rich 
condition, in this case, at idle.

Poor transitional performance (you mash on the gas) is either the accelerator pump 
or the secondary circuit on a 4V (or both). Make sure the pump is correct first 
(right size, diaphram isn't cracked, check valve - if fitted - is working, linkage 
is adjusted correctly). You can look down the carb throat and work the accelerator 
linkage by hand to see if gasoline squirts into the primary carb throats as a quick 
and dirty "is it working?" check. If the pump has a check valve, make sure it's 
clean and functioning properly. 

Secondary on a 4V is controlled by vacuum and a spring. If the vacuum part doesn't 
work the secondary won't want to open. If the vacuum is working but the spring 
is too weak it will open too soon; if it opens too soon there will be a large 
hesitation (called a BOG). The standard spring should be fine, and the kit should 
have the parts to rebuild the secondary vacuum bit. 

Power valves are designed to enrichen the mixture when the load becomes heavier 
but the engine is running at the same speed (you're cruising on a level road and 
come to a slight incline, or any other activity that will increase the load, like 
pulling a trailer). As engine speed begins to slow under load, vacuum in the carb 
drops, and the valve opens. Power valves are manufactured with different opening 
characteristics; if it opens too soon your gas milage will be poor, if it opens 
too late your car will seem sluggish on an incline. Normally, stick with the rated 
valve your car came with, or go one size smaller (leaner). Note that power valves 
can leak if dirt keeps them from closing, or if they have "blown out" (usually caused 
by a backfire through the carburetor); if they leak your car will be too rich.

Any "too rich" or "too lean" condition may keep a car from starting.

Don't be afraid to tackle a rebuild on your carb. Make notes when you take it apart 
(take pictures or draw diagrams if you have the time) and follow the instructions 
carefully. Put all the small parts in a container so you don't loose anything. 
Get everything really CLEAN. Don't overtighten screws and bolts, and use a little 
vaseline on screws that go through diaphrams to keep the threads from tearing the 
rubber. Make sure none of the mechanical linkage binds.

Make sure mating surfaces are flat (not warped), and when you mount the newly rebuilt 
carb use a new gasket and tighten the nuts to 10 or 12 foot pounds. If the carb 
base IS warped (someone overtightened it), ture it up carefully with a file.

Good luck...