SERVICE AND TROUBLESHOOTING
The Carburetor shop LLC
Often the accelerator pump gets the blame for other problems. It is very
easy to test the function of the accelerator pump. Start the engine, and
warm to normal operating temperature. Shut off the engine. Remove the air
cleaner. The choke butterfly should be fully open, as the engine is warm.
Observe the pump jet in the carburetor, and with your hand, work the
carburetor throttle to the wide-open position. You should observe a healthy
squirt of fuel from the pump jet. A single barrel carburetor will normally
squirt a single stream; while a two or four barrel carburetor will normally
squirt 2 streams. If you see the stream(s) of fuel, the pump is working. It
is important to start the engine prior to doing this test. With modern
gasoline, it is quite possible the carburetor will be completely dry prior
to starting. If there is no gasoline in the bowl, the pump will not work;
and this would give a false result.
This paragraph applies to an instantaneous bog, hesitation, or stumble upon
acceleration. Constant hesitation is covered under “surging”. This paragraph
also applies to relatively stock engines with the original carburetor. We
will discuss two types of bog: the first is bog when the vehicle is
accelerated from a stop; the second is bog when the vehicle is accelerated
from cruise. Bog from a stop is virtually always (and generally erroneously)
diagnosed as a faulty accelerator pump (see the section on “accelerator
pumps” for testing). Most modern carburetors are designed to function with
roughly 0.020 (20 thousandths) clearance between the center of the throttle
plate edge, and the throttle body at a point equidistant from the throttle
shaft bearing areas. This clearance allows for maximum velocity of idle air
past the idle ports. Exceptions to this are GM carburetors with the idle
speed air screw, and end carburetors on tri-power. Setting the idle for the
highest vacuum idle reading will result in too little clearance of the
throttle plate; forcing too much of the idle mixture through the lower idle
port and too little through the idle transfer slot. This will cause a
phenomena called “puddling” where little droplets of gasoline adhere to the
intake manifold runners. When the throttle is opened, there is now
sufficient velocity of air to sweep all these droplets into the cylinders,
creating a mixture which is too rich to burn, hence the bog. As soon as the
over-rich mixture is pumped out the tailpipe, and a normal mixture is
ingested by the cylinders, the bog disappears. A defective advance mechanism
can also cause bog; as can a defective accelerator pump. If bog exists only
from an idle, not when accelerating from a constant speed, the idle
adjustment is probably the culprit.
from a cruise RPM may be caused by a defective advance mechanism, but on 4
barrel carburetors is often caused by the secondary side opening too soon.
Most original equipment 4 barrel carburetors have “on-demand” secondaries (I
use this term rather than vacuum, as some early 4 barrels used vacuum to
actuate the secondary, while most 4 barrels from about 1960 up used either
spring tension or weights to control the secondary). The Carter AFB uses
weights, and therefore never goes out of adjustment. Other 4 barrel
carburetors such as the Carter AVS, Carter TQ, Rochester 4GC, and Rochester
Q-Jet have a tensioned secondary spring. As the spring fatigues, the air
valve will open too soon, creating an instantaneous lean condition, and a
bog. These units, when rebuilt, should virtually always have the tension
spring replaced, and adjusted to factory specifications. A defective
accelerator pump will rarely cause bog from cruise.
Many mechanics have been conditioned to ask for a
float each time they rebuild a carburetor, due to the reasonable price of
modern, mass-produced floats, and the propensity of nitrophyl (foam) floats
to absorb gasoline after time. In dealing with older, NON-CURRENT-PRODUCTION
brass floats, neither of the above are true, and a mechanic should attempt
to 'save' the float if at all possible.
The first step is to clean the float and inspect it
for obvious damage. Small dings and dents are quite common, even in unused
floats, and occurred when the manufacturer shipped the floats in bulk. Major
dents (generally caused by water freezing in the carburetor) are not
generally repairable. If one can hear liquid sloshing around inside the
float, skip to the next paragraph. If the float looks to be reasonably
damage-free, it should be tested. Testing is accomplished by grasping the
float arm with a pair of needle-nose pliers, and submerging the float in
very hot water. The hot water will pressurize the air inside the float, and
a leaky float will blow a stream of bubbles.
If the float should need repair, it is important to
understand how the float was originally produced. Virtually all brass float
pontoons (the floating part) are composed of two pieces (a few are more) of
brass soldered together. The pieces differ in the seam area, as one piece
has a male seam and the other a female seam. One float piece will also have
a small hole for temperature equalization. This hole will be covered by a
small drop of solder, and will be as far from the seam as possible. The
manufacturer would solder the two pieces together, allow the float to cool
completely, AND THEN close the equalization hole. Soldering MUST be done
using a soldering 'iron'. Repair should not be attempted using either a
torch, or a soldering gun. If you plan on disregarding this advice, read the
next paragraph first! The following procedure works for us (no, we will not
repair your float unless we restore the entire carburetor): First, if liquid
is present inside the float, find the hole, and remove the liquid by placing
the hole down inside the hot water. The pressure will force the liquid from
the float. If the float has much liquid, it may be necessary to remove the
float from the hot water, allow the float to cool, and repeat the hot water
dip. Once the liquid has been removed, and the leak has been marked, open
the equalization hole by removing the solder. Solder the leak closed using
as little solder as possible. A small piece of tape over the equalization
hole will allow the hot water test to be preformed. If there are no leaks,
remove the tape, and ALLOW THE FLOAT TO COOL COMPLETELY before closing the
equalization hole. A final test, and you have 'saved' a valuable float.
area of the carburetor generally misunderstood is the function of the
automatic choke. Automatic chokes use a bimetallic coil to close the choke
plate, and vacuum to open the choke plate. It is important to understand
that the bimetallic coil does NOT open the choke. Automatic chokes are of
two types: integral, and divorced (also called remote). The integral choke
is an integral part of the carburetor. The divorced choke resides on the
manifold (divorced or remote from the carburetor) and has an operating rod
from the choke to the carburetor. In general, carburetors with divorced
chokes use a separate choke-pulloff to open the choke. Integral chokes have
a piston inside the choke housing. In general, the bimetallic coil rotates
when cold to close the choke. As the bimetallic coil is heated, it relaxes,
and the choke is pulled open by vacuum. In the case of the integral choke,
there will be a tiny vacuum passage from the throttle area (vacuum source)
up to the choke housing where vacuum is exerted on the piston. If this tiny
passageway is clogged (often), no vacuum is applied to the piston, and the
choke does not fully open. A problem with divorced chokes is the use of an
incorrect thickness carburetor to manifold gasket when the carburetor is
rebuilt. This will change the required length of the choke operating rod,
and may result in the choke either not closing, or not fully opening.
Setting an automatic choke is quite simple, even if an aftermarket choke is
used. For integral chokes, loosen the retaining screws such that the choke
will rotate freely. Adjust the choke such that the choke plate just touches
closed at 68 degrees F. (65~70 degrees is close enough). Tighten the
retaining screws. For the divorced choke, the same setting applies, but bend
the operating rod to set the choke plate.
Many of the less expensive carburetors from the
beginning up through about 1940 were originally equipped with floats made
from cork. Most of the floats were coated with orange shellac, and then the
finish was baked, creating a finish fairly impervious to the gasoline of the
day. A few of the manufacturers did not coat their floats, and used a cork
material that seemed to work fairly well with the gasoline then being sold.
The gasoline of today cuts orange shellac like a hot
knife in butter, and also will permeate the natural cork material!
This poses a severe problem for the restorer. It is
not economically feasible to attempt to mass produce brass floats to replace
the cork floats. Also, the company producing the poly-nitro fill foam floats
has been most un-cooperative unless orders of very large magnitude are
placed with them. We are currently machining float pontoons from this
substance, to be used with the original float arm.
For those who are independently wealthy, individual
brass floats can be made. This also may be a solution for a retired
machinist with access to a good machine shop. This is a very time-intensive
remedy, expensive if one must pay for the time.
For the rest of us, it becomes imperative to attempt
to use a replacement cork (or foam) float, and seal the cork (or foam)
against the permeation of the gasoline. The procedure we at The Carburetor
Shop are currently using is as follows;
Detach the original brass arm from the original cork float.
Clean the arm (we use a glass beading machine)
Attach the arm to the polynitraphyl pontoon included with this kit.
Submerge the pontoon, and the portion of the arm in direct contact
with the pontoon into a product called ‘POR-15’. This product is available
from POR-15, Inc, P.O. Box 1235, Morristown, NJ 07962. They have a website
www.por15.com. READ THE DIRECTIONS. ACCORDING TO
POR15, ONCE THEIR PRODUCT DRYS, YOU MUST WEAR OFF ANY YOU SPILL ON YOU! I
Remove the float from the liquid and slowly rotate to eliminate any
Suspend the float with a suitable hanger, and allow to air dry for 72
hours prior to use.
This procedure seems to be working with the current
mixture of gasoline.
If anyone comes up with a better procedure, we would
certainly wish to be informed!!!
Concerned about the price of gasoline? Beyond our
control; however most can make their vehicle use fuel more efficiently, in
many cases MUCH more efficiently. All of the following will help your
vehicle to use less fuel per mile traveled.
Clean out the trunk. Weight uses fuel.
Air up the tires, and check the air in the tires periodically. As a
general rule, the manufacturer listed tire pressures which will give a “soft
ride”. Talk to professionals at your local tire shop to see what they
recommend. Do NOT exceed the pressure listed as maximum pressure on the tire
sidewall. Lower pressures create more rolling resistance. Higher pressures,
in addition to being more fuel-efficient, tend to improve vehicle control,
AND prolong the life of the tire. The figure of a 2 percent reduction for
each pound under inflation should be sufficient incentive to monitor ones
Turn off the cruise control! A good driver should average 10 to 20
percent better fuel economy than the cruise control unit.
Pay attention to the condition of the vehicle. A dirty vehicle has
more “drag” in the air; a brake disc or drum which is dragging or a front
end out of alignment causes more rolling resistance. A clean, waxed vehicle
Keep your vehicle’s drive train in good condition. Tune the engine,
check the transmission and final drive at the recommended intervals in your
owners/operators manual. Spark plug wires are an often-overlooked culprit.
(Of course, if you need a carburetor rebuilding kit, we would be pleased to
Adjust your driving habits. Everyone knows that full throttle
acceleration wastes fuel, so we will not discuss this. However, anticipating
a stop sign and gradual slowing (traffic permitting), will save both fuel
and extend the life of your brakes.
Adjust your driving cycles. Take a little time and think; idling in
traffic wastes fuel: can I change my route and avoid signals or stop signs?
How about trips to the store; can I pay the water bill, go to the post
office, and then come by the grocery store instead of making 3 separate
Avoid the use of ethanol (if possible). Ethanol has less energy and
will thus deliver worse fuel economy. It may also require carburetor
Carpool – the most efficient vehicle is the one unused in your
Buying a new vehicle? On the same vehicle, a manual
transmission will result in 10 to 25 percent better fuel economy. However,
until the rest of the buying population learns this fact, expect to take a
beating when you trade in your vehicle. In fact, a good used second vehicle
with a manual transmission might just pay for itself over your existing
HARD STARTING, COLD
Difficult starting a vehicle that has been allowed to
sit for a number of days (that will then start well the rest of the day) is
often caused by modern fuel. Modern fuel begins to vaporize (evaporate) at a
much lower temperature that fuel before the 1970’s. Once the engine is shut
off, the fuel in the carburetor bowl begins to evaporate through the bowl
vent. If there is no fuel in the carburetor, the engine will not start. If
you have this problem, try priming the carburetor by using an eyedropper and
filling the carburetor bowl through the bowl vent prior to cranking the
HARD STARTING, HOT
Difficult starting of a hot vehicle from 5 minutes to
an hour after the engine has been operated, can be caused by the volatility
of modern fuel. If you have this problem; try using the following method to
start the engine: DON’T touch the foot-feed (VERY important). Crank the
engine over from three to 5 seconds (different vehicles will respond to
different times); and then GENTLY (so as not to activate the accelerator
pump) press the foot-feed approximately 1/3 of its travel. The engine should
start, and may run rough. Run the engine at a high idle for about 10
seconds. This issue is caused by volatility of modern fuel. Once the engine
has been shut off, the gasoline is heated by the latent heat of the engine,
and percolates the fuel from the bowl into the throttle area, forming a
mixture that is too rich to fire. If you push the foot-feed to the floor (as
has been the traditional method of “unloading” a flooded engine) the
gasoline continues to flow into the engine (again due to the volatility). By
not touching the foot-feed, you do not open the throttle plates, and the
engine will pump the overrich mixture out of the tailpipe. Once the over-rich
mixture has been alleviated, gently opening the throttle will allow the
engine to start.
THROTTLE BODY GASKETS
Beginning with the 1957 carburetors, Rochester started
using throttle body gaskets (the gasket between the throttle body and the
bowl assembly) that have slots in the sealing surface, thus not making a
complete seal of the two castings ON CERTAIN CARBURETORS ONLY! Not all
carburetors use them. This is only one of the reasons we want a tag number
when we supply a kit, as our kits are manufactured (by us) to the original
Rochester bill-of-material. The correct gasket will be in the kit. The slots
were provided to allow pressure in the venture area to be bled to the
outside of the carburetor during hot city driving thus helping to prevent
stalling during hot city driving. For all you die-hards (or hardheads) –
there is no vacuum leak! These slots are above the throttle plates!! Are you
still a die-hard? Here is a link to a reproduction of the original
Rochester bulletin introducing the slotted gaskets.
ROCHESTER BULLETIN Here is another link showing a
slotted gasket, a regular gasket (often used for marine carbs) and some
other hot idle compensation devices
HOT IDLE DEVICES
TRI-POWER TUNING TIPS
This section is for use in tuning FACTORY GM tri-powers
with ROCHESTER CARBURETORS. DO NOT ASSUME THAT THIS SECTION WILL HELP IF YOU
ARE USING A ‘HOME-BREW’ TRI-POWER, OR ONE USING AFTERMARKET CASTINGS!!
Tri-power was used by General Motors on Cadillac (1958-1960); Chevrolet
(1958-1961); Oldsmobile (1957, 1958, and 1966); and Pontiac 1957-1966). More
often than not, there are more than one tri-power per year and make for
different engine/transmission configurations. The information below is
general. The factory shop manual is an excellent resource when working on
Get the correct parts!!! With many generic “one kit fits all”, repair kits
on the market; it is difficult for the novice to know what to purchase.
Components that one might not consider which can cause issues are: fuel
valves, accelerator pumps, gaskets, and power valves. Discussing these
Fuel valves - I am aware of 4 styles of fuel valves that are being sold: (A)
the conventional pointed fuel valve (our second favorite type); (B) the
aluminum plunger with a neoprene disk inserted in the plunger that seals on
a inverted flare seat (our favorite, but unfortunately, the manufacturer is
now out of business and no new complete units are available); (C) the 2 ball
valve (these tend to hold pressure well, but we have had issues for full
fuel flow in high performance applications, and also have had these flood
profusely on vehicles not driven daily – we will not use these valves); and
(D) an imitation of the valve (B) where a wafer containing the neoprene disc
is placed between the seat and the plunger (we have seen the wafer get stuck
causing profuse flooding, we will not use this valve). If, when redoing a
setup containing valve (B), we can include new neoprene discs in our kits.
Since the neoprene disc is the only wear item, replacing this disc and
cleaning the plunger and seat will restore the unit. Others may have
differing opinions of the various valves.
Accelerator pumps – in the good old days, accelerator pumps were made from
leather. Somewhere along the way it was determined that accelerator pumps
could be made much cheaper with neoprene, rather than leather skirts. BE
(before ethanol) the neoprene pump would last maybe 3~5 years, while leather
will last indefinitely. Neoprene pumps used with ethanol will fail rather
quickly, while the leather pump will still last indefinitely. If at all
possible, purchase kits with leather pumps. In fact, if your old accelerator
pump is leather, try soaking it in light machine oil rather than replacing
it with a modern neoprene pump.
Gaskets – during the 1957 model year, Rochester began using a slotted
throttle body to bowl gasket ON SOME MODELS ONLY! For the carburetor to
function properly, it is imperative that the PROPER throttle body gasket is
used. Using a solid gasket on a carburetor designed for the slotted gasket
WILL result in hot idle issues.
Power valves – Rochester used a number of different calibrations and two
different plunger lengths for power valves on tri-power carbs along. Using
the incorrect valve will create mixture-timing issues.
UNLESS YOU HAVE PRIOR KNOWLEDGE, ALWAYS BUILD THE CARBURETORS TO STOCK
SPECIFICATIONS. NOW YOU HAVE A BASELINE IF MODIFICATIONS ARE NECESSARY!
OK, you rebuilt the carbs using correct parts to stock specs and now you are
ready to install and tune the carburetors. Unless you are a carburetion
specialist, install the center carburetor ONLY and install block-off plates
to block off the end carburetors. If you are a carburetion specialist, you
already knew that, and I didn’t need to tell you.
Adjust the idle. Using a vacuum gauge and setting for the highest vacuum can
cause hesitation (see the paragraph on BOG). You cannot adjust the idle
unless the engine is fully warm. If you blocked off the intake crossover,
this could mean 30 minutes or more. When the engine is warm enough to
properly set the idle, the choke butterfly will be in the vertical or
wide-open position. It is important to understand the idle circuit to
properly adjust the idle. Contrary to popular belief, the idle mixture
control screws DO NOT adjust the mixture. The mixture delivered by the
carburetor is controlled by the idle tubes (gasoline jets), and the idle air
bleeds (air jets) in the carburetor. The idle mixture control screws control
the VOLUME of the preset mixture. An analogy would be a shower where you
first set the temperature and then adjust the pressure. In this analogy the
temperature (mixture) would be preset in the carburetor, and the pressure
(volume) is set by the mixture control screws. For BEST results, the
clearance from the throttle plates to the throttle body will be about 0.020
(20 thousandths) at idle. If the tri-power is being used on other than the
stock engine (455 instead of a 389, or a very radical cam), it may be
necessary to modify the idle circuit. There are two common possibilities in
the modification of the idle circuit (if others are needed, your engine is
too radical for the scope of this discussion). REMEMBER BEFORE MAKING ANY
MODIFICATIONS THAT THEY PROBABLY ARE PERMANENT!!!
Idle modifications - the two common modifications are: enrichening the fuel
mixture and increasing the idle air supply. Enrichening the fuel mixture MAY
be necessary when using ethanol or if the engine has been built to a
slightly higher tune, or headers have been added. Increasing the idle air
supply MAY be necessary if the engine has been built much more radical than
stock or if the displacement has been significantly increased. The goal of
either modification is a steady idle with the mixture screws from ¾ turn to
1 ½ turn from fully seated, and about 0.020-inch clearance from the throttle
plates to the throttle bore. The idle mixture control screws in these
carburetors are the pre-smog short taper. 1 and ½ turns from lightly seated,
and the valves are WIDE OPEN.
To enrich the idle mixture, one must first measure the inside diameter of
the idle tubes. One can then drill these tubes oversize. We recommend NO
MORE than 0.005-inch increase in the diameter. The first attempt may be made
at plus 0.002 inch. If this is not sufficient, then subsequent attempts
should be made in 0.001-inch increments not to exceed 0.005 inch total.
To increase the idle air supply, one may drill small holes in each of the
throttle plates of the center carburetor. If one observes the throttle
plates while attached to the throttle shaft, the throttle plate will appear
as two hemispheres. For best results, the holes should be drilled in the
center of the hemisphere AWAY from the idle mixture control screws. One
should start with a 0.060-inch hole (60 thousandths). If necessary, the
holes may be increased in size, not to exceed 0.125 inch (125 thousandths).
This modification does not change the idle mixture, rather this modification
is done to control the clearance of the throttle plate to throttle bore.
This clearance is important to minimize or eliminate bog from a stop.
Once the idle circuit has been tuned it is time to direct attention to the
main metering circuit. IF A MORE RADICAL CAM HAS BEEN INSTALLED, a vacuum
gauge should be connected, and a reading of idle vacuum obtained. If the
idle vacuum is less than 12 inches Hg. then it will be necessary to install
a weaker spring on the power-valve actuating-valve. A kit with a number of
different calibrated springs is available from The Carburetor Shop LLC. The
purpose of changing the spring is to allow the power valve to remain closed
at high vacuum cruise and open at W.O.T. Once the power valve is operating
properly, one can calibrate the main jetting of the center carburetor. This
is best done with one of the portable air fuel ratio meters. Both the main
jets and power valve should be calibrated on the center carburetor.
Once the idle, main metering, and power circuits have been calibrated on the
center carburetor, one may install the two end carbs and tune them for W.O.T.
Again, this is best accomplished with the use of an air fuel ratio meter.
Different applications will have different desires for air/fuel (power or
economy). These setting will be left to the tuner; but I would highly
suggest consultation with one’s engine builder for suggested ratios.
The sales pitch: repair kits with the proper gaskets, leather accelerator
pumps, etc., as well as additional jets, power valves, vacuum spring kits,
and other parts may be obtained through
The Carburetor Shop LLC.