The Carburetor Shop LLC
204 East 15th
Eldon, Missouri 65026
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TRIPOWER TUNING TIPS
This section is for use in tuning FACTORY GM tripowers
with ROCHESTER CARBURETORS. DO NOT ASSUME THAT THIS SECTION WILL HELP IF YOU
ARE USING A ‘HOME-BREW’ TRIPOWER, OR ONE USING AFTERMARKET CASTINGS!!!
Tripower was used by General Motors on Cadillac (1958-1960); Chevrolet
(1958-1961); Oldsmobile (1957, 1968, and 1966); and Pontiac 1957-1966). More
often than not, there are more than one tripower 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 use. Components
that one might not consider which can cause issues are: fuel valves,
accelerator pumps, gaskets, and power valves. Discussing these components:
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 neopreme 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 neopreme 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 neopreme discs in our kits.
Since the neopreme disc is the only wear item, replacing this disc and
cleaning the plunger and seat will restore the unit.
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 neopreme, rather than leather skirts. BE
(before ethanol) the neopreme pump would last maybe 3~5 years, while leather
will last indefinitely. Neopreme 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 neopreme 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 tripower carbs along. Using the
incorrect valve will create mixture-timing issues.
UNLESS YOU HAVE PRIOR KNOWLEDGE, ALWAYS BUILD THE CARBURETORS TO STOCK
SPECIFICATIONS FIRST! 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 blockoff plates to
block off the end carburetors. If you are a carburetion specialist, you
already knew that, and would have done so without being told..
Adjust the idle. Using a vacuum gauge and setting for the highest vacuum can
cause hesitation (see the paragraph in “Troubleshooting” 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 tripower 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.
Idle and cruise settings of the center carburetor SHOULD NOT CHANGE when the
end carbs are installed. If installation of the end carbs change either the
idle or cruise settings, then there are probably throttle shaft clearance or
adjustment issues with the end carbs.
Different applications will have different needs for air/fuel (power or
economy). These settings 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, power valves, etc., as well as additional jets, power valves, vacuum
spring kits, and other parts may be obtained through The Carburetor Shop LLC.