204 East 15th Street
Eldon, Missouri 65026
SERVICE AND TROUBLESHOOTING TIPS
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 thousanths) 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 tripower. 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 overrich 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.
Bog 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 accuate 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 equilization. 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 equilization 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 equilization hole by removing the solder. Solder the leak closed using as little solder as possible. A small piece of tape over the equilization 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 equilization hole. A final test, and you have 'saved' a valuable float.
An 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-nitrofill 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;
(A) Detach the original brass arm from the original cork float.
(B) Clean the arm (we use a glass beading machine)
(C) Attach the arm to the polynitraphyl pontoon included with this kit.
(D) 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 at www.por15.com. READ THE DIRECTIONS. ACCORDING TO POR15, ONCE THEIR PRODUCT DRYS, YOU MUST WEAR OFF ANY YOU SPILL ON YOU! I BELIEVE IT!
(E) Remove the float from the liquid and slowly rotate to eliminate any bubbles.
(F) 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!!!
This section is not designed to turn a novice into a professional mechanic, rather to suggest to the novice a step by step approach to troubleshooting.
Engine not starting (engine you have PERSONALLY heard run, but has been not started for some amount of time).
(1) Compression test (3 percent)
(2) Ignition test (90 + percent)
(3) Fuel delivery system test (less than 1 percent)
(4) Carburetion test (3 percent)
Engine not starting (engine you have never personally heard run)
(1) Compression test (3 percent)
(2) Valve timing test (less than 1 percent)
(3) Ignition test (90 + percent)
(4) Fuel delivery system test (less than 1 percent)
(5) Carburetion test (3 percent)
So if the probability of failure is 90 percent with ignition, why should I do a compression test first? The tests are numbered in the order of if the one above is faulty, nothing you do in a subsequent test will fix the problem. If you have bad compression, adding a new carburetor, and/or completely replacing the entire ignition system will result ONLY in a thinning of your wallet!
Compression test - my compression ratio is a number, and the compression gauge reads pounds, what gives?????
There are mathematical formulae to determine the number from the compression ratio, but they get complicated because of factors such as altitude, etc.; so best to simply research the suggested maximum cranking values for your engine. If you cannot find your engine, find an engine with an equivalent compression ratio.
What you really are looking for are 3 things: (A) the average should be at least 70 percent of the suggested cranking value, (B) no cylinder should deviate from the average by more than 15 percent (10 percent would be better), and (C) the value read on the first revolution of the engine is at least 80 percent of the final reading for that cylinder.
Valve timing test - very low probability of failure, but if you bought someone else's project, you do not know their capabilities, or why they abandoned the project. The distributor COULD be off 180 degrees (remember in a 4-stroke engine the crankshaft makes 2 revolutions for each revolution of the camshaft) OR a timing chain COULD have jumped a tooth. Testing for these items may save you a ton of time down the road.
Ignition test - so many possibilities here. If you have access to a diagnostic oscilloscope - USE IT! If not, you are looking for a really good spark at each plug at the proper time. Research how to check the various components in the ignition system. And when you finally set the points gap (also known as the dwell angle) and the timing, SET THE DWELL FIRST! If you set the timing before setting the points (dwell), changing the point gap (dwell) WILL change the timing.
Fuel delivery system test - basically, you are checking to make certain the fuel delivery system is delivering fuel to the carburetor. Possible items to fail - (A) the sock a.k.a. sending unit filter can clog in the tank, (B) a fuel line may be filled with dirt-daubers, (C) a fuel line fitting may have cracked, allowing air to enter the system, (D) the flexible hose from the hard line on the frame to the fuel pump may be collapsed, (E) the fuel pump may be defective, the fuel filter may be clogged.
If you get to this point, the carburetor probably needs to be removed, and rebuilt.
The sales pitch - if the carb needs rebuilding, we can probably help you with a rebuilding kit.
Fuel overflowing from the carburetor may be caused by a number of issues:
(1) Float/fuel valve incorrectly adjusted
(2) Defective fuel valve
(3) Defective float
(4) Excessive fuel pressure
(A) incorrect or defective fuel pump
(B) clogged or missing tank vent
(5) Cracked housing
Discussing in order:
(1) Make certain that you have the float adjusted to the manufacturer’s original specifications, and make sure you know HOW the manufacturer measured the adjustment. Some manufacturers specify the distance from the float to the casting without the gasket, some with the gasket, some the distance to the fuel level in the bowl at a specified pressure, and Rochester was probably the most creative, often measuring to a “dimple” on the float itself or the float seam. If you buy an aftermarket rebuilding kit, NEVER rely on the generic specification sheets which come in the kits. Check your factory shop manual or the carburetor manufacturer’s manual.
(2) If the fuel valve is defective, replace it.
(3) If the float is brass, check this link: Brassfloats , if a material other than brass, and there is any doubt, replace the float.
(4) A fuel pressure gauge placed right at the carburetor will confirm/deny the presence of too much pressure. However, one should consider the clogged or missing vent. The fuel tank MUST be vented, or no fuel can exit the tank. Many older vehicles were vented through the gasoline cap. Vented caps were obsoleted because of smog emissions regulations. If the tank vent is clogged or missing, normal ambient temperature change will create either a positive pressure or negative pressure (vacuum) on the tank. Some tanks are located close to the exhaust. As the engine warms the hot exhaust heats the fuel in the tank, and can create excessive pressure in the tank.
(5) Cracked carburetor housings are quite rare; in more than 50 years, I have only seen a hand-full, but the issue CAN exist, ESPECIALLY if the “mechanic” feels the necessity of using some form of Teflon (either tape or paste) on a tapered fuel fitting. Teflon is a fabulous lubricant, and will allow anyone to apply excessive torque to the fitting threads and crack the casting.
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.
(1) Clean out the trunk. Weight uses fuel.
(2) 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 underinflation should be sufficient incentive to monitor ones tires.
(3) Turn off the cruise control! A good driver should average 10 to 20 percent better fuel economy than the cruise control unit.
(4) 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 reduces drag.
(5) Keep your vehicle’s drivetrain 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 help).
(6) 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.
(7) 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 trips?
(8) Avoid the use of ethanol (if possible). Ethanol has less energy and will thus deliver worse fuel economy. It may also require carburetor re-calibration.
(9) Carpool – the most efficient vehicle is the one unused in your garage.
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 vehicle.
A common complaint today is fuel dripping out of the throttle body by the throttle shaft AFTER the engine is switched off. While a number of issues may cause this problem, by far the most common issue is the volatility of modern fuel. Mechanical fuel pumps have a check valve which prevents fuel from moving back to the fuel tank. The problem is as follows:
(1) After the engine is switched off, heat from the engine heats the fuel in the fuel line.
(2) The expanding fuel (increased volatility) creates pressure in the fuel line from the pump to the carburetor.
(3) The check valve prevents the fuel backing up through the fuel pump.
(4) The pressure increases to a point the float/fuel valve combination in the carburetor cannot withstand the pressure.
(5) An amount of fuel (usually from a teaspoon to a couple of tablespoons) flows into the fuel bowl of the carburetor.
(6) This raises the fuel level in the bowl above the main discharge nozzle(s).
(7) Fuel flows through the main discharge nozzle(s) and drips onto the throttle plate(s) which is/are closed, and exits out beside the throttle shaft(s) dripping onto the intake.
(1) IF POSSIBLE, AVOID ETHANOL LACED FUEL! Sometimes you can buy real gasoline at a marina
(2) Buy the lowest octane name-brand fuel that does not ping or detonate in your engine (the higher grades often have more ethanol)
(3) Install a “vapor return line” (take a look at return lines used on many factory air-conditioned cars)
(4) Learn to live with the issue.
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 than 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. Pumping the footfeed during this time simply prolongs the agony, as the accelerator pump will pump the fuel into the engine, but in amounts insufficient for starting. 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 engine. If you do not wish to prime the engine, crank the engine for 15 to 20 seconds WITHOUT PUMPING. Then stop cranking, pump the footfeed 3 or 4 times, release it, and then reattempt to start the engine. Priming eliminates excessive wear on the starter. Another possible solution is the installation of an electric fuel pump. If an electric pump is installed, check local, state, and federal laws about wiring; and pick a pump with pressure not exceeding that of the original pump.
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 footfeed (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 footfeed 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 footfeed 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 footfeed, you do not open the throttle plates, and the engine will pump the overrich mixture out of the tailpipe. Once the overrich mixture has been alleviated, gently opening the throttle will allow the engine to start.
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 venturi 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
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, 1958, 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 these carburetors.
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 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. 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 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. 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 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 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.
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.