Carburetors
First off, make sure the correct size, (cfm) carburetor is installed on the engine and be sure the idle mixture screws are properly adjusted (see page 6 of the Owner's Manual). You can always make your carburetor leaner, but this will not always improve gas mileage. Since every engine is different, there is no way of knowing how much of an affect leaning down the carburetor will have on your engine. You can refer to your carburetor owners manual and the calibration chart inside to lean down the carb. Start off light, but keep an eye on the plugs to make sure you aren't taking it too lean. You will see on the chart that you can lean the carburetor down in different percentages in different modes, cruise and power.
There is no "pre-determined" combination of rods and jets for any given motor, except for altitude corrective applications, which is 4% for every 3,000 ft of elevation. Every engine is going to require a different fuel curve. The best thing to do is run the carburetor on the engine right out of the box to build a baseline, then check the spark plugs and see how they look. You need to have a light brown, or tan color on the plugs. If they are too white or too black, you need to make adjustments, refer to the tuning chart in your carburetor owners manual. Plug reading gives you an accurate idea of what is actually taking place inside the combustion chamber. Please note, today's re-formulated fuels can leave a lack of accurate colorization. Purchase carburetor replacement parts online.
Fuel in the oil can be caused by several conditions including a ruptured fuel pump diaphragm, incorrect fuel line hook-up, and carburetor flooding. If the problem is carb flooding, check the following: Fuel pressure should not exceed 6.5 psi. Float level should be verified to be at 7/16. Refer to supplied carburetor owners manual. Verify needle & seats are free of any debris, and have a smooth range of operation and travel. Verify floats are free of any fluid by shaking the float, or submersing it in a safe, non flammable fluid.
This may be caused by a rip or tear in a base gasket, air horn gasket, or adapter gasket or some foreign material stuck in an air passage of the carburetor. Inspect the gaskets to make sure you have no vacuum leaks of any kind. Check to make sure carburetor is seated correctly, and is fastened with the correct hardware. You can perform a quick check to find external vacuum leaks (such as a leaky carb base gasket) by spraying starting fluid on suspected areas while the engine is idling. If the engine speed changes for a second or two after you spray an area of the carb, you've found the leak.
Check for correct choke operation and adjustment. Make sure that the fast-idle cam is not causing this. Verify the throttle arm rests on the idle speed screw. It is important to verify throttle linkage, and or throttle blades are not binding and have a free range of operation. Throttle return spring should be correctly located and adjusted. In most cases, the return spring should be positioned on the top of the throttle arm forward. This should have been confirmed when the Wide-Open Throttle test was performed during the initial installation of the carburetor. Make sure all of the vacuum ports on the carburetor are being utilized or blocked off.
The 1406 is calibrated lean for fuel economy. In some applications, re-calibration of the carburetor may be necessary for optimum performance. See page 22-23 in the supplied carburetor owners manual. Step # 23 on the calibration reference chart is a good starting point for enriching the air/ fuel mixture.
All of our Performer Series and AVS carburetors feature the PCV port on the front center of the carburetor; it can only be used for PCV. The fuel inlet is on the passenger side rear of the carburetor. Incorrect installation of the fuel line can cause severe engine damage.
The Electric choke on our Performer Series and AVS carburetors needs to be hooked to a "keyed" (while the key is in the on position) 12-volt power source, with a good ground. Verify there is no voltage when the key is in the off position. Do not connect the positive wire to the Ignition Coil, ballast resistor or Alternator.
Carburetor flooding can be cause by the following conditions. First, verify, with a fuel pressure gauge that the pressure does not exceed 6.5 psi. Floats should be set to the factory spec of 7/16, (see carburetor owners manual). Needles & Seats can be clogged with debris, not allowing them to close. They can be cleaned and checked when the float level is being verified. Floats can be sinking due to a leak. Verify floats are free of any fluid by shaking the float, or submersing it in a safe, nonflammable fluid.
We do not recommend routing the PCV line to the rear of the carburetor. We recommend that the PCV line go to the front, if you have power brakes, they go to the rear.
Both Idle mixture screws on the front of our Performer Series and AVS carburetors control air/fuel mixture at idle. The left screw controls the left venturi and the right screw controls the right venturi. Refer to your carburetor owners manual to properly tune the idle mixture.
Generally the distributor vacuum line goes to the timed (pass side of carburetor) port. This is mandatory on emission controlled applications.
An incorrect float level can cause this to occur. Make sure that the float levels are set properly at 7/16" per the carburetor owners manual. Excessive fuel pressure can also contribute to this condition.
Too much fuel pressure usually causes this and often times dirt in the float bowls can do the same. Make sure your fuel pressure does not exceed 6.0 psi, optimum pressure is 5.5 psi.
There are many different circumstances that would cause fuel to leak out of a carburetor. Below are a few reasons why you may see a fuel leak along with some possible solutions.
- Fuel pump pressure is too high. The fuel pump pressure should be no more than 6.5 p.s.i.
- Take the top of the carburetor off and check for debris in the needle & seat, also inspect the needle tip to be sure it has not been compromised.
- While the float is off, shake the float for any signs of fuel in the float. If there is fuel in the float replace float.
- Check float level, the float level should measure 7/16”. Measure the float level by turning the carb lid upside down and placing a 7/16” drill bit between the airhorn gasket and the top of the outer end of the float just past the inboard float radius.
- With today’s fuels, another cause of leaking fuel could be heat soak. Heat soak is when the heat from the engine compartment heats up and can boil the fuel in the float bowls. When this happens, the fuel can get forced through the venturi booster and drip into your engine. This can also cause fuel leaks through the airhorn gasket.
If the choke is not opening all the way this is usually caused by either a misadjusted choke or improper voltage supply. The best time to set your choke is in the morning when the engine is cold and the outside temps are on the lower side. With no power supply to the choke hit the throttle, the choke should close and you should have about an 3/16” gap between the choke valve and the airhorn. If your choke is out of adjustment just loosen the 3 choke cap screws and rotate the black choke thermostat until you achieve a 3/16” gap (counter clockwise leaner). If you must adjust the choke be sure to hold the throttle lever open about a ¼ of the way. This will ensure that the fast-idle cam is out of the way and will not interfere with your adjustment. If the choke is adjusted properly and it still will not open fully, you may have a problem with the voltage supply to choke. The choke needs a key-on 12 volt power source. This means it is necessary to have 12 volts while engine is running and no power when the engine is off. With the wire disconnected from the choke check to be sure you have 12 volts when the key is on. If not find a new power source. Do not connect the positive wire to the Ignition Coil, ballast resistor or Alternator.
There are many reasons why your idle mixture screws do not affect the engines idle, this is usually caused from a flooding condition. Below are a few circumstances that may cause your idle mixture screws to not function properly along with some possible solutions.
- Be sure you have no vacuum on the timed port on the front of the carburetor (passenger side vacuum port). If you have vacuum on this port then the throttle blades are open to much. This will cause the idle mixture screws not to function as you are now pretty much out of the idle circuit and the primaries are feeding the engine with fuel. To correct this the throttle blades must be closed more, so you will need to adjust your idle speed screw until there is no vacuum on the timed port. If your engine is now idling to low in some cases you can advance your timing to bring engine speed back up. Also for non-emissions use you can run your distributor vacuum advance to the full vacuum port on the driver’s side front of the carburetor.
- Fuel pump pressure is too high. The fuel pump pressure should be no more than 6.5 p.s.i.
- Take the top of the carburetor off and check for debris in the needle & seat, also inspect the needle tip to be sure it’s not compromised.
- Take the carburetor lid off and remove the floats. Shake the float for any signs of fuel in the float. If there is fuel in the float replace float.
- Check float level, the float level should measure 7/16”. Measure the float level by removing the lid and turning lid upside down. Then place a 7/16” drill bit between the airhorn gasket and the top of the outer end of the float just past the inboard float radius.
The top hole closest to the pivot point will give you the biggest pump shot of fuel. The bottom hole furthest away from the pivot point will give you a smaller pump shot of fuel. Making these adjustments can be a great way to fine tune your off idle performance.
At an idle you should never see any moving fuel at all. The fuel delivered to the engine at an idle is delivered below the primary throttle blades so you can’t see the fuel at an idle. If you are seeing any fuel at an idle then you are dealing with a flooding condition. Below are the things to check to resolve a flooding condition.
- Fuel pump pressure is too high. The fuel pump pressure should never exceed 6.5 p.s.i.
- Take the top of the carburetor off and check for debris in the needle & seat, also inspect the needle tip to be sure it’s not compromised.
- Take the carburetor lid off and remove the floats. Check the float for any signs of fuel in the float. If there is fuel in the float replace float.
- Check float level, the float level should measure 7/16”. Measure the float level by removing the lid and turning lid upside down. Then place a 7/16” drill bit between the airhorn gasket and the top of the outer end of the float just past the inboard float radius.
A ticking sound from the carburetor would be caused by the metering rod step-up piston assembly hitting the metering rod over plates. The only way this would happen normally is due to the engines vacuum being at or near the same as the rating of the step-up spring. To correct this a lighter step-up spring would need to be installed. As a base line, you want the step-up spring vacuum rating to be half of what your engines vacuum is at an idle for best performance.
The Performer/Thunder series carburetors do not need to be removed from the intake manifold to do jet and rod changes. Just use caution and not drop any parts down the throttle bores. Metering Rod and Step-Up Spring changes can typically be made in less than five minutes and without removing the carburetor. First, loosen the Step-Up Piston Cover Screws and twist the Step-Up Piston Cover Plates to the side. The Metering Rods and Step-Up Springs can now be removed and replaced if necessary. Be sure to replace the Step-Up Piston Cover Plate and snug the Step-Up Piston Cover Screw when finished. CAUTION: Do not overtighten the Step-Up Piston Cover Screws! They should only be tightened to 12 to 17 inch/pounds. Excessive torque will weaken or snap off the screw heads. If this happens, they may fall into the carb causing serious engine damage. If an inch/pound torque wrench is not available, snug the screw until it just touches the plate, then tighten 1/16th turn more.
To replace the Primary or Secondary Metering Jets, first, remove the Metering Rods and Step-Up Springs as outlined in the preceding paragraph. Next, disconnect the fast idle Cam Connector Rod, Accelerator Pump Connector Rod, and Electric Choke Connector Rod (when applicable). Finally, remove the 8 Airhorn Attaching Screws (early models have 9 screws) and remove the Airhorn from the carburetor body. A standard screwdriver can now be used to remove the appropriate Metering Jets. Once desired Metering Jets have been installed the carb may be reassembled by reversing this procedure.
The Step-Up function, which moves the Metering Rod to the Power Mode, is controlled by the Step-Up Spring. The base calibration has a spring which “stages” rich at 5" Hg. If your vehicle has a mid-throttle driveability problem that is encountered as the throttle is gradually opened, but then goes away upon further opening, it may be possible to eliminate the lean spot by using a stronger Step-Up Spring. It is best to select a new spring based on vacuum readings, but in the absence of a gauge, try the strongest spring (highest vacuum rating) to see if the problem goes away. If the drive problem is cured by the strong spring, try the next weakest spring as well. If the strong one does not help, then the calibration problem is related to the A/F metering stage of either the Cruise or Power Modes. As a general rule for best performance run a step-up spring that is rated at half of what the engines vacuum is at an idle.
The Edelbrock Performer Series/Thunder Series carburetors have conventional Idle Mixture Screws (IMS) that provide a leaner A/F when turned clockwise and richer A/F when turned counter clockwise.The idle air flow is controlled by a conventional idle speed screw that opens the Primary Throttles. The following procedure should be used to set the idle mixture and speeds.
- Fully warm engine and ensure choke is fully open.
- Air cleaner in place.
- Set desired speed with the idle speed screw.
- Adjust the IMS on ONE side to get the maximum possible RPM.
- Do not go rich beyond the maximum speed point.
- If the above changed the idle speed more than 40 RPM, then re-adjust the speed.
- Adjust the side OPPOSITE of that in Step 4 to get maximum RPM.
- Reset the speed.
- Carefully trim each IMS to again get the maximum idle RPM.
- Go leaner just enough to get a 20 RPM drop in speed.
- Reset the speed via the idle speed screw to the desired RPM.
- This is a Lean-Best Idle Set. Setting richer than this will not
- Improve idle quality or performance, but could tend to foul plugs.
Another way to adjust the idle mixture screws would be with a vacuum gauge. With your idle RPM set hook up your vacuum gauge to a full manifold vacuum port. Next adjust the idle mixture screws one at a time until you get the highest and steadiest vacuum. If your idle speed is now off reset idle speed via the idle speed screw and re-adjust mixture screws until you get the highest vacuum.
There is no requirement to run a fuse for the electric choke on the Edelbrock Performer/Thunder series carburetors as the choke has a very low amp draw (.5amp). If you would feel more comfortable running a fuse you can use a 10-amp fuse for the choke power wire.
For a first step, you can pull out the carburetor idle mixture screws and blow some compressed air into the idle mixture screw holes. This can be a temporary fix since you’re just blowing the debris right back into the carburetor. To fully clean out the idle circuit you will need to remove the carburetor from the intake manifold, then remove the top of the carburetor and remove the primary venturi boosters. Clean out all the passages on the venturi booster especially the small idle jet, this is where the fuel travels through to feed the idle circuit. Take out your idle mixture screws and clean out that passage, you should have free flow from the mixture screw hole up to the passage where the venturi booster mounts.
In most cases the airhorn gasket can be reused, just inspect for any tares in the gasket before re-installation. When removing the airhorn be sure the gasket comes up with the airhorn. If the gasket is sticking to the base slide a thin flat object such as a gasket scraper between the gasket and the base to break the gasket free.
To help eliminate heat soak problems your best bet is to use a heat insulating spacer under the carb base. A heat insulating spacer material plays a huge role in insulating the carburetor for heat. The best spacer material to use is either wood fiber, phenolic plastic or one made of gasket material. Its best to run a spacer with a 4-hole design, this will help shield more heat from the carburetor. Some other tips for heat soak, be sure the fuel line is routed away from any hot heat sources like intake, exhaust and so on. You can also verify that the float levels are correct, if the floats level is too high this can compound the problem. Be sure you don’t have restrictions in the exhaust, some vehicles have heat risers in the exhaust and if they get stuck in the closed position this will cause more exhaust crossover heat under the carb.
Edelbrock does not recommend the use of an open adaptor on a stock intake manifold for a few reasons. With an open adaptor on a stock intake manifold, you can run into several problems. Installing an open carb spacer on a stock intake manifold will likely result in uneven air/fuel distribution due to the open space. With a lot of stock intakes, they have 4 holes and with an open adaptor you will have a lot of positive steps in your air/fuel charge, plus fuel will puddle on the intake carb pad. You will also lose throttle response when using an open adaptor. The only time open adaptors should be used is on single plane intake manifolds.
The main reason an accelerator pump shaft would break on an Edelbrock Performer/Thunder series carb, is that the accelerator pump connector link has been installed backwards. When you’re looking at the front of the carburetor the accelerator pump connector should look like the shape of the letter S. If your pump connector looks more like the letter Z then the connector is on backwards. If the connector is correct then check the bore of the accelerator pump well for any damage. Also, be sure that the accelerator pump passage to the nozzle is free and clear. If the passage is plugged it can cause a hydraulic effect and break the pump shaft.
If your engine is hard to start cold please know that is not normal and there are a few items to check. The first step is to be sure you’re getting a pump shot of fuel out of the accelerator pump nozzle. If the vehicle has not been running in a few days, you could be having a problem with today’s fuels evaporating out of the float bowls. If the vehicle is ran every day and you’re not getting a pump shot of fuel you may need to rebuild the carb or at least change the accelerator pump. If your accelerator pump is working properly, you will now need to see if your choke adjustment is off as this could also be the cause. Be sure when the choke is set that there is about an 3/16” gap between the choke blade and the airhorn. If the choke is closed all the way or if you have to big of a gap the engine will be hard to start cold. If the choke needs to be adjusted.
In a past FAQ we talked about how to lean your carburetor out for better fuel economy, this time I would like to give some other tips that can help improve fuel mileage. Adding a 1 inch 4-hole spacer can help improve signal to the carburetor which will help atomize the fuel better and can increase your mileage. Be sure your running a large enough air cleaner and it is clean. A EGR valve can help with fuel economy at cruise speeds. You also want to be sure you have a good tune-up, good sparkplugs, wires, cap and rotor.
Carb spacers can definitely change the performance of an engine. A 4-hole spacer can improve your low end response, but also limit your top end performance some. A divided spacer would do the opposite, you will lose some bottom end performance but you may pick up some performance on the top end. Open spacers are only for use on single plane intake manifolds and will cause your engine to lose some low end response but you should pick up on the top end. Open spacers are very common in racing applications when allowed. You can change the power band of your intake pretty easy, using 1 to 2-inch spacers. The bigger gains will be seen when your intake manifold is a little too small for your application adding this additional plenum volume (open spacer) can really wake an engine up at higher RPM’s.
If this is carb related your calibration might be a little off. First try the different positions on the accelerator pump arm. The top hole closest to the pivot point will give you the biggest pump shot, the bottom hole will give you the smallest pump shot of fuel. In most cases this will take care of the problem. Also, be sure your idle mixture is adjusted properly, the idle mixture still plays a roll right off idle. If these things don’t solve the problem then you will need to recalibrate the carburetor. If you can notice a difference in performance with the different pump arm positions then this will help you. If the top hole closest to the pivot point works the best the engine wants more fuel and if the bottom hole works the best then the engine wants less fuel.
This is normally due to a choke that is slightly out of adjustment, this is common as the temperatures get colder into the fall months. As the temperatures decrease the choke valve will start to close more, when this happens the fast idle cam will engage. The best way to resolve this is to readjust your choke. If the choke is not the cause, then check to be sure the return spring is not catching on anything.
Altitude has a direct effect on the operation of most carburetors. As the altitude increases, the air becomes less dense so a carburetor, originally calibrated at low altitude (sea level), delivers too much fuel and the engine runs richer. If the preceding tuning procedure is performed on an Edelbrock Performer Series/Thunder Series carburetor, a proper high altitude calibration will result. If the vehicle was calibrated at lower altitude, however, and is to be driven at high altitude temporarily, it is not necessary to repeat the complete calibration procedure. A good rule to go by is to adjust your carb “2% leaner per 1500 feet” of elevation you will be traveling to. For example, with a #1405 at baseline calibration (location #1 on the chart) and intended operation at 6000 ft. altitude, you would want 6000 divided by 1500 x 2% = 8% leaner calibration. That would be location #24 on the chart which would require only a rod and jet change.
There can be a few things that can cause no pump shot of fuel through the accelerator pump nozzle. First be sure the pump connector rod is hooked up; this rod connects the throttle lever to the accelerator pump arm. Next, remove the airhorn of the carburetor and inspect the accelerator pump for any obvious signs of a bad pump, like a torn pump cup. Be sure the float level is set to 7/16, if the float level is to low, there will not be enough fuel in the float bowls to fill the accelerator pump well. If float is to low see adjust float procedure. Next, be sure that the accelerator pump passage is clear use some carb cleaner. If the pump passage needs to be cleaned use some carb cleaner with the supplied nozzle spray the carb cleaner through the hole in the bottom of the pump well. You should see the spray coming out of the accelerator pump nozzle. If that passage is plugged take off the accelerator pump nozzle and turn carb upside-down. A check ball and weight or check ball and spring should come out depending on the model of carb. You want to be sure the check ball comes out, if it does not then it may be stuck in the bore and causing your problem. This can be common especially with engines that are not ran much. Use some carb cleaner/penetrating spray to try to free up check ball if needed, then clean passage.
The most common reason the secondary’s do not open is due to an improper choke adjustment or the choke is not opened all the way. When the choke is engaged there is a secondary lock-out that will not allow the secondaries to open. Some other causes could be a throttle cable not adjusted properly or a possibility that the secondary throttle blades are hitting the intake or adaptor. There is also a link on the passenger side that ties the primaries to the secondary’s, be sure this linkage is installed. Note: The secondary air door MUST be opened to see the secondary throttle blades. The air door is not a mechanical device and will not open when you actuate the throttle.
When off-roading in an uphill off camber situation the fuel from one float bowl spills over to the other float bowl due to a passage that ties the 2 float bowls together at the rear of the carb and then the carburetor floods out the engine. This passage can be seen with the top of the carburetor off, its located at the top rear of the carbs base. Here are a couple things that you can do to help make this better. First be sure the floats are set to the recommended 7/16” gap, if float needs to be adjusted see float adjustment procedure. It is also recommended to use the Edelbrock off-road needle & seat assembly #1465, this could also help during off-road use. To help prevent the fuel from flowing through the rear passage it can be blocked off. With the top off the carburetor place a fuel resistant putty or epoxy to fill in the rear passage. We don’t recommend modifying the carb just use something to prevent the fuel flow. Our carburetors are not ideal for extreme off camber situations when off-roading, but if you do these steps it should greatly improve the flooding problem.
Here are a few things that would prevent fuel from entering a Performer/Thunder series carburetor. A stuck Needle & Seat can cause this, first try tapping on the rear of the carburetor with a small screwdriver handle while cranking to try and knock them free. Check the fuel pressure you must have at least 2 p.s.i. of fuel pressure to run the carburetor. If you still can’t get fuel to enter the carburetor, then remove the lid from the carburetor and pull the needle out of the seat. Inspect the needle tip and seat for any obvious signs that would cause the needle to get stuck. Next remove the seat assembly and inspect the screens behind the seat. These screens could be plugged preventing the fuel from entering the carburetor.
If the choke valve will no longer close, then more than likely the choke thermostat is bad and the choke cap needs to be replaced. First, loosen up the 3 screws and rotate the choke cap clockwise to see if the choke valve will close. Also check to see if you can move the choke valve by hand, if you can move the choke valve by hand but can’t move it by turning the choke cap then replace the choke cap #1474.
If you are in an area that does not require a choke it is possible to not hook up the push pull cable. If you’re not using a manual choke push pull cable, then you must wire or secure the choke open in some way. If you do not secure the choke in the open position it can close upon acceleration. One of the best ways to lock the choke open is to use a piece of wire in the hole where the push pull cable goes, then secure it somewhere on the carburetor.
The float level should measure 7/16”. Measure the float level using a 7/16” drill bit between the airhorn gasket and the top of the outer end of the float just past the inboard float radius.
Edelbrock Carburetors can be used with a draw through blower, such as the positive displacement 6-71 type blowers. Edelbrock carburetors can’t be used on blow through blowers because of the hollow float design and non-sealed throttle shafts. Edelbrock has done some testing with #1405 carburetors on 6-71 blowers and below is a combination that worked well during testing. Two model #1405 carburetors should be used on engines with positive displacement superchargers, such as GMC 6-71 blowers or equivalent. The following calibration changes make an excellent starting point: Primary Jets - .101" (#1429); Secondary Jets - .101" (#1429); Metering Rods - .070" x .042" (#1450); Step-up Piston Springs - 5" (orange, stock); Needle & Seat assemblies - .110" (#1466). This calibration has been tested on engines ranging in size from 350 c.i.d. Chevys to 440 c.i.d. Chryslers with good results.
There is no fuel filter inside the Edelbrock Performer/Thunder series carburetors. Screens are installed into the needle and seat assembly, but this is not a fuel filter, the screens are used to trap large pieces of debris. An external fuel filter must be used with Edelbrock carburetors.
The Edelbrock Performer/Thunder series carburetors have vented float bowls so it’s common to smell some fuel fumes, especially under the hood with the air cleaner off. A couple things can compound the smell of fumes and heat soak could be a big reason. With heat soak the fuel in the float bowls heats up expands and can even boil the fuel in the float bowls. When this happens fuel can get forced up through the primaries and float bowl vents. This can cause an extreme fuel vapor smell after the engine is shut down. To help prevent this Edelbrock recommends the use of a heat insulating spacer to help keep carburetor cooler. Another cause of a heavy fuel smell after the engine is shut down is flooding. With flooding you will normally have a carburetor running problem as well. Here are some things to check to resolve a flooding condition.
- Fuel pump pressure is too high. The fuel pump pressure should be no more than 6.5 p.s.i.
- Take the top of the carburetor off and check for debris in the needle & seat, also inspect the needle tip to be sure it’s not compromised.
- While the float is off, shake the float for any signs of fuel in the float. If there is fuel in the float replace float.
- Check float level, the float level should measure 7/16”. Measure the float level using a 7/16” drill bit between the airhorn gasket and the top of the outer end of the float just past the inboard float radius.
- Another cause of leaking fuel could be heat soak. Heat soak is when the heat from the engine compartment heats up and can boil the fuel in the float bowls. When this happens, the fuel can get forced through the venturi booster and drip into your engine. This can also cause fuel leaks through the airhorn gasket. Add link to heat insulator gaskets.
The Edelbrock Performer series #1406 & #1411 carburetors are calibrated lean for fuel economy and in most cases, will not function properly on Performer RPM & Torker style manifolds. Most people think you can just change the calibration of the carburetor and turn it into one of the Edelbrock performance carbs. This is not the case as the Venturi boosters are also different and Edelbrock does not sell Venturi booster separately. In most cases when using one of our economy style carbs on a Performer RPM or Torker syle intakes will cause hesitations and surging in the lower RPM range.