Rebuild each side at a time, in case you forget where something goes.
Performance Gain:
Cleaner running engine, better throttle response
Complementary Modification:
Install jets optimized for your car
This article is one in a series that have been released in conjunction with Wayne's book, 101 Projects for Your Porsche 911. The book contains 240 pages of full color projects detailing everything from performance mods to changing your brake pads. With more than 650+ full-color glossy photos accompanying extensive step-by-step procedures, this book is required reading in any Porsche 911 owner's collection. See The Official Book Website for more details.
Check out some other sample projects from the book:
The early 911s were equipped with Zenith, float-less Solex, or Weber triple choke 40 IDA 3C carburetors as stock equipment. The design of the carburetor is relatively simple: as air flows down the center venturis, gasoline vapors from two fuel bowls mix with the air and feed the engine. Accelerator pumps provide an extra kick when first stepping on the pedal, and idle screws allow air to bypass the throttle valves when the car is at idle.
The Weber triple barrel carburetors are the most popular for installation on a late-model 911. This project will cover the rebuilding of the Weber carburetors in detail. The procedure is similar for the Solex and Zenith carburetors and differs mostly in the method that the floats are set.
The key to maintaining good functioning carburetors is keeping them clean. There are many very small passages within the carburetor that can become clogged from dirty or old fuel, and prevent the car from running properly. Typical carburetor problems include backfiring, flat spots, missing, lack of power, and general uneven performance. You can usually figure out that your carburetors need rebuilding when they become insensitive to adjustments that are made when attempting to tune and balance them (see Pelican Technical Article: "Carburetor Adjustment, Balance, and Tuning").
The first step is to remove the two carburetors from your 911. They should be mounted to each intake manifold by six studs on each side. Be careful when removing the fuel lines that are attached, as there will most likely be fuel in the lines. Likewise, when you remove the carburetors from the car keep in mind that the fuel bowls might still be full of gasoline which can easily spill out.
When you get the carbs on your bench, you should carefully inspect them to make sure that they are not worn beyond repair. Very often the throttle valve bushings become worn, and the valves themselves begin to wear a groove in the throttle body. If the throttle shaft is loose in its bearings, then you should probably take the housing to a professional rebuilder. They will need to drill out and remove the old bushings, then install and ream new ones in their place. The alignment and tolerances of this procedure are critical, and should probably be left to a professional. If the shafts are loose and you dont replace the bushings, then the throttle valves will most likely permanently damage the throttle body in the very near future.
The task of rebuilding is very straightforward. The entire process is simply a teardown and reassembly with new gaskets. It is recommended that you rebuild each side one at a time, so that you have a reference to check in case you forget where a particular part is installed.
Carefully remove all the parts and pieces of the carburetor, and clean them in parts cleaner. It is not necessary to remove the throttle valves and the linkage from the bottom of the carburetor housing. These parts can be cleaned in place. Make sure that you flow carb cleaner through every passage and area of the carburetor. When finished, blow compressed air through all the passages. If you dont have an air compressor, use one of those compressed cans of air that are designed to blow out dust from computer equipment. Its also important to note that you should always use eye and hand protection when working with any solvents or cleaners.
Some of the fittings are made out of brass, and extra care should be taken when removing them. Brass is a soft metal, and the fittings may easily strip out. If any of them seem very difficult to remove, try soaking them overnight in WD-40.
When you reassemble the carbs, make sure that you keep track of every part and gasket that you use. With so many small tiny parts, its very easy to forget an important item. After installing the floats, check the height according to the procedure outlined in the photos.
A properly rebuilt and adjusted carburetor should deliver optimum performance, and should be able to be adjusted quite easily when tuning the motor.
Figure 1
Disassembly begins with the removal of the intake funnels (sometimes called velocity stacks), and the upper mounting plate for the air filter. Removing the small locking hex nuts will now allow the top cover assembly to be removed from the carburetor body.
Remove the air correction screw in order to gain access to the channels and passages within the carburetor body. Make sure that you spray plenty of carb cleaner and compressed air down all the passages to clean them out. The cleanliness of these passages and chambers are directly related to the ability to tune and balance the carburetor.
Its not wise to disassemble the bottom part of the carburetor that contains the throttle valves. If the bushings are heavily worn, then additional more expensive rebuilding will be required. You can tell that the bushings are worn if the throttle valves stick in the throttle bodies. Very often when the shaft bushings wear, the throttle valves will actually dig a groove into the body of the carburetor. If this is the case, the carburetor body is worn beyond repair and should be scrapped.
The venturis are held in by a set screw that secures them within the inner housing. They can be gently coaxed out of the main carburetor body after removing this screw, and then cleaned. Make sure that you are careful to keep track of all the small parts that you remove from the carburetor. Many of these parts are very difficult to find these days, and can only be swiped out of older, junked carburetors.
Place the main carburetor body into a parts cleaner, and force cleaning solution into all the passages in the carburetor body. Its also wise to let the housing sit for a while in the cleaning solution. Years of dirty fuel and deposits need to be dissolved and swept out of the internal passages.
A cotton swab dipped in carb cleaner makes an excellent tool for cleaning hard to reach places. Make sure that you rinse each part under fresh carb cleaner before letting them dry. It is important not to get cotton fibers stuck in the small passageways of the carburetor.
The complete carb kit contains all the gaskets, sealing rings, and floats that you need to completely rebuild one carburetor. In many cases, the carburetor kits are universal across different sizes of carburetors, so dont be worried if you have a few extra gaskets left over when youre finished. When installing the paper gaskets, make sure that you match up the proper sized ones for your size carburetor.
An important step to perform is the adjustment of the float height. Wedge a paper towel in-between the float and the housing to elevate it off the bottom of the housing. Using a straight edge, measure the height of the float tab to the carburetor upper face, and adjust it until the measurement is 18mm. If you happen to have one, you can also use a specialty float gauge to measure the float height.
With the float tab exactly 18mm from the carburetor upper face, the height of the float should be between 12.5mm and 13mm from the upper face. Do not measure the height from the weld seam of the float, instead use the top surface of the float. If this measurement is incorrect, then bend the float tab slightly until you obtain the required measurements. Additionally, check the height of the needle valve. When installed with its sealing washer, the needle valve should hang down exactly 18mm from the top of the carburetor housing.
The accelerator pump assembly can be a little tricky because it the parts are spring loaded and compressed. Make sure that you dont damage the new rubber diaphragm when you install it. Also make sure that you install all the parts correctly and in the right orientation. Its quite easy to get confused here.
The completed and rebuilt carburetor. Place a little lithium grease on the linkage assembly to reduce the wear in this area, and make sure that you use a new air cleaner when you reinstall them in the car. Make sure that you install a new fuel filter in your car as well. You do not want dirt to get inside your newly rebuilt carburetors.
Comments: I have a 40 IDF on my high performance Harley; bored, stroke, cam, ported, etc., home made manifold. I need a three choke Weber In useable condition so I can make a manifold to mount to the Harley ports. I have the expertise and tooling to accomplish this. Let me know if I can buy one for $300-400, I'll work out the venturi and jetting details later.
April 16, 2012
Check out some other sample projects from the book:
