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Weber Carburetor Triple-Barrel Tuning

Italian contribution

Tuning the Weber triple-barrel carburetor might seem intimidating, but it really is not that difficult. However, time and patience are required to make sure the carburetors are properly tuned. Time is well spent for the do-it-yourself mechanic because your local Porsche mechanic may not be a whiz at tuning Weber carburetors either, and he gets paid by the hour. $$

Before attempting to tune the carburetors, make sure your engine is in good working order with valves adjusted, distributor timing correct, clean spark plugs, clean air/fuel filters, and throttle linkage in proper repair. Even consider rebuilding the carburetors to ensure they are good working condition.

Disconnect the carburetor fuel lines and remove the carburetor air cleaners
Make sure the throttle plates are exactly vertical when full throttle is commanded with the accelerator pedal. They throttle plates should not go past vertical. The accelerator pedal stop (behind the accelerator pedal) is the full throttle stop for the carburetors.
Check that the throttle linkage is assembled completely, and then unsnap the throttle linkage ball joint rod end at each carburetor throttle shaft-actuating arm.
On both carburetors, turn the idle speed screws out until they no longer touch the throttle shaft actuating arms.
Turn the idle speed screws back in until they just touch the throttle shaft actuating arms, then screw them in about half a turn more (This is simply a starting point for the idle speed adjustment).
Adjust the throttle linkage ball joint rod’s length such that they snap onto the throttle shaft actuating arms without changing the arm position and, hence, they won’t interfere with throttle shaft position.
Loosen the air correction screw lock nuts and carefully close the air correction screws completely, then lock the screws with the nut.

tuning adjustments

Carefully close the idle mixture screws, and then back them out two complete turns as a starting point for adjustment.
Re-attach carburetor fuel lines.
Start the engine and warm it up to at least 140 degrees.
Unlock the rod end from the right hand carburetor.
Evenly adjust both idle screws until the cockpit tachometer reads 1100 RPM.
Check the balance between each carburetor by using the uni-syn tool or the Weber Synchrometer STE SK tool (at right). If necessary, adjust one of the idle screws to even the vacuum between carburetors.
Adjust each cylinder by turning the corresponding idle mixture screw in until the engine RPM drops, then turn it out until it runs smoothly again. Listen carefully for a change in RPM and smoothness of operation. As the idle mixture screw is turned in, the mixture is leaned to the point of idle air/fuel starvation; that is why the engine will run roughly. Then, when the idle mixture screw is opened again, idle air/fuel flows into the intake allowing the cylinder to fire properly and the engine will run smoothly.
Use the uni-syn tool or the Weber Synchrometer STE SK tool to check the three carburetor throats in each carburetor for vacuum draw. Put the tool into the air horn for each throttle body throat and find the carburetor throat that has the highest vacuum draw (for each carburetor). Leave that air correction screw alone. The air correction screw should be completely closed and locked with the nut. That means one carburetor throat out of three in each carburetor will not be adjusted.
Adjust the air correction screws on the remaining two throttle bodies (for each carburetor) until all three carburetor throats have equal vacuum draw within each separate carburetor. Make sure to lock the air correction screws with the nut.
Return to the idle mixture screws and adjust them again as before. Listen carefully to the engine RPM and operating smoothness. Turn the idle mixture screw in until hearing the RPM drop, then back out the screw enough to cause RPM increase and smooth operation. Turn the idle mixture screw out an additional half turn for the final setting.
Evenly back the idle speed screws out to attain 850-950 RPM.
Check the balance again.
Adjust the right hand ball joint rod end that attaches to the throttle shaft actuating arm so it will snap on without disturbing engine RPM and attach it.
Check balance at 3000 RPM.
Install the carburetor air cleaners.

After finishing Weber triple-barrel carburetor tuning, congratulate yourself on a job well done. Take the savings from not paying a mechanic and purchase a tank of gas for some real fun…drive your car!

Please reference a proper maintenance manual when performing work on an automobile, especially your pride and joy early Porsche 911.

Accelerating Pump Injection Quantity

Accomplish the accelerating pump injection quantity check before adjusting the carburetors, if possible.

With the engine ignition off, check accelerating pump injection quantity using a small, narrow measuring vile.

Be sure to start and run the engine in between each check (six pump checks) in order to clear fuel out of the carburetors.
Wrap a wire around the vile to hold the vile while hanging it under the accelerating pump jet.
Actuate the throttle and catch the gas as it pumps out of the accelerating pump jet.
Approximately .8cc of fuel should be in the vile after actuating full throttle.
Adjust the injection quantity using the nut at the end of the accelerating pump threaded shaft actuation rod.

Please reference a proper maintenance manual when performing work on an automobile, especially your pride and joy early Porsche 911. Don't forget to purchase the coolest Weber triple-barrel t-shirt available at right. >>>>>

Weber triple-barrel >> carburetor theory >> carburetor glossary >> carburetor tuning >> jetting chart