Weber Carb Tuning


Base line Settings

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Base line Settings

 Speed Screw 1 to 1 1/2 turns

 Mixture Screw 2 turns

Your settings with engine running

 Speed Screw______________

 Mixture Screw______________

It is most important to verify all linkage and levers are installed without binding and the linkage opens to full throttle position and is allowed to close to the Idle Speed Screw. This is the number one and two reasons for tuning errors, improper linkage installations and over tightened linkage nut, causing binding in the linkage assembly.




“LEAN BEST IDLE” Procedures

After confirming the linkage allows the throttle to seat against the Idle Speed Screw. Back off the Idle Speed Screw until it doesn’t touch the throttle lever. Turn the Speed Screw in until it contacts the throttle lever and turn it in 1 ½ turns. Repeat this with the other carburetor. Turn the Mixture Screw in until it “LIGHTLY” seats, then back out the screw 2 full turns.

a.      Start the engine, it will run slow and like a tractor. As long as it will stay running, the idle speed is not important at this point.

b.      First, turn in each mixture screw until the engine runs worse, then back out the screw ¼ turn at a time. The engine should start to smooth out. Continue to back each screw ¼ turn at a time until the screw does nothing or runs worse. Then turn it back in to the point where it ran best. You want to tune the engine by sound. Adjust each mixture screw to the best, fastest and smoothest running point. Do this procedure with each mixture screw.

c.      Now you may adjust the Idle Speed Screw. It should be sensitive and only “require” ½ turn out to lower the idle speed. The Speed Screw should never be in more than 1 ½ turns in.

d.      These carbs are commonly used in pairs, this makes the synchronization important, be sure to bring the high flowing carb down to the low flow carb. Then bring them both up to “proper” Idle speed. The Idle Speed Screws are never opened more than 1½ turn in maximum.

e.      After synchronizing multiple carbs, reconfirm steps b. c. & d.

Pump By-pass Valve:

The pump by-pass valve is designed to allow fuel into the pump circuit and when depressed,  by-pass a percentage of the fuel delivered to the accelerator pump nozzle/jets. (by-pass through a hole on the side of the valve back into the fuel bowl). We have included the zero by-pass valve. (no hole in the side) This will deliver all of the available fuel to the engine and not by-pass back into the fuel bowl. This will increase the duration and fuel volume of the pump shot with the original pump nozzle/jets. The accelerator pump by-pass valve is located in the bottom of the fuel bowl. One is required per carburetor.

Simple Rules for low speed calibration

If the mixture screw is more than 2 1/2  turns out turns then the Idle jet is too lean (too Small). When the mixture screw is less than 1 1/2  then the Idle jet is too rich (too large). These assumptions are based on the fact that the speed screw setting is not opened more than 1 1/2 turns. If the speed screw has to be opened 2 or more turns then this is also an indication of a lean condition usually requiring greater change. At times it may appear to be showing signs of richness or flooding it is really a lean condition. See pictures and notes in the tech 2 article supplied in the kit instructions, view and please understand the need to keep throttle plate as near to closed as possible so as not to prematurely expose the transition holes. This is what causes the visible rich condition, and confirms the need to increase the jet size. JET KITS are available if needed.

EXAMPLE      With the speed screw set at no more than one half (1 1/2) turns in after contact with the stop lever; and the best idle occurring with the mixture screw set at  3 turns from bottom, indicates the need for a larger Idle jet. Achieving the best idle at under 2 turns indicates the need for a smaller idle jet.

The secret  to understanding the critical nature of the carburetor set up and the advantages of a WEBER over other carburetors is the Idle circuit. Referred to as the low speed circuit by Weber, this circuit is responsible for 80% of the driving operation. This is the reason that the Weber should give a fuel economy improvement over most factory carbs along with significant  performance gains. In the worst case you should not see a significant fuel economy loss over stock, while improving HP & Drivability.

The Weber Carburetor is a sequentially timed device to the motor like the distributor. Time taken in the setup will provide more fun later.