Changing or Cleaning Main Jetting on Webers Made Easy!

Howdy all
Thought I would add a corallary to Birdman's now epochal thread on carb tuning - how to change and/or clean your Weber main jetting in 40 minutes.

1. Take off air cover lid and remove air filter. Put them where they won't fall down or pieces get lost.

2. You're looking at the carb intake air horns or trumpets, one for each barrell. Next remove the sixteen (16) 8 mm self locking nuts holding the 4 air horn on and remove. Sears makes a cool ratcheting wrench for this.

THE #1 RULE OF WORKING WITH WEBER CARBS IN SITU: EACH OF THESE NUTS, AS MANY OTHERS, SHOULD BE REGARDED AS A LIVING COGNIZANT BEING THAT DOES NOT WANT TO BE REMOVED. EACH WILL WAIT UNTIL YOU ARE NOT CAREFUL, THEN DO IT'S BEST TO JUMP FROM YOUR HAND DOWN INTO THE PERCIEVED SAFETY OF THE CARBURETOR THROAT. DO NOT EVER ASSUME THEY WILL NOT TRY TO DO THIS, AND TAKE PRECAUTIONS. IF THEY DO OUTFOX YOU, DO NOT OPEN THE THROTTLES OR THEY WILL FALL DOWN INTO THE CYLINDER. DUE TO THE GRACE OF A MERCIFUL ENGINEER, THESE BASTARDS ARE MAGNETIC, AND CAN BE RETRIEVED WITH A COMMONLY AVAILABLE MAGNETIC PARTS PICKER-UPPER FROM THE TOP OF THE BUTTERFLY PLATE. Ask me how I know.

#3. In a small well at the part of the carb facing the front car, you'll find the brass looking main jetting looking up at you with a slot to unscrew it using a short fat screwdriver. Gently unscrew each until loose, then remove, best done using a curved set of needle nose pliers that you bought at the dollar store, you cheapskate.
IMPORTANT: THE #1 RULE APPLIES HERE ALSO

#4 Take the jets to your desk and place them on a soft cloth. Each is composed of three pieces (see brilliant drawing attached), and is press fit onto the other in a way where they will only go on one way:

The 'top' part with the screw slot is the air corrector jet. It modifies the mixture strength to lean or rich in the high end of the rev band at power, usually from 6000 rpm on up. Smaller numbers (e.g. 180) give richer mixtures, while larger numbers (e.g. 210) give leaner high end mixtures

The middle part is the mystical 'Emulsion Tube'. This is where air from the air corrector jet mixes with gas from the main jet before going to the carb throat and magic happens. They sit in the a well of fuel that goes into the carb throat. I've included in the photo two common ETs (call them this if you want to be percieved as 'cool'), the F24 used on earlier Dinos (6s & 8s) and the F36 used in the later cars. Each is shown in pairs turned 90 degrees so you can see all of the hole pattern. Not only are the hole patterns different, but notice the diameters are different as well, regulating the amount and flow of gas in the well. It is a combination of these characteristics that give a Weber much of it's response 'personality'. It's a WAY more complicated than this, but let's just say an F24 is more racy than an F36. OBTW - do not assume there is an understandable nomenclature for ETs - there isn't.

The small nub at the bottom is the main jet, it regulates the flow from the float chamber fuel resevoir into the ET. They are also stamped with their size. Larger (e.g. 145) is richer while smaller (e.g. 125) is leaner. VERY important to keep this clean. One trick I have is if the main jet fits too loose to be snug in the ET, then just ever so slightly squeeze and tighten a wee little bit the bottom of the ET before putting in the jet to make it a tighter fit. It's important that it fit snugly so it doesn't fall out when removed or installed.

#5. Using the carb cleaner of choice jently soak and clean Compressed air is preferable for cleaning the main jets.

RULE #2 DO NOT USE A METAL OBJECT TO CLEAN OUT THE MAIN JETS. THEY ARE SOFT AND VERY SENSITIVE AND YOU MAY INADVERTANTLY MAKE THEM BIGGER.

6. Once you've checked, cleaned the pieces and/or changed them, reinstall them in their wells and tighten just finger snug, NOT HARD TIGHT, in their place.
AGAIN REVIEW THE #1 RULE

7. Now re-install the carb air horns
AGAIN REVIEW THE #1 RULE

Put the filter and lid on and you're ready to go. See! All done soup to nuts in less than an hour! Wasn't that easy? That's one reason why Webers are favorites of racers - very quick adjustmants are possible.

Hope this helps - corrections or other suggestions welcome!
rt
Image Unavailable, Please Login
Image Unavailable, Please Login
Image Unavailable, Please Login

 

Chris
Yes - they are a puzzlement. It's a minor miracle mine have not come off in a bad place. Only advice I have is to check the barrels again.

While you do not have to take off the airbox to do the jet change I describe here, the airbox should come off for about everything else including synchronization, idle jet change, accell jet change, venturi change. setting the floats. In the second picture in post #1 you can see the tops of the small metallic bushing/spacers around the mounting studs for the air horn. They are flanged on the LOWER side next to the carb, so they only can come out going down.

If you haven't seen these before, after you disconnect the intake vent and oil breather hose and anything else, lift the air box up a little and make sure before the studs clear that either
a. all of the spacers are still in the rubber gaskets, or
b. they have come all the way our and will remain on the studs

My rubber gaskets are still pretty pliable and hold the spacers pretty well. If it was very problematic, I might dab a bit of that two part epoxy glue (not super glue, or cyanoacrylate) around them before inserting and let it dry overnight.

As to where to look when they fall, well... you know as much as I. Would just rule out the obviously bad places.

As a trivia note, if you install separate air cleaners or air horns for each carb (see photo below of separate airboxes and chrome meshed trumpets), do not thow these rubber gaskets away - you will need them to space the new airboxes over the reach of the studs, so they will need to be removed from the old airbox. Fortunately, they are easy to remove and re-install. Regular tuning and maint. is much much easier with the separate filterboxes as everything is out in the open, but they do add significant sound - a great sound, when you're in the mood.

best
rt
Image Unavailable, Please Login
Image Unavailable, Please Login

 

Bump

Just in case anyone wants to re-jet while they are changing Venturis

 

Most individual filters come with a little doofer to attach the crankcase breather to one of the individual filters.
Great question.

 

Holy crap!!!!!

What a nightmare scenario!!!