Finishing the Weber 40 IDTP Carburetor Assembly
In our last lesson, we left you at the accelerator pump circuit and all of your jetting installed. In this lesson, we go from floats to engine ready.
We set the floats with math, you can check them later with the fuel level tool. For us, we have found the floats to be spot on when using math and do not use the bowl method.
This lesson will have you ready for a run test.
There are two types of float available for the Weber 40-46 3C carburetors. The brass type and the plastic or Spansil type.
Spansil® is an NBR expanded Ebanite formula with cells 100% impermeable to liquid. Spansil® floats have been in production for over 50 years. We have had less trouble with the spansil floats than the brass floats.
We showed how to clean the brass floats in the carburetor cleaning video. Catch that here. The Most Important Step
Checking your Floats
You can check the weight on your new floats. Typically they run a little lighter than the brass. They do perform slightly differently, in that they may be a little slower in their movements but we have not experienced any fuel starving from this.
There is a copper seal ring used on the float axles or pivots. We will set the float and wire the pivot screws a little later on. For now, we jump to setting up the top plate. We need the top plate to mount the needle and seat to measure the drop.
Weber 3C Carburetor Top Plate Vent Tubes
We removed the vent tubes in our top plates so they could go to coloration. You cannot mix steel parts in the coloration. Check how we did this here. Teardown a Weber Carburetor. If you need to flatten the top plate, do this before you send it to color.
Cutting tools used on the Top Plate
To replace the vent tubes that we destroyed in removal, we start by cutting a thread into the top plate. We cut a thread with an M8x1 Tap. This solves the problem of these tubes coming loose over time.
We use our CC lathe to make the threaded tube. The tube is already plated in yellow zinc. Use some strong blue loctite to lock them in place.
Setting the Float Height on a Weber Three Barrel Carburetor.
Setting the float height on your carburetor is essential. It is how the fuel will be shut off in the needle and seat valve. We have always used the method of setting the float up and measuring down from the top plate to see where the ball valve is.
The height the float needs to be set at is 12.5mm without a gasket. 12mm with a gasket. That’s why there are two sides to the float tool. After setting the float, we measure down to the tang on the float. Our first measurement is 17.94mm.
Needle and Seat Valve
You can raise the needle and seat by adding a shim under it. We start with a 1mm thick shim and tighten the valve into the top plate.
Measure the needle and seat valve where it comes out of the top plate. Use a depth Vernia and stay square to the top plate. We come up with a measurement of roughly 18.25mm.
That means we need to lower our tang on the float to 18mm to 18.25mm. You do this by gently bending the tang. Remeasure.
Installing The Auxiliary Venturies
Make sure the fuel hole lines up in your auxiliary venturi. One side is a clip and one side is a hole for fuel. In the Weber 46 carburetors, you may find the tall venturi.
The booster needs to be flush with the body of the carburetor. If you had to machine the surface of the carburetor body, you will need to file the venturi to fit flush. Be careful when installing the paper gasket, often they are tight as they shrink. You cannot have a tear in this gasket.
Putting on the Top Plate
Use new Nylock 5mm nuts and 5mm flat washers. Don’t forget to put the return spring purchase on your longer stud. Work the nuts down in a circular pattern, do not over tighten them. The stud cuts into the nylon in the Nylock nut and they are one-time use.
Needle and Seat Sealing Nut
Needle and seat nuts are very prone to damage because they have such a low hex head. We like to replace them as they can be an area of fuel leaks.
The crush washer for the nut can be aluminum or copper, but it is also only used one time.
Air Bypass and Mixture Screws on a Weber Carburetor
Air bypass screws should be gently seated into the carburetor body, then the locking nut screwed down. The air-bypass screws are to adjust for individual airflow at idle speed only. If you leave these turned out you will create a lot of tunability issues. Best to start out with all by-pass screws seated.
Mixture screws have a spring, O-ring cup retainer, and an O-ring. Feel the screw when it seats. Be careful as you seat the small tip of the mixture screw. You need to feel the contact then we set them 2 1/2 turns out.
This setting is a bench set and will be a starting point for the engine run test. Two and a half turns should let the engine start and be a good reference.
Safety Wiring the Weber Carburetor
Using some stainless steel wire now is the time to safety wire the five places on the carburetor. The choke retaining screws and the float pivot screws all wire to the carburetor body.
Create a few turns then thread the wire and finish off the turns with the spinning pliers. Cut off the extra wire and push the tail end against the nut. Be careful as the wire has a way of poking you and it hurts to get stabbed.
Finishing up the Fuel Inlets
Replace the fuel hose and use fuel line ferrules to keep the ends of the hose neat. Fuel pressure in a carburetor is low, 2.5 PSI with a max of 3. The line would stay without anything but the ferrules help lock it all in place and stop the cloth from fraying.
The fiber fuel washers are different sizes so make sure they are the right ones in the right place. Smaller on the outside, larger on the inside of the fuel inlet pipes.
Make sure you face the fuel inlet in the right direction for the engine. Center the washers on the fuel inlets so they seal perfectly.
You set your Floats! What’s Up Next
So I hope you adopt this way of setting your floats from now on!
These Weber 40 IDTP 3C Carburetors are now bench set and ready to run-test. We will talk about how to make the fine adjustments when they are on the engine.
Tuning for altitude and performance are questions we get a lot. Carburetor tuning is a matter of understanding what you are doing. Working through your decisions and only making one change at a time.
It takes time and practice to get good at carburetor tuning. So start out right, but don’t beat yourself up as you learn to tune.