Carb Operation and Cleaning

Carb Operation

I'm not sure how to present this information, so I'll just throw something out there, and let everyone help make it better.

Main Jet / Full and Part Throttle Operation

I expect that everyone knows that the fuel is pulled up into the carburetor venturi area. The fuel travels up from the bowl, through the main jet, into the emulsifier where it mixes with air, and then into the venturi area. How much of the fuel/air mixture is drawn up depends on how much vacuum in created in the venturi area. The amount of vacuum is dependent on the amount of air flow controlled by the carb butterfly.

The air flow path that is provided to the emulsifier is shown below:

Emulsion Tube Air Supply Path 2.jpg

Idle / Tip-In Operation

This operation is a little harder to explain. We'll start with an air and fuel supply path first:

Idle and Tip-In Air and Fuel Supply 2.jpg

When the throttle is closed, a vacuum signal pulls both air and fuel through various passages, through the idle mixture adjustment screw. The amount of air is limited by a brass orifice, and similarly the fuel amount is limited by another brass orifice. The location of the orifices will be shown in the carb cleaning section.

When the carb butterfly is opened slightly (from its idle position to maybe as much as 1/8 open), a number of small holes provice additonal fuel/air. I call this first 1/8 of throttle Tip-In. The following picture shows the Tip-In holes (the two that are side-by-side) and also the idle mixture screw tip:

Idle and Tip-in Orifices in Carb Throat.jpg

Carb Cleaning

OK - first my philosophy. I think its best to blow air or cleaning fluid in the reverse direction of what is normal. Dirt comes in from outside the carb, and comes in a definite direction. If you clean in the forward direction, you could just move the dirt where you don't want it - further in the carb.

In addition, I recommend removing your welch plug on the side of the carb. You can get by without this, but removing it is better. In some of the pictures below, the welch plug is removed, and for others it is installed. I have found that welch plugs from Walbro work fine (WALBRO 188-171-8. 5/16" OD. STENS PART NO. 615-690). I bought some off of ebay - very cheap.

Having said that, let's get cleaning!

Idle / Tip-In Circuits

I am not including disassembly steps, just a method to clean the carb . . . . THE FOLLOWING STEPS ASSUME THAT YOU HAVE THE CARB ALREADY DISASSEMBLED.

Remove the Idle Mixture Screw, and leave in the screw which is just below (and behind) the Idle Speed Screw.

Use your cleaning fluid (I use Brake Kleen) and blow the fluid into the hole for the Idle Mixture Screw. See picture:

Clean through Idle Mixture Screw Hole.jpg

Try to "seal up" the end of your cleaning fluid tube (whatever) so that the force from the can really pushed through the drillings. What drillings? See picture:

Drillings behind welch plug.jpg

Your pushing fluid down behind the welch plug, out the tip-in holes, and out the hole to the left, which is the fuel and air mixture for the idle and tip-in holes.

After you've got this circuit flushed out real well, now put the idle mixture screw back in, and at its normal position. Then force the cleaning fluid in through the air supply for the idle and tip-in circuit. See picture (its the right hole):

Idle and Tip In Step Two.jpg

This action forces the fluid back through the circuit to the idle mixture hole, and the tip-in holes. You should see fluid coming out of these holes inside the carb body. In addition, fluid is being forced down through an orifice that limits the amount of fuel available for the idle and tip-in operation.

Next, remove the screw which provides access to the idle / tip-in orifice. See picture:

Idle and Tip-In Orifice.jpg

You can try forcing cleaning fluid down through the orifice (fluid will come out of the main jet area). See picture:

Fuel Source for Idle and Tip-In Circuit.jpg

This is a very small hole, only about 0.014". I've found it best to run a small drill bit or a small wire down through this hole. Get it good and clean! A plugged or restricted orifice means no idle, and a poor tip-in.

You're done with the Idle and Tip-In Circuit!

Cleaning the idle hole in a Dellorto SHA carburetor.

There is a very small hole below the emulsion hole on the body of a Dellorto SHA . It becomes plugged over time and if it does, your motor will not idle at all. It will just stall at slow speeds. It is very hard to see and i missed it several times while working on a minarelli V-1 . To clean it , pluck a wire from a wire brush and poke the hole. While you are at it poke the 2 emulsion tube holes. Then spray all 3 with carburetor cleaner and blow all of them out. I use a sports needle from a ball inflator that I cut the tip off with a cut off wheel. It now blows out the tip. Here are pictures of the hole from a thread I created. If someone can embed the pictures i would appreciate it .,4080881

Emulsion Tube Air Supply

This one is easy. Force fluid through the hole on the left:

Idle and Tip In Step Two.jpg

Some will notice that I'm going against my philosophy here - but there is not an easy way to force the fluid backwards. When cleaning, you should see the fluid coming out from this area:

Emulsion Tube Air Supply.jpg

Float Bowl Air Vent

Another easy one. Force your fluid through the bottom hole in this picture:

Float Bowl Vent.jpg

Emulsion Tube

Just clean it really good. Clean the holes in the sides (these are air inlet holes), and the holes on the top and bottom.

During a recent cleaning, I found that using a drill bit slightly smaller than the hole works really good. The flutes on the bit work well to clean the sides of the holes.

Emulsion Tube One.jpg

Emulsion Tube Two.jpg

Emulsion Tube Three.jpg

Main Jet

Another simple one - clean it good! The jet hole should be completely round and clean.

Main Jet.jpg

Fuel Inlet

Fuel Inlet.jpg

Force the cleaning fluid out through this hole. The fluid will come out of the fuel inlet for the carb.