by Alex Varv
After re-publishing my article "Tuning a Diaphragm Carburetor" on the Inventions Page, I received several requests to describe how these improvements on the Idle,LO and HI screws were performed.
Below, those interested in this topic, will find step by step illustrations of these modifications.
The modifications below will allow the pilot to tune a membrane carburetor while the engine is running. This process is faster and more accurate than stopping the engine, turning a jet (screw) and re-starting the engine several times.
Before the modifications, I strongly recommend that the carburetor is disassembled, cleaned, dried and new membranes, fulcrum and needle valve are installed. This will ensure that the carburetor will work perfectly thus will allow flawless tuning.
We will modify: the HI Screw, the Idle Screw, and the LO Screw as follows:
The High Screw (HI screw)
All membrane carburetors have a HI screw that has a handle forming a "T" at the end.
In other words, the little handle is symmetrically out on both sides of the screw.
This is a definite disadvantage because it is hard to remember the HI screw setting.
I suggest that the pressure pin is moved to the side so it will become like the hand of a dial.
This way, we will know the exact position of the screw compared to a reference point located on the body of the carburetor.
The pictures below show how this operation was performed:
Note: It is important that we push the pin to the correct side.
In other words, we push the pin in such a way that while the HI screw is 1 and 1/2 turn open, the handle is in an accessible position so we can turn it easily.
I recommend that this position is upwards. See picture to the left.
Idle screw and LO screw:
Before we start the modifications, we will have the widen the slot in both screws.
For this purpose, we use a "dremel" with a separator attached.
This is needed to be able to accommodate the handles that are: a flexible steel cable for the Idle Screw and a thin "L" shaped nail for the Low Screw.
Idle Screw improvement
After having enlarged the slots on both screws, we can start making the flexible handle for the Idle Screw.
For this purpose, we use a stiffer braided steel brake cable that can be purchased at a local motorcycle parts store or a go-cart store.
Our handle, attached to the cable, will be made from a male or female electrical spade connector.
After cutting about 2 inches of this cable, we have to transform it into a small "screwdriver" so it can be soldered into the slot of the Idle Screw -that we already widened with the dremel tool--
Using a hammer, we flatten the end of the cable.
Next, we put some soldering compound on the cable end:
Now we can flatten the cable end one more time with a hammer to achieve a mini "screwdriver"
In order to make a "perfect screwdriver", we trim the edge of the flattened cable:
Drilling a small hole in a piece of wood (board) we will make a support for the screw while soldering:
NOTE: We will have to reinforce this soldering by building more solder around it until we achieve a cone shape. To prevent the solder from melting, thus freeing the previously soldered cable in the slot of the screw, we must use some heat sinking process.
This we can achieve by putting the screw on a piece of bigger metal (in this case a vise-grip)
Note: The heat shrink tubing is only for aesthetic purpose only.
Having this operation finished, the flexible cable will allow us to set the idle even if the steel cable is slightly bent. Also, the flexibility of the steel cable will prevent the handle to break off the small Idle Screw (due to engine vibrations)
Low Screw improvement
For building the Lo screw handle, we can use 2 inch long steel nails like those employed in nail guns. The nail head should be flattened with a hammer.
With the LO screw already having a widened slot, we turn it into the pre-drilled hole on the piece of wood. This way, the nail will be firm and will allow us to work easier while soldering the nail (see picture below)
Next, a generous amount of flux and soldering should be applied until the whole head of the nail (including the slot) is covered.
It is very important that the nail is soldered as perpendicularly to the screw as possible. This will require several attempts and the best way to center the nail is to hold the upper end with a finger while soldering it to the screw. If it needs adjustments, still holding the top end of the nail, we re-heat the soldering while trying to center it.
When the nail is centered, we can add more soldeing compound to build a cone shaped reinforcement.
We use the same "heat sink tool" as above: in our case a vise-grip.
Before we bend the very end of the nail -for a temporary handle only- we may want to cover the "cone" with heat shrink tubing.
This temporary handle is made only to be able to turn the LO screw into the carburetor.
The entry level setting for most of the engines is about 1/2 of a turn.
Holding the temporary handle we turn the LO screw fully in (do not force it past the stop because the carburetor jet may be damaged!!)
Now we turn the LO screw out until we achieve 1/2 of a turn (again, from fully seated position)
We will notice that most probably, the temporary handle on the screw will not be in a very easy to reach position. The picture on the left illustrates the ideal positions of the handles on both HI and LO screw.
Now we will bent the nail again making the final handle pointing upwards.
Next we cut off the first bend (temporary handle) on the nail.
Important note: The LO screw with its handle must stretch out past the HI screw so the LO screw has a good travel (it must be longer than the HI screw)
In the same time, we should pay attention that at the minimum setting (1 and 1/2 of a turn)
the handle of the HI screw is close to the LO screw handle.
In other words, the handle on the HI screw is in such position that it allows us to open the Hi screw past 1 and 1/2 of a turn but we can NOT close it past 1 and 1/2 of a turn because the LO screw will stop it.
If you carefully look at the pictures below all the above statements will be very clear and easy to understand.
Finally, the above configuration of the HI screw will also be a safety feature because until the LO screw is in its place, the HI screw handle will never allow us to close the HI Screw past 1 and 1/2 of a turn.
Having done the above modifications on the Idle Screw, HI screw and LO screw, we will find that tuning our diaphragm carburetor with the engine is running, is very easy and safe.
Alex L. Varv