Measuring carburetor circuits with a pin gauge.

Measuring carburetor circuits with a pin gauge.

June 29, 2020 0 By Ray Bohacz

Collecting, maintaining, and running antique iron often rewards a newcomer with a level of success that can bewilder the seasoned tractor man or woman. 

Call it beginners’ luck or the quiet before the storm. Those with busted knuckles and gray hair can attest that these wonderful machines can sometimes get the better of us. 

Thus, I have coined the phrase, “One tractor few problems; many tractors many problems.” 

As your exposure to machinery grows, you will be faced with a mechanical issue that seems to have no answer. 

In the same fashion that a rotary hoe is a rescue implement to get a seed to emerge through crusted soil, this can be considered a rescue primer. 

Much like the rotary hoe, the theory represented here is not something you will use every day, and hopefully, you will never need it, but when you do, you will be glad that you have it. 

The scenario

With exceedingly rare exception, any tractor you get will have had a multitude of owners that employed it for just as many uses. 

It left the factory and started life as an essential member of a farm, and as time went on, it changed hands, and its relevance declined. 

As it transitioned from a working machine to a collector item, that journey was often met with a lack of financial commitment by its owners. 

During the less than glorious time in the tractor’s existence, if a part failed or was thought to be problematic, it was swapped, often with a used lookalike from a myriad of sources. 

Nowhere on a farm tractor was this sin perpetrated more than with the carburetor. If the engine has an issue, swapping out the carburetor became the magic elixir for all ills. 

The problem being this often did not fix the issue due to:

  1. The carburetor was not the cause.
  2. The carburetor was the culprit, but the one that was swapped looked the same on the exterior, but the passages, jets, and bleeds inside were of a different dimension.

It now caused a dilemma since the tractor still did not run correctly. The owner does not want to put any more money or time into it. Guess what? The “for sale” sign goes up, and you bring it home, giddy with the thrill of ownership. Now you unknowingly are faced with the problem.

Where the problem may lie

It must be recognized that the manufacturer will develop a model line such as the Zenith TSX or Model 62, and then modifies that family to fit the requirements of the tractor manufacturer. 

These changes may include a larger throttle plate, linkage design, or, more commonly, alter the dimensions of the internal passages. 

These would include every circuit and are not limited to the needle and seat, idle and power circuits, air bleeds, emulsion holes, and jet size. 

The challenge being the dimensions that are identified as the carburetor calibration are not able to be determined with the naked eye, a tool is required to measure them accurately. That tool is a pin gauge. It can be considered a feeler gauge for round holes and is just as easy to use.  

A carburetor works on the pressure differential in the venturi versus that of atmospheric pressure on the gasoline in the float bowl.

Via this difference, gasoline and air is introduced in the atomized form to create the desired air/fuel ratio for the engine to operate under varied loads and speeds. 

The carburetor designer considers the needs of the engine as mathematical computations based on the specific gravity of standard gasoline.

They then alter the physical dimensions of the internal carburetor orifices to provide the proper amount of fuel or, in the case of the emulsion circuits, air to mix with the fuel. 

If the carburetor installed on the engine has the improper internal circuit dimensions, the engine will never run properly regardless of what you do.

Depending on where in the carburetor the sizing error is, will determine the effect. 

If a carburetor that has too small a power enrichment and idle circuit, it is possible that you can richen the air/fuel enough through the mixture screw to achieve a perfect idle. But when the engine is put under load, it may buck and or be lazy, lacking power. 

The amount of fuel in mass that is required to idle a small or large engine is not that different since, under that condition, little power is being produced. 

That is why the smaller (leaner) idle circuit may function. 

Usually, a telltale sign of an issue is the orientation in turns from the seated position of the mixture screw.

Most carburetor’s idle circuits are designed to create the proper mixture strength with the mixture screw between two and three complete revolutions from being seated.

If you need to have the mixture screw out five turns or only one-half turn, that is a good indicator that the passage sizes are not correct and that the carburetor, though visually the same, is from a different application. 

A good rule is if you do not know the tractor’s complete history and with routine cleaning and servicing of the carburetor, you cannot tune out the problem, it is time to start to look inside. 

The caveat that I attach to this is you must be confident that all other aspects of the engine are correct such as the ignition, compression, and the lack of any vacuum leaks.

The right tool for the job

When shopping for this tool, you need to decide the dimensions that you will be checking and the increments in the size you require. 

My set has gauges ranging from 0.011 inch up to 0.060 inch in 0.001-inch increments. 

This is a good range for a carburetor along with other parts. 

A high-quality gauge set can be ordered on the high or low side of the tolerance. You can also get pin gauges that are certified to the exact dimension.

That is a level of accuracy that is not required in a mechanics shop. It is an unnecessary expense. 

A low-cost gauge set will not offer a low or high tolerance option when purchasing it. The set I own is of high-side tolerance and costs only $54.00. 

The pin gauge set is straightforward and easy to use, similar to a feeler gauge.

Start with the pin you believe will be close to the proper size and then go either up or down in dimension until you find the one with a good deal of even drag when you move it in the hole.

That is the dimension of the carburetor orifice. There is nothing more to it!

Where the problem lies

Measuring hole size is quite easy; determining what the proper size should be is not. 

Sadly, most tractor shop manuals do not list circuit dimensions.

Their mindset is that you are working on the proper part from the factory and not trying to determine if the carburetor is the appropriate calibration. 

All you need to do is apply some investigative processes with the following protocols.

As fuel passages are increased in dimension, more fuel is supplied, and the mixture is richened throughout the engine operating range. 

In contrast, an air bleed will serve two functions: it shapes the fuel curve and alters the mixture. 

A larger air bleed size will lean the mixture and, in most carburetors, delay the enrichment of the fuel curve.

It will be leaner at lower speeds and then trend richer at high speeds. 

To apply a practical application to this, assume that you have two of the same model carburetors. 

Take them apart and, with the help of a shop manual, identify the different orifices, measure them, and record your findings on a note pad.

Then do the same with the other carburetor that you have or are considering buying. 

Now you have quantifiable data as to the similarity or differences in the two carburetors. 

If you need to and the orifice is too small, use a drill chart to determine the new and sharp bit you need to achieve the desired dimension. 

If possible, mount the carburetor on a drill press in an orientation that allows the metal chips to fall out and drill it to size, confirming the dimension with your pin gauge. 

Clean out all the debris, and you have just made the carburetor into what it needs to be for your engine.

That is what hot rodders did back in the day.

Many of them were young farm boys with a good dose of old-fashioned American ingenuity!