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Dial-A-Jet: Superfluous or Virtuous – Phil Mickelson

-Phil Mickelson, Tech Professor

In the August issue of Snow Goer, I wrote a column discussing fuel control systems that adjusted to varying atmospheric conditions and altitudes,  That column prompted a number of letters from readers asking about an aftermarket system known as Dial-A-Jet.  I had heard about the product, mostly in reference to motorcycles, but had never worked with the product myself.

Very basically, Dial-A-Jet is an additional fuel delivery circuit that can be added to any carburetor.  It is similar to, but different than, a power jet system.  The Dial-A-Jet system includes an adjustable air metering orifice and a mixing chamber.

Realize that no add-on to a carburetor is going to be able to increase peak horsepower provided the carburetor is properly calibrated.  Larger bore carbs or throttle bodies for injectors that flow more air can increase peak power, and systems dumping in additional chemistry such as nitrous oxide certainly can move the peak up, but additional circuits added to an existing carburetor won’t move the peak power.  What additional circuits like the Dial-A-Jet can do is allow for more accurate calibration throughout the powerband of the engine and in the case of Dial-A-Jet, provide a simple “in the field” mixture adjustment.

The Dial-A-Jet system requires drilling and tapping the main jet access plugs at the bottom of the float bowls and installing adapter fittings for the fuel lines that will supply the Dial-A-Jet bodies.  Fuel delivery from this lowest point in the carburetor float bowls means that the carbs will be constantly purged of any water/alcohol accumulation.

The Dial-A-Jet has a plastic body with a connector for the fuel supply line that leads to a small mixing chamber.  Air is metered through an adjustable dial and added to the mixing chamber.  There are five different sized orifices in the dial that allow for mixture adjustment in the circuit.  What amounts to an emulsion tube in the chamber delivers the fuel-air mixture from the device to the carburetor bore.  The Dial-A-Jet body mounts on the carburetor inlet flange after a hole has been drilled to accept the emulsion tube.  In some cases, the Dial-A-Jet body must be mounted on top of the air silencer connector boots or the boots must be trimmed to allow clearance for the Dial-A-Jet body.

Once installed, main jetting on the stock carbs will usually have to be decreased three jet sizes; two if you’ve jetted closely for your local conditions.  Because the added fuel delivery circuit add fuel from mid-range to top-end, pilot and needle jetting won’t require much, if any, changing.  My brief testing with Dial-A-Jet showed that this system is effective over a far greater power range than the standard power jet systems.  Probably because an emulsion rather than pure gasoline is delivered by the circuit, lower venturi velocities will allow discharge from the Dial-A-Jet circuit.  On the engine I ran, dropping the needle two positions was required and indicated the system was delivering additional fuel well down in the mid-range.  According to the manufacturer, the Dial-A-Jet circuit will deliver 10 to 14 percent of the fuel delivered to the engine when the circuit is functioning.

Checking the spark plug color as an indication of the mixture condition is not as easy as some suggest.  If you really know where to look for the color change – if you’re working with new plugs and if you run the sled up properly – you can achieve accurate jetting results be reading the plug color.  Exhaust gas temperature gauges make the task of jetting much easier.  Generally you want to achieve 1,100 to 1,200 degrees F at each throttle position or the highest safe exhaust gas temperature allowed by the engine manufacturer.  With the Dial-A-Jet circuit installed, flatlanders would establish jetting that would be correct for the conditions with the dial in the middle position, allowing two richer and two leaner positions for the adjuster dial.  On a warm, humid day with low barometric readings, dial the adaptor to one of the leaner positions.  A cold day with high barometric pressure could be adjusted more easily by dialing to a richer setting.  For the drag racer or tuner looking for an easy, fine tuning system, the Dial-A-Jet is a pretty slick system!

Dial-A-Jet offers an interesting feature for those operating at varying altitudes.  With the device installed, jetting would be optimized at the lowest altitude the machine would be operated at when the adjuster dial is set at its richest position.  Fuel delivery can then be reduced as you go up in altitude by dialing to the leaner positions.  In talking with those installing Dial-A-Jet systems in the western United States they claim one position change on the adjuster dial will correct jetting through about 2,000 feet of altitude change.  The system is not automatic, but those operating in the mountains, correcting jetting becomes almost as easy as opening the hood on your machine!

Because the Dial-A-Jet system maters air through its own inlet, the circuit will be influenced in itself by air density.  The manufacturer suggests the circuit will compensate for changes of approximately 3 percent in air density.  The little time I spent playing with Dial-A-Jet proved its ability to allow for quick fine tuning and I think its final worth to me will be found in the improvement of mid-range operation of the engine.

If you want more information about this product and you local dealer doesn’t  handle Dial-A-Jet, write:  Thunder Products, Inc., 21676 Deep Lake Road, Richmond, MN 56368 or call 320-597-2700.