Carburetor tuning on street bikes used to be simple. But with multiple cylinders, tighter packaging and more carb complexity, jetting a set of carburetors for optimum performance can be difficult, confusing and frustrating. Intimidation, both legal and technological, causes many owners to live with from-the-factory jetting, and since aftermarket exhaust systems generally require mixture changes, the potential horsepower gains that could have been realized with correct jetting go right out the window. Dial-A-Jet has introduced a unique adjustable jetting system that works in harmony with stock carburetors, stock or aftermarket exhaust and the machine’s stock airbox.
The Dial-A-Jet is simple in design. The heart of the unit is a small mixing chamber that attaches to the air boot behind each carburetor. Fuel supply to the chamber comes from the float bowl; specially machined fittings replace the stock float-bowl drain bolts, and a fuel line connects the fittings to the mixing chamber. Each chamber has a ratchet-type dial with five mixture settings and is held against its air boot by straps that replace the stock metal clamps on the rear bell-mouth of the carb.
Fuel from the float bowl is carried to each mixing chamber via tubing. Once in the chamber, the gas is mixed with air to form an emulsion that’s then introduced through each delivery tube, through each air boot and into the center of the intake air-stream where the air is moving quickest. This emulsion can be carried easily into the intake tract by the higher-velocity air, especially at throttle settings above half open.
The Vance & Hines dyno provided some initial figures. Mounted on a box-stock GSX-R750 Suzuki, the Dial-A-Jet kit produced impressive numbers. The stock GSX-R without the kit made 84.1 horsepower at 9500 rpm. With the Dial-A-Jet installed, the Suzuki made 87.8 horsepower at 10,500 rpm, a difference of 3.7 horses. Peak torque also increased by 1.7 foot/pounds. At 10,500 rpm, the point at which the stock motor’s power drops significantly, the difference between the kit and box-stock machine was 14 horsepower. Suzuki’s GSX-R1100 was next on the agenda, and the numbers were equally impressive. The box-stocker made 12.6 bhp at 10,000 rpm. With a Vance & Hines pipe and a two-step-richer main jet, the optimum jet for power production with that particular pipe, the 1100 produced 118.2 bhp. The Dial-A-Jet was then installed, leaving the V & H header in place. This combination netted 123.7 bhp at 9500 rpm, a jump of 11.1 bhp over the box-stock machine. The testing impressed Vance & Hines enough to become distributors of the system.
We then installed the Dial-A-Jet on two other test machines, Yamaha’s FZ750 and Honda’s Nighthawk S. Installing the kit was relatively simple. Installation on our FZ was easy due to its carburetor placement. The cramped quarters of a normal in-line four engine takes more time, however. Installation time ranged from 45 minutes on the FZ to an hour and a half on out Nighthawk S. To install the kit, you need a flat-blade and a screwdriver and wire cutters, along with the supplied cutting tool that cuts a small hole in the air boot.
The Yamaha motor is phenomenal in stock form, and we weren’t too surprised to find that the FZ with the Dial-A-Jet ran identical times to the stock version. The FZ’s rideability, however, was improved with the Dial-A-Jet. Midrange power seemed stronger, and the transition from midrange to top-end was smoother and less abrupt. The Nighthawk S also benefited from the kit. The slight midrange flat spot nearly disappeared, and top-end power was stronger from a seat-of-the-pants point of view. Fuel consumption did increase slightly with the system, however. Without the Dial-A-Jet, our Nighthawk averaged 42.7 mpg. With the kit installed, the mileage numbers dropped to 40.3 mpg.
The Dial-A-Jet system does offer advantages. First, the dyno numbers don’t lie, and even though the horsepower gains are near the top of the rev band, the increases themselves are significant. Secondly, the kit offers easy adjustability, a trait that’s especially important for those who ride in varying altitudes. The dial, which is located on the face of the mixing chamber, can be turned with a screwdriver to any one of five positions. Turning the dial counterclockwise results in a leaner mixture. A clockwise turn richens the mixture. At higher elevations, the leaner settings worked best. Nearer to sea level, the richer settings were optimum.
Certain machines will benefit more from the kit than others, especially in terms of sheer performance, but the rideability of all machines we sampled improved, and our riders felt the system was a worthwhile addition to the machines. The Dial-A-jet kit fits more than 230 models at this time. Check with Thunder Products, Inc. for complete application information. For more information contact Thunder Products at 21676 Deep Lake Road, Richmond, MN 56368 or call 320-597-2700.