Gently insert the hydrometer...
Gently insert the hydrometer and take a reading of the level. Our sample tested 0.740 at 57 degrees F.
Using the supplied chart,...
Using the supplied chart, look up the specific gravity at the test temperature, and then cross that value over to the standard 60-degrees-F value. At the standard temperature, the subject fuel had a specific gravity of 0.736, slightly better than the lowest acceptable density of 0.720.
Big Brother Is Watching At...
Big Brother Is Watching At The Pump - In a perfect world, you get what you pay for, but as we all know, that is not always the case. When it comes to gasoline, you trust that the retailer is pumping your Poncho full of Premium and not Regular. Well, most state and local government agencies want to make sure, so they take random fuel samples. Zeltex Corporation (www.zeltex.com) markets a portable fuel analyzer for this purpose. With a base price of $10,500, it's not practical for a hobbyist, but it's nice to know this technology is available.The ZX-101 offers automatic fuel selection to detect the type of fuel being analyzed. A Delron sensor head firmly positions the sample bottle into the instrument, and a two-piece optical filter tray provides extended service life. The ZX-101 features a warm-up time of less than one minute and an analysis time of only 20 seconds. A screen supplies the actual octane number, and a printout is available.
Once the specific gravity of the fuel is identified, a decision for jet tuning can be made. As with determining and using air density ("Weather or Not You Go Faster," April '06 issue), good notes and a baseline need to be determined. For example, if you ran your Pontiac at the track with 0.712 fuel and the best performance was obtained with a certain jet, use that as a baseline. On your next visit, the fuel checks at 0.736, so a jet change is in order or the engine will be richer than required for peak power. The calculation is:
|0.712 – 0.736 |
| –0.024 |
|– 0.0326 (–3.26%) |
Using a carburetor manufacturer's jet-area chart (Demon Carburetion has one in the company's catalog), decrease the jet area by approximately 3.25 percent.
If your Pontiac requires dedicated race fuel, it is also important to acknowledge the specific gravity of the gasoline. Upon request, racing fuel dealers provide an information sheet on the fuel they sell. Due to the small-batch production of race gas, along with a higher level of quality control, huge differences in specific gravity on a load-to-load basis are unusual. If you're a traveling racer, then it's important to check and adjust for variations in specific gravity of different brands of race fuel. For example, if you race in New Jersey using Sunoco Cam 2 and travel to an event in Ohio where only VP is offered, it's good practice to check the density of the unfamiliar gasoline.
The actual fuel-check procedure is better explained with images, so follow the photos and captions as HPP shows you another trick to keep your Pontiac ahead of the competition.
Carburetor icing occurs when the intake air is chilled below the freezing point of water by the vaporization of the gasoline. The ice forms on the throttle blade(s) and in the venturi and can cause the engine to stall as soon as the gas pedal is released to idle. This ice formation restricts any air from passing by the throttle plate(s), choking the engine.
Icing can be acute when the air is moist (70 percent or higher relative humidity) and the ambient temperature is between 35 degrees F and 55 degree F. These weather conditions are common during the fall, winter, and spring in many parts of the country and can last into early summer in coastal regions. Carburetor icing is lessened when the intake air is much below freezing since it's common for the humidity to be less.
The extent of carburetor icing doesn't depend on the weather alone. It also involves carburetor and vehicle design and the mechanical condition of the engine, in particular, the components that affect warm-up time. Thermostat, choke, intake-air heaters, and heat risers all help to eliminate carburetor icing. Part of the solution also involves gasoline volatility.
A good index of the tendency of a gasoline to cause carburetor icing is the 70-percent evaporated-temperature specification in the distillation profile. The lower this temperature, the more severe the icing.
Carburetor icing is not as big a problem as it used to be. For emission control reasons, most carburetor engines built since the late '60s are equipped with intake-air heating systems, which generally eliminate carburetor icing. But keep in mind that many aftermarket intake manifolds and headers eliminate these components. Also, race gasoline, which is not designed for cold-weather startability, has a tendency to create more icing than street fuel.
The symptom of carburetor icing is the engine loading up shortly after its started, much in the same manner as with a faulty choke pull-off or stalling when cold and the throttle is released.