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Carb Tips

Posted in Features on August 6, 2003
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Dinosaur juice. Petrol. Go-juice. Gas. Whatever you call it, fuel is the stuff that moves your truck. We all know combustibles are cool, but sometimes parts in your 4x4's fuel system can be the source of problems on the trail, especially the carburetor. Sputtering while climbing a steep incline, stalling in an off-camber situation, and belching black smoke out the tailpipe after a brief stop are all common problems when 'wheeling a carbureted rig. Many people accept these problems as facts of naturally aspirated life and live with them, but there are several techniques to eliminate or minimize the effects of 'wheeling on a carb.

Sean Murphy at Jones Performance Fuel Systems showed us a multitude of tips and tricks for Carter, Holley and Quadrajet carburetors to minimize the effects of four-wheeling. A few of the parts and pieces require some cash, but most of these tips cost only pennies to perform. Following these simple fixes will add performance reliability to your trail rig.

Fouled-Up Floats

Most carburetor problems that occur during 'wheeling stem from the carb's sensitivity to angles deviating from horizontal. All carbs have a small bowl (or bowls) that fills with fuel and acts as a reservoir. A float in the bowl regulates the fuel level by opening and closing a needle valve. On the street, your truck sees gradual inclines and very little bouncing so a carburetor remains close to level and the float accurately regulates the amount of fuel entering the float bowl. But, as we well know, 'wheeling causes your truck to tilt and bounce to extremes. As your 4x4 tilts, so does the fuel inside the float bowl. If your 4x4 bounces, the float does too. This causes the float to make inaccurate readings of the fuel level in the bowl. Too high a level causes an overflow, sending fuel gushing out the vent tubes. Too little in the float bowl starves the jets and leans out the mixture causing erratic performance.

Lower Level

These are the three most common carbs found on 4x4s: a Carter (bottom right), a Holley (left), and a Quadrajet (upper right).

Often, lowering the level of fuel in the float bowl can net you mountains of improvement for off-road use. As we said before, the float bowl acts as a reservoir for the jets. When you stomp on the accelerator, fuel is pulled through the jets quicker than the fuel pump can supply it. Also, all the fuel sloshes to the back of the bowl. The jets feed from the fuel in the float bowl until the pump catches up. When a float bowl is full during serious off-roading, raw fuel sloshes around inside the bowl and often escapes through the vent tubes and leaks into the venturis. Also, with the exception of the Quadrajet, float bowls aren't completely sealed off from the venturis. Lowering the fuel level reduces the chance of the reserve gas going where it's not supposed to. However, there is a catch to this adjustment. When you lower the fuel level in a float bowl too much, the bowl can be sucked dry during acceleration--hesitation and power loss result. When rockcrawling, there is hardly ever a situation when you need full throttle, so usually the fuel level can be reduced with no adverse effects on engine performance.

Last Resort Regulation

Shown here are the two different types of a float bowl mechanism found in a Holley: a center-pivot and a side-pivot float.

Some applications are stubborn enough to refuse help in all of the forms we've mentioned so far. When this happens, many 'wheelers need fuel regulators. A properly operating mechanical fuel pump should supply between 3 and 8 psi to the carburetor. In most rockcrawling situations the engine operates at idle or just off idle, so the pump may force more fuel into the carb than necessary and flood it. Installing an adjustable fuel regulator between the pump and the carb may solve this problem and enable you to maintain highway performance without tuning the carb. A fuel regulator allows you to control the amount of pressure going to the carb. At a trail head, the regulator can be turned down so that as little as 3 psi reaches the carb. After you complete the trail, the regulator can be returned to normal pressure for the trip home.

Vapor Lock

Excessive heat and vapor lock are often problems for trail trucks. At slow speeds, there isn't enough air being forced through the engine compartment to keep things cool. Vapor lock occurs when fuel reaches a certain temperature and changes from liquid to gas. Once the fuel becomes gaseous, it can no longer be pumped into the carb or atomized in the venturis. Vapor lock also occurs when fuel sits in a hose that's extremely hot. The most common source of vapor lock is improperly routed fuel lines. Simply routing the hoses farther away from engine heat or using insulation over the fuel line are good solutions. The Carter and Edelbrock carbs are more sensitive to vapor lock because they are constructed of aluminum, which conducts heat well.

PhotosView Slideshow
PhotosView Slideshow
128 9704 fuel 19 z
128 9704 fuel 20 z

Hose Tech

Sometimes fuel system problems aren't caused by specific components but instead by the hoses that run between them. Most people overlook a fuel hose unless it's leaking, but there are a couple of factors that are important. First and foremost is that hose diameter needs to be large enough to provide a sufficient volume of fuel to the engine. In some extreme cases, a small-diameter hose may cause an insufficient amount to reach the carb. Steel line is more reliable than rubber, but steel hose can transfer heat, while rubber line acts as an insulator. Steel can work-harden and break when exposed to prolonged vibration, but rubber can crack and decay with age. Keep these things in mind when you're routing fuel lines.

The following chart gives you some general guidelines on correct hose diameter and fuel pump volume according to engine horsepower.

PhotosView Slideshow


Jones Performance Fuel Systems
Huntington Beach, CA 92647

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