Carb Tuning

Back in the March newsletter I told you about fuel mixtures and how they can change with density altitude. Itís always better to keep your mixture on the rich side. Many racers use the tried and true method of plug reading to see what the carb is doing. Unless youíre very good at reading plugs itís almost a hit or miss since there are many adjustments that can be done to a typical carb. One of the easier ways is to use an O2 sensor. If your headers have an O2 sensor port on them then youíll only need to get a sensor. If they donít then youíll need to have a bung welded onto at least 1 header, preferably the left one so the PCV valve doesnít give the sensor a false reading. Installing a sensor in both headers will allow you to see what each cylinder bank is doing but this isnít really required. Purchase a GM single wire O2 sensor and install it into the header. Once you have the sensor installed and wiring run up to the inside of the car, attach a digital voltmeter (you really should have one of your own, but you can sometimes borrow these from friends if you don't have one) to the sensor and a good body ground. The sensor is positive. The readings you'll get once the sensor has heated up will be from 1.1 volts (1100 millivolts, or mv) down to about 100 mv. The high readings are rich, the low readings are lean. The perfect mixture for cruise is 400 mv. Most cars run well at about 700-800 mv. Once it gets below that, it tends to get into a lean misfire. Your results may vary. Here is a general idea at what the O2 sensor voltage output looks like. As you can see, the slope around 400mv, which is 14.7:1, or perfect combustion, is very steep. This is why only computerized fuel injection systems can really hold anything close to 400mv. If you're wondering about how a sensor can read oxygen content in rich mixtures where there is no extra oxygen, the sensor begins to act as a temperature sensor above 400mv.

Vacuum gauges - You will need a gauge to read the manifold vacuum on your engine. The more accurate the gauge, the better your results will be. The manifold vacuum is measured at the base of the carb or on the intake manifold somewhere. Don't tap into just one runner, though, as sometimes this will give funny readings. You need to check the signal that sees all the cylinders. Most Holleys have a manifold vacuum port in the front on the passenger side under the primary float bowl.

Road Tuning - Start by taking your carb apart and writing down the sizes of the jets, the actuation point of the power valve, and the size of the accelerator pump squirter. Put it all back together, check for leaks, then drive it until the engine is warm. In order to get good readings, you will need to drive at a constant speed of 45-55 mph, accelerate lightly, and accelerate heavily.

Idle - The best way to set the idle mixture is to lean the carb out until the vacuum just starts to drop, then richen the mixture by about 1/4 turn. If you have a bit of a stumble in very light, low speed operation, sometimes it helps to richen it up by another 1/4 turn.

Cruise - Going down a flat road at a rate of about 45 mph or higher will give you a good indication of your main jet sizing. Shoot for between 400 mv to about 700 mv. Since carburetors are not as exact as computer controlled electronic fuel injection, keeping at the perfect 400 mv will be tough to impossible. You always want to go a bit rich, as excessively lean mixtures will cause damage to your engine, create pollution, and give you bad gas mileage and performance.

Power Valve - Once you have the main jets set, it is time to play with power valves. Going up a slight incline at highway speeds, or accelerating slowly at highway speeds, you will notice the vacuum reading falling. As it falls, it will come to the opening point of your power valve. You can tell when the power valve opens because the meter will go lean for a while, then the valve opens, and the meter begins to show rich. You'll probably notice the power increase right when the power valve opens. About 700 mv to 900 mv is a good reading for light loads with the power valve open. Wide open throttle readings should be between 850 mv and 900 mv. Tip: Use a power valve that is about 2 inches of mercury below the LOWEST manifold vacuum reading you get on cruise and idle (in gear for automatics). If the power valve flutters open at idle, it can act as a pump, and push extra fuel into the main well, causing a drip from the booster venturis. If it comes on a lot while you're driving down the road, your gas mileage will suffer.

Blown Power Valve - You may notice a bad power valve because you will not see the jump from lean to rich when your vacuum gauge gets to the power valve setting. You will see a rich mixture at cruise, when the power valve should be closed. You may also notice a badly BLOWN power valve as an overly rich condition at idle. This occurs only if the diaphragm has a big hole in it that lets fuel from the float bowl into the power valve vacuum passage and into the engine. There are some hand held vacuum pumps and adapters out there to check power valves. I just use my mouth after making sure all the gas is dried off. If you suck a lot of air through the diaphragm, or it won't hold a vacuum, then replace it.

Accelerator Pump - You'll probably need the help of a spotter on this, because things happen fast when you punch the throttle from a dead stop. If your meter goes rich right away when you punch the throttle, then it leans out gradually, you need a smaller pump nozzle to make the fuel come out slower. If the meter doesn't go rich right away, but gets richer later, you may need more fuel earlier. A bigger nozzle is in order. Try some different cams, too, but be aware that bigger is not always better. If your car still has a lean stumble with the biggest cams and the biggest nozzles, you may have to go with the 50cc "Reo" pump diaphragm and the corresponding cam. You must use the big black cam on 50cc pumps to get the full shot of the 50cc pump! If you put that big old pump under there but don't move the lever any farther than before, what's the use? Also, don't put the 50cc cam on small pump diaphragms! You'll break them! Most manifolds will require a 1/4 inch spacer under the carb to run the big pumps. So if you add a spacer, don't forget to check your hood clearance BEFORE you slam the hood! Another thing to think about when you start getting into the big pump cams and nozzles is to use the hollow nozzle hold down screw. This is due to the fact that the little passage cut in the side of the internal threads may not be big enough when you're way up in shot size, over .040, as recommended by Holley