Carb recommendations

[k wright]

I've been spending some time tuning my carb prior to ditching it for Mass Flo fuel injection and have found that I'm getting pretty close to a good fuel curve using my O2 sensors as a guide for tuning my carb. If your interested these are some of the things that I've learned:

1) If your driving on the street get a spark plug that is made for street driving. Go to the website for the cylinder heads that your using and start with their recommendation. Don't do any tuning with a set of fouled racing spark plugs. If you do you will scratch your head wondering why your mixture is so rich but your car won't run (the reason is that the plugs are not igniting the mixture in the cylinder and the raw gas passes unburned to the exhaust). The importance of getting the proper plugs for the street can not be overstated.

2) Get a carb with annular boosters. Downleg boosters are what most racing carbs come with because they occupy a smaller volume of the venturis and may result in a larger amount of air flow at full throttle. The downside of downleg boosters is that they have a single hole that discharges large droplets of gas, the annular boosters have as many as 12 small holes that gas is discharged from. In addition the annular boosters are shaped to begin flowing gas at a much earlier point than the downleg booster. Example: my carb initially had a 750 with 74 main jets (Quick Fuel), after replacing the downleg boosters with annular boosters I have been able to decrease my main jet sizes to 68 as well as decreasing the amount of fuel delivered via other carb circuits. These boosters provide a much finer dispersion of fuel and make for a worthwile change. Specify them when getting a new carb or motor.

3) Get a metering block that has adjustable emulsion bleeds for the powervalve restriction channels and idle circuits. These circuits if tuned well will make your idle much smoother and prevent the bug killing haze that your probably have now. Enlarging the power valve channel restriction will allow you to lean the main jets and get a much smoother and more efficient cruise without getting a lean mixture on acceleration.

4) Demon carbs has a new feature that allows adjustment of idle speed without altering the angle of the thottle blades (Idle Eze). This is a very nice feature and it will allow keeping the throttle blades in the right position on the transition slot. However Demon does not offer annular boosters or quick jet change bowels. The other option is to gradually drill small bypass holes in the throttle blades on the Quick Fuel or ProForm carb.

5) Get the book "how to build horsepower vol.2" and the video from the Holley website and refer to them frequntly. A great website for carb tuning is "carburetor tuning the scientific way".

6) Get a good O2 sensor that monitors each bank.

All of this sounds like a lot of work but the difference it makes is giant. Nobody puts fuel injection on a car and doesn't tune it, you should expect to follow the same plan with a carb. The carb has some advantages; simplicity, cost and dependibility. It can never match fuel injection efficiency.

ken