Carb Class: Horsepower in the Details

Carb Class: Horsepower in the Details

Billet metering blocks are a standby if you purchase a Quick Fuel Technology race carburetor - what does that really mean to you? The word billet means precision, quality, and maybe even good looks. To us precision is more important than beauty and that's why our CNC machined blocks are your key to horsepower. Today's modern race engines require an induction system that has a wide range of adjustment. Carburetors as they advance become nearly as adjustable as fuel injection and still win out on the peak dyno numbers in most cases. Factor the peak power increases with affordability and it's a win-win situation. So what can you do to make the most of your carburetor? Settle in and read on....  

Metering blocks are where the action is when talking carburetors. What your metering block is made from or how it's made is of no consequence without a good knowledge base. Understanding how a metering block works will help you understand what your carburetor and race car ultimately want. In the case of a QFT carburetor (and many others out there), metering blocks take air from the air bleeds in the main body and mix it with fuel from the float bowl. The "push" that gets fuel through the metering block is provided by atmospheric pressure (remember our last class) and without it a carburetor will not function. Fine tuning your metering block is done with patience and a good base-line to refer to - we often talk to people over the phone that find themselves out in left field and need the base calibration of their carburetor. 

So what is a baseline? Most of carburetors when purchased include a specification sheet that has some basic details of how your carburetor is calibrated. If the carburetor was built for your application is should be in the ball park and we would use this as a baseline tune-up. What if your carburetor was previously used or didn't include a specification sheet? Simply take apart your carburetor and "pin" (measure) each orifice with a pin gauge and mark them accordingly in a notebook. It might help to draw illustrations and if pin gauges are not available we recommend using the backside of drill bits to get close. 

With a base-line setting you can now experiment with orifice sizes and see what your car reacts to. A good rule of thumb to go by is any hole on the fuel bowl side of the metering block will take in fuel while ones on the main body side will deal with air. With the fuel bowl and metering block removed, look at the main body and see where the air bleed channels lead to; match these holes up with orifice openings on the metering block and then you can distinguish what circuit goes where. Opening up passages that flow air will lean out that circuit while replacing restrictions with smaller ones will richen the circuit. The gas orifices are exactly opposite and will richen when they are enlarged (think of your main jet). 

So now that you know what it takes to move things around we should talk about limits and how for to go. Usually when changing orifice sizes in a metering block we suggest going .002" - .003" at a time. Moving more than .003" at a time on any given orifice can potentially throw things off - especially if we are talking about leaning out a high RPM engine. Remember, take baby steps, document what is being changed and how the car reacts and above all, be patient. 

This is a very broad topic and difficult to fit in the space of our column. Hopefully it points you in the right direction and when in doubt consult your local engine dyno shop or give us a call, we love talking carbs and want to see you get the most out of your car!

Phillip Oakley