Raptor Restricted Carburetor Analysis
In the last article, related to carburetor blue printing, a matrix chart for R&D analysis was said to be forthcoming. It has been developed and is available but requires one to forward a snail mail address. Due to limited skills in working with HTML, we have not been able to get it into a proper format to forward, and open successfully, via the internet.
Flow values for Rookie and Junior carburetors were not discussed on the previous article. This article outlines a process for selecting the best carburetor from what selection you may have available. There may be other processes, but what follows has worked well for us over the past 10 years or so.
It = s an absolute must to secure a good known performing carburetor to serve as a standard, or calibration carburetor, for any and all test. The calibration carburetor selected for machine set-up should be a good performing Sr. class carb. A Sr. carb will permit a wide range of comparison and analysis possibilities for all Jr. & Sr. classes without ever going back and changing to another calibration carburetor. Once a calibration carburetor is selected and installed it should never be removed from the flow machine.
Exclusive purpose of a permanent calibration carb is for adjusting and setting-up flow machine values before initial, comparative, or sequential testing begins. This will establish an accurate set-up flow standard to serve as a fixed standard throughout the testing and analysis process.
Any change in atmospheric conditions will have an affect on data accumulated for comparison. Air density, atmospheric pressure, temperature and humidity should be recorded each time a test is conducted. Changes can occur between test and from one day to the next. Accurate calibration is always required to insure valid test results for any flow test data.
Most commercially produced, or home built, carburetor comparators are relatively inexpensive and can provide very accurate data if set up, calibrated, and used in a consistent manner. Modifications should be made to permanently install a calibration carburetor. This will require a block off plug, or valve, to switch from the calibration to the test carb for each machine re-calibration, check, or re-adjustment. Instillation will also require fabricating a carburetor mounting flange with an opening into the machine = s air chamber.. This modification will eliminate calibration and test carb attaching, removing, and re-attaching each time a test is conducted. Yes, this means that two carburetors will be mounted on the machine at the same time!
A U-tube water manometer, usually 16", one small diagonal oil manometer, and variable speed blower, are common place on most comparator flow testers.
For fuel draw comparison and analysis, two additional U-tube oil manometers need to be permanently installed. Once installed they will be used to measure fuel draw and make comparative analysis. During testing, the test carb and calibration carb, will have a set of connecting hoses running from the pick-up tube fuel draw holes to a fuel draw manometer simultaneously. Each connecting hose should be of equal length and diameter. .
High Speed Test Procedure:
After mounting a test carburetor, close the block off valve, or plug, to the test carb air passageway. Open the block off valve to the permanently attached calibration carburetor. For additional information refer to the first Rix article written and review instructions related to 1/4 and full open test set-up analysis.
Specific/exact, air and fuel draw, machine calibration values are not that important due to the fact that they will vary form one machine and from one calibration carburetor to another. The important thing is to duplicate set-up values, and calibrate the machine to exact identical values, before conducting each test. Never re-adjust throttle stop or change jets on the calibration carburetor. Doing so would change the consistency of the machine set-up calibration value.
For setting up to conduct high speed checks, open calibration carburetor throttle to maximum and adjust blower speed to 8" of water column. Once water column is established, note the fuel draw value as indicated on the U-tube manometer. Record these values on the machine set-up calibration section of the matrix analysis form. This value will serve as the standard for test carb analysis.
Block off the calibration carburetor and open the carburetor to be tested (test carb). Start the machine and adjust blower motor to duplicate inches of water column previously established in and recorded from step 3.
Record test carb fuel draw values as indicated from the attached fuel draw U-tube manometer. Do not make any adjustments to the test carburetor at this time. The first step is to note carb characteristics. Modification work should always be performed after pre-testing the carb unaltered. This will show potential carb characteristics of what one has to work with.
Low Speed Test Procedure:
1/4 draw fuel draw should be conducted next . Some type of system needs to be developed to insure that the calibration carburetor is opened exactly the same, near 1/4opening, to insure duplicate accuracy in setting up the machine. One way is to remove the spring on the throttle stop screw, lock tite, and run the screw down until it bottoms out. This will prevent the throttle valve from completely closing and should provide for a near 1/4 opening position. If not, a install a slightly longer throttle stop screw in place of the OEM part.
Block off the test carb air passageway. Open the calibration carb air passageway and adjust the blower speed to slow before starting the machine. Failure to do so will result in sucking the water out of the compoaritor machine = s manometer.
Slowly increase blower speed until about 10 - 20% fuel draw is indicated on the calibration carb = s fuel draw manometer. Record fuel draw and reduced water column values in matrix chart = s machine set up section. Water column should be about 30% less than for the high speed test.
Next block off the calibration carburetor and open the the test carburetor air passageway DO NOT READJUST THE BLOWER MOTOR SPEED!.
Hold the test carb throttle wide open, start the machine, and slowly close the test carb throttle until the set-up standard of 30% less water column is indicated on the manometer. At this point note and record the fuel draw as indicated on the test carb = s manometer. Trick is to always match water column then record fuel draw. Once recorded comparison data can be analyzed.
Calculating maximum air flow has not been included with this procedure Air flow values can be noted and recorded off the diagonal oil filled manometer that is usually attached to outside of the inner blower box chamber.
Once these steps are completed, other carburetors can be simultaneously flowed, characteristics of each recorded on the matrix form. Upon analyzing a number of carbs, the best selection can be made that matches known values for a good performing combination. This needs to be down before any work begins to fine tune fuel and air draw for maximum performance on a final selected test carb. .
Specific set-up values were not suggested, they will vary from machine to machine, and from one calibration carburetor to the next. R&D on the engine builders part is required to set the standard that fits ones specific equipment, resources, and circumstances.
This procedure does not flow test restricted carburetors with a plate. The plate destroys most of the ability to detect fuel draw characteristics due the major decrease in air flow. High/low fuel draw, rather than maximum air flow characteristics, is most more important on restricted class engines. Some of the best carbs show strong 1/4 throttle fuel draw and lean high speed fuel draw during initial (less plate) flow testing and analysis. Exact calibration numbers/values are something you will have to work out for yourself.
After a carb is flowed and prepped plates are used for WOT checks for block/carb/plate assembly flow analysis for porting purposes. If fuel draws checks were to be taken with carb/block/plate assemblies it would require a entirely different set-up and calibration process.
Fuel/air draw and mix is more important than maximum air flow no matter what class you may be running. Headers, cams, air cleaners, ignition time, spring tension, etc. will affect ideal carburetor characteristics. There are no easy answers of fixes, it takes a lot of time and hard work to experience a good performing combination.
Yours in karting,
Rick at Rix