factory jetting infohttp://www.holley.com/data/TechService/ ... isting.pdfhttp://www.holley.com/data/TechService/ ... erence.pdfhttp://www.bgsoflex.com/holley.htmlhttp://www.angelfire.com/fl4/pontiacdud ... lator.htmlhttp://www.angelfire.com/fl4/pontiacdud ... lator.html
these come in handyhttp://aeromotiveinc.com/wp-content/upl ... ter-NN.pdf
first youll want to verify theres no vacuum leaks,youll use a un-lighted propane torch not any flammable sprays as its far more precise, as you can stick the torch tip exactly where you suspect leaks,and it less messy and leaves no residual crap on the engine.
ITS important to understand how and why things work,IF you don,t have a timing light, generally you use a vacuum gauge on a STOCK engine to set timing by hooking it up to plenum vacuum , set the idle to normal idle speed (usually 670rpm-850rpm)and adjust timing to max vacuum reading, this DOESN'T,T ALWAYS result in the best possible timing but it will generally be fairly close on a DEAD STOCK ENGINE
YOU CANNOT JET OUT A MOMENTARY LEANNESS CAUSED BY AN INSUFFICIENT FUEL SUPPLY IN YOUR CARBS FUEL BOWLS, THE WRONG POWER VALVE,OR ACCELERATOR PUMP CAM, or the CARBS TRANSITION CIRCUIT THAT WILL NOT SHOW UP IN A NORMAL PLUG COLOR READING. !!!
Common Fuel Delivery Mistakes and How to Correct Them
by Barry Grant Inc. Â©2004
Fuel lines, filters, pumps, and regulating devices exist to deliver gasoline or alcohol from the fuel cell to the carburetor, and in a bypassing system return it to the fuel cell. Pressures and volumes vary depending upon the kind of fuel used (gasoline or alcohol) and the type of fuel system employed. With routine maintenance, a good fuel system will pay dividends; yet many cars are fitted with systems that fall below the necessary standards. With a little help from BG Fuel Systems of Dahlonega, Georgia, here is a list of the most common mistakes in competition fuel systems and a guide as to how to correct them:
1. Incorrect fuel lines
Do not use 1/4" - 3/8" o.d. lines on a racing fuel system between the fuel cell and the pump. Racers often joke about their first race car and how the fuel line was so small it functioned as the main jet. Ensure the fuel is supplied through lines that are of the correct size for the application. Remember fuel line size is determined by the system, not the vehicle! Push-Lok, stainless steel braided hoses, and aluminum tubing are the most common fuel lines used on race cars.
2. Right-angle hose-end fittings from the pump to carburetor
Avoid forged 90*elbow fuel fittings as much as possible. Although they are inexpensive and readily available, they're restrictive and frequently cause fuel flow troubles. Hose ends with angles of 90* & 45* should also be avoided if possible. Nonetheless, should it become necessary to use one, use radiused hose ends (90* bends) as they have much better rates of flow. They're manufactured from aluminum, equipped with swivel ends for a positive seal and are easy to install.
3. Fuel pumps unsuitable for alcohol
An alcohol fuel system differs from the gasoline alternative in several crucial respects. Fuel pressures in a gasoline system are typically maintained between 7- and 9-psi throughout the rev range, whereas alcohol carburetors require low pressures of around 4- to 6-psi at idle and 9- to 11-psi at fully open throttle. This is necessary to prevent the carburetor from flooding at idle and under light engine load, yet maintain the extra volume necessary for maximum acceleration. Engines producing around 500-hp can be fueled with a 15-psi mechanical pump in conjunction with a throttle bypass valve. However, for engines over 500-horsepower, a belt-driven system with a diaphragm valve, or poppet bypass should be considered. For overall reliability and performance, a belt-driven system is usually the better choice. When using a diaphragm bypass, BG Fuel Systems recommend it be installed close to the carburetor for faster response.
4. Incompatibility between bypass and pump
Throttle bypasses were designed to operate with block-mounted pumps and, similarly, diaphragm bypasses with belt-driven pumps. Never use a throttle bypass with a belt-driven pump - they must function in pairs and are not to be mismatched.
5. Unsuitable fuel filter
Fuel filters with conventional paper elements must not be used with alcohol. Because alcohol absorbs water, the paper and the bonding materials deteriorate quickly. As a consequence, particles can enter the float bowls, or get stuck in the needles-and-seats and main jets. Further, it's equally important on alcohol applications, to avoid filters with inadequate flow rates, and inlet and outlet sizes that are too restrictive.
6. Neglecting routine maintenance on an alcohol system
Although alcohol fuel additives can help prevent corrosion and provide lubrication for pumps and other components, alcohol can severely corrode metal objects, especially aluminum, if allowed to remain in contact too long. There is no substitute for a strict maintenance program, it will not only prolong the life of the carburetor and fuel system, but also keep it trouble-free. Alcohol will naturally absorb water from the air. This not only dilutes the fuel, but also adds to the corrosive effects of alcohol. After every race, the alcohol should be drained from the race car and stored in air-tight containers; a vented fuel cell is not considered an appropriate storage container. The fuel system and carburetor should also be thoroughly drained and flushed of any residual alcohol. Some racers will add gasoline to the empty fuel cell and run the engine until they are certain the carburetor is filled with gasoline. Other methods include removing the carburetor and flushing it with a cleaning solvent or lubricating aerosol sprays. Removing inlet and outlet fittings from the pumps, bypasses etc. and lubricating the internals is also an acceptable practice. Whatever the method, maintenance on an alcohol system is crucial; ignore it and the system will fail.
7. Failing to use a high-flow air cleaner
Race engines rely on receiving air as well as fuel, but are frequently starved by the use of thin, small-diameter air filters that are detrimental to their performance. If possible, use a filter that is 14"diameter x 4"tall. If clearance is at a premium get one with a recessed pan, which allows for deeper filter elements. Get a good quality air filter. If you're constantly cleaning lots of dirt from the surface of your race car the chances are your air cleaner is struggling to filter the dirt and debris from your engine.
8. Deterioration of foam-filled fuel cells
Modern military-spec foam-filled fuel cells, are compatible with conventional fuels, racing fuels and alcohol. However, alcohol can cause the foam to deteriorate and it must be renewed once a year. To check the condition of the foam, simply remove the cap and pinch it between finger and thumb. If pieces come away the foam must be replaced.
9. Not having proper linkage travel and return springs
Make sure that, at wide-open throttle, the butterflies of the carburetor are fully open. Use a minimum of two, good quality return springs - preferably of stainless steel and employ them, if possible, in two different places on the linkage. Ensure the linkage and the return springs operate without interference throughout the full range of throttle travel.
10 Failure to use an adjustable pedal stop.
It's amazing to consider the large numbers of race cars that have no form of throttle-pedal stop, and inexcusable that so many are permitted to compete. Excessive loadings on the linkage, carburetor shafts and butterflies can cause the mechanisms to distort and jam, and the consequences are usually grim. To avoid the inevitable, use an adjustable pedal stop and, at the fully open throttle position, synchronize the stop on the carburetor with the stop on the pedal.
11. Not having the proper size of carburetor for the application.
Having the proper venturi sizes for a given application ensures the carburetor generates sufficient air speed. Air speed creates the necessary depression (low pressure) to draw fuel through the metering systems and booster venturii into the air stream to be atomized. The Race Demon, which is equipped with removable venturi sleeves and boosters etc., overcomes most of the sizing problems.
12. Inadequate fuel cell venting.
If the fuel cell vent is too small the fuel system can malfunction. In extreme cases, inadequate ventilation can cause permanent damage to the system. As the fuel pump draws fuel from the cell it needs to be replaced by air. If the vent on the cell is too small the pump will try to draw the fuel from the cell faster than the air replaces it. This can create a vacuum
in the cell, distort its shape, and starve the pump and engine of the fuel it requires.
13. Filtering of fuel cell vents.
Just as an undersized vent will adversely affect your fuel system, not having a filter on the vent will cause dirt and debris to enter it. As air replaces the fuel, the vent hose will attract anything that's in the air, including dirt, sand, or debris. These particles will eventually destroy a fuel system as well as an engine.
14. Fuel filter location.
To protect the fuel pump and carburetor, use a good filter before the pump, and filtered fittings at the carburetor.
Steel braided fittings and aluminum fittings have a life cycle. Over a period of time, the rubber bore will deteriorate causing the line either to collapse and starve the engine of fuel, or disintegrate and possibly block the passage. Radiator hoses and fan belts are routinely replaced and so, too, should fuel system components. Aluminum fittings will wear out over time and fail to seal properly. From a safety and performance standpoint regularly check the lines and fittings. Try to detect soft or weak spots by feeling the outside of the lines, and visually inspect the internals each season. Keep the connections tight.
16. Relays for electric fuel pumps
On cars equipped with electric fuel pumps, use a relay to ensure the pump is provided with the proper voltage in order to maintain proper fuel flow.
Barry Grant, Inc.
we have all seen those formulas that allow you to calculate the required carb flow rates on an engine,
CFM = engine size (cid) x maximum RPM / 3456 x VE
So, assuming 6000 RPM and 85% VE: on a 383 cid engine
383 x 6000 / 3456 x .85 = 565 CFM.
Even at 90% VE, CFM = 598
At 90% VE and 7000 RPM, CFM = 680
that indicates 650 CFM is all a stout 383 will ever need. By going to a 750cfm, you give up some better peak hp and slightly reduced throttle response due to the larger primary bores.
the formula would be just fine if the engine was 100% efficient and port flow in and out of the cylinders was at a constant air speed and the valves were not a factor, but theres a 720 degree repetitive cycle and ports generally flow only about 1/3 of that time, now divide the flow into a plenum and swap which port is running a negative vs a positive pressure about 30-60 times every second and you find true flow efficiency drops to the point where you need a good deal more potential flow capacity to match the engines requirements.
ever think of what those flow numbers mean on cylinder head flow charts, if for example the heads flow 260cfm at .600 lift and youve got 8 cylinders, it sounds like youll need almost 2100cfm of flow to keep up, but because the port only flows when the valves open and the flow rate changes with piston speed,port inertia, cam timing and exhaust scavenging,intake plenum and runner design, port reversion rates, altitude,etc. a simple formula can,t give you more than a semi-accurate guess as to the engine requirements.
the best gauge
IVE found is a vacuum gauge
, if your pulling more than 1 psi of vacuum
at WOT your more than likely going to benefit from a larger carb or a carb spacer or a different plenum design.viewtopic.php?f=55&t=2994viewtopic.php?f=55&t=4362