LIST # CFM STOCK JETTING POWER VALVE TYPE Secondaries
1850-2 600 (F)66,(R)plate 134-9 6.5 4160
1850-3 600 (F)66,(R)plate 134-9 6.5 4160
1850-4 600 (F)66,(R)plate 134-9 6.5 4160
2818-1 600 (F)65,(R)76 6.5 4150
3124 750 (F)70,(R)76 8.5,8.5 4150
3247 750 (F)70,(R)76 8.5,8.5 4150
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Holley carbs are calibrated for sea level operation and an inlet air temperature of 70 degrees Fahrenheit. Once you know the correct stock jetting for your particular Holley carb, you can determine whether you live or race at an altitude above sea level. For every 2000 foot increase in altitude, you can reduce the jet size by one size. If you had a carb which has a stock jet size of 80 and you live or race at 2000 feet above sea level, then you would use a #79 Holley jet in the carb. Similarly, a change in the carb's inlet air temperature may require a change in the jet size from the stock calibration.
There is a lot of misunderstanding concerning power valves in Holley carbs. Many 4-barrels come with a particular power valve depending on the carb list# and application. Some carbs have two power valves, while others only have one. The power valves are numbered by the amount of engine vacuum in inches at which they will open and add additional fuel to the power circuit. In other words a 6.5 power valve will open when the vacuum signal on the engine drops below 6.5" and will remain closed above that amount. One of the misconceptions is that they can't be trusted to work because an engine backfire or "belch" can "blow out" the power valve. Many Holley performance carbs models and list#'s now come with built in power valve "blow out" protection which eliminates this problem. If you have an older model carb you can purchase a small, inexpensive, easy to install kit from Holley (PN - 125-500) that will also protect the power valves in case of an engine blowback thru the carb. CENTEK in Redmond, Oregon, (see their website at
www.powervalveshield.com ) also sells an inexpensive Holley power valve blow out protector, "Power Valve Shield", which takes about two minutes to install and does not require any drilling