Offshoreonly.com - backfire thorough carb..

You say the engine runs fine except for an occasional backfire out the carb when accelerating.

Holley made two types of four barrel carb designs. The first was their famous "vacume secondary" design which had no basic connection between the front primary barrels and the rear secondary barrels - these were opened by a large vacume diaphram that was controlled by the engine. The second design was the Holley "mechanical secondary" carburator which lookes a great deal like the other. This mechanical secondary carb does away with the vacume chamber and uses hard linkage to open all four barrels. This carb is also unique because it has two accelerator pump systems.

Now when a carb is opened - the engine experiences a drop in intake manifold vacume. This drop activates a "power circuit" in the carb. This is also a small vacume diaphram valve that dumps in more fuel for the power condition. These screw into the fuel metering block which is between the float bowl and the main body of the carb. Some Holley carbs like the mechanical four barrel use two thick metering blocks for front and rear bowls (vacume carbs just have one on the primary) - these thick metering blocks may have idle mixture screws and full power valve circuits for the rear barrels.

Accelerator pumps are used to mechanically squirt a shot of fuel into the carb throat when the barrels are opened to keep the fuel mix where it should be. The vacume secondary carbs only had this on the front barrels while the mechanical secondary carbs needed it on the rear. Now Holley did this a couple of ways. When the mechanical secondary carb first came out they put a large accelerator pump on the front barrels and a squirter in the center of the four barrels that squirted into all barrels. Later they went to a front and back squirter and a accelerator pump on the rear float bowl.

Holley accelerator pumps use four screw to attach them to the bottom of the float bowl where they have a rubber diaphram to move the fuel by a mechanical arm which works off of a plastic cam on the throttle shaft. This plastic cam is held in one of two positions by a screw. If you move this cam or if it breaks you have to get it back in the right position because one screw location pumps it earlier than the other and it is timed to be at a particular position. There are also two check valves in each accelerator pump one allows the pump to draw fuel into the pump chamber and one opens to flow it into the carb.

Caliberations of this system are the size of the squirter, the size of the pump, the timing of the cam.

Holley is a great racing carb - makes a lot of horsepower and is adjustable with a great deal of factory jetting and equipment. Its drawbacks are its vertical float bowls which tend to leak fuel badly if not kept up and many rubber diaphrams and plastic parts that deteoriate. It is definately a carb that requires constant attention. If you take it apart be sure you get back together right.