Auto Parts: Vacuum secondaries, diaphragm pumps, vacuum secondaries

Question
What can cause the vacuum secondaries on a 450CFM Holley carb not to open?  It is on a mastercraft boat.  It is a marine carb.  It is 2 years old.  The spring looks fine.  The diaphragm pumps air.
Any help would be appreciated.

Answer
Hi Robert,

Two things will cause a set of vacuum secondaries to not open. First is the choke-lock. Your engine can damage itself if, while it's 'cold' it is spun too fast, too quickly, so the choke has an arm that attaches to the choke coil that blocks the secondary throttle plates from opening until the choke is fully opened.

Second, the secondary throttle plates are mechanical whereas the choke and metering rods are vacuum retarded with a dashpot meaning that they are held shut by the introduction of vacuum to the little diaphragm that you state is pumping air (I know what ya mean), when the engine is idling or running up to about 1/3 throttle they are held closed and when there is a lack of vacuum, such as when you are more than 1/3 to 1/2 throttle and the engine needs the additional air, the secondary choke plates will open which also opens the metering jets allowing more air and more fuel to get dumped down the intake.

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Thanks for the question, I hope this information was helpful to you.
Fred
Knucklebusters Used Auto & Truck Parts, Inc.
www.knucklebusters.us
570-453-0910
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