The Holley 20-13 is a vacuum secondary spring kit for Model 4150, 4160, and 4500 carburetors, containing seven color-coded steel springs of different tensions. The spring controls how quickly the vacuum secondaries open, so swapping springs lets a tuner match secondary response to the engine and vehicle -- a lighter spring opens sooner, a heavier spring later. It is a must-have for dialing in a vacuum-secondary carburetor.

Holley 20-13 Kit Contents:

• Seven color-coded vacuum secondary springs
• Assorted tensions to change the secondary opening rate
• Fits Holley 4150, 4160, and 4500 vacuum-secondary carburetors
• Steel construction
• Sold as a set of 7