Mechanical or vacuum secondaries?
Holley, Edelbrock or Quadrajet?

Dan'l

you wont get enough load on the trailer to work the secondary's. sorry.

I've heard that you can place a small zip-tie around the secondary rod loosely so that it can move with little force, then when the secondaries open you tell how far. 

Or mount a GoPro.

was testing my brothers commander with a holley 750 vacuum sec carb.  we did have the engine cover off but was pretty fun to just reach back and grab the secondary's open by hand.  you will definitely know when they come in (hang on! ) haha

Ya I was doing the same it made me realize my secondaries had not been opening at all the second I cracked it open I took off like a rocket been running the damn thing with 50% power lol

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No he means to place the zip tie around the rod that goes from the carb to the diaphragm. Slid the tie up to the bottom of the housing.  I have used a little paper clip as well, then after your full throttle run you can look back and see how far it was pushed down the shaft.  This will tell you how far the secondary's are opening while the motor is running.


Also, i read the instructions for that unit --

https://www.holley.com/products/fuel_systems/carburetor_components/choke_and_vacuum_secondary_components/vacuum_secondary_parts/parts/63-1QFT --

it says it still has a "purple" rate spring in it and the screw only operates how fast it can open.   

Have you ever had it open to verify that the spring hasnt been swapped out?

Holley makes an adjustable vacuum break for retrofit.  Not prohibitive priced either.

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