I see a lot of threads on various forums where someone is trying to figure out if their Holley vacuum secondaries are really opening or not. Since they operate off I’d venturi vacuum and engine load trying to verify their operation is not really something that can be done in the driveway easily. This is one way to find out. This is on my ERA Cobra but all vacuum secondary Holleys operate basically the same way. Here is a zip tie fastened on the secondary throttle shaft and at rest, leaning forward with the secondaries closed. The zip tie needs to be pulled tight on the shaft so it has a friction fit.
At the primary throttle is opened up the bent link rod pulls forward in the slotted arm which lets the secondary throttle blades open up when venturing vacuum is high enough to activate the diaphragm pull on the other side that opens them. If after a drive and a good hard acceleration pull (probably 2nd or 3rd gear is best) the throttle shaft will rotate CCW inside the zip tie since it is resting on the carb body and cannot move. And when the throttle is released and the shaft will rotate back CW, the zip tie will now ride back with it in the CW direction and end up sticking approximately straight up if the secondaries opened fully. An easy tattletale to verify operation.
I would like to take credit for thinking this up but I learned it from a frustrated lawyer who would rather dabble with electrical circuits and mechanical gadgets. So if someone else has already posted this tip on here I wasn’t trying to plagiarize it.
At the primary throttle is opened up the bent link rod pulls forward in the slotted arm which lets the secondary throttle blades open up when venturing vacuum is high enough to activate the diaphragm pull on the other side that opens them. If after a drive and a good hard acceleration pull (probably 2nd or 3rd gear is best) the throttle shaft will rotate CCW inside the zip tie since it is resting on the carb body and cannot move. And when the throttle is released and the shaft will rotate back CW, the zip tie will now ride back with it in the CW direction and end up sticking approximately straight up if the secondaries opened fully. An easy tattletale to verify operation.
I would like to take credit for thinking this up but I learned it from a frustrated lawyer who would rather dabble with electrical circuits and mechanical gadgets. So if someone else has already posted this tip on here I wasn’t trying to plagiarize it.
