Rochester Sekundärstufe

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Technical Explanation: Help! My Secondaries Don’t Work!

Here’s one of the most common “complaints” I hear. “My secondaries don’t work. I don’t get that jerk when they open, and the diaphragm is holding them closed!”

There is almost nothing that can prevent the secondaries from opening on a Q-Jet. As described in the previous section of this paper, the secondaries are purely mechanical. If the throttle cable pulls them open, they will open. The airvalve will open if there is mass flow demand to open the airvalve. The choke pulloff will relax and control the airvalve opening rate upon loss of manifold vacuum and allow the airvalve to open. Very few things can prevent this sequence of events.

When “testing” for secondary operation, many people make a couple of basic mistakes. First, many will flick the throttle momentarily wide open with the engine running in neutral. In this condition, the secondary airvalve will seldom open since manifold vacuum never drops enough, or long enough, to allow the choke pulloff to relax: The engine must be under load with a high air mass flow rate through the carb in order for the secondary airvalve to open. You cannot “wing the throttle” in neutral to get much, if any, indication of secondary operation.

The second presumption of secondary operation is a “kick in the butt” upon secondary opening. Actually, if the secondaries are operating correctly, the transition into the secondaries will be so smooth that there should be no “kick” feeling at all – only smooth acceleration. What many people regard as “that secondary kick” is an improperly set up secondary airvalve which actually causes a momentary hesitation before “catching” and pulling. This incorrect operation will first throw you forward, and then throw you back in the seat. Although you may be able to impress your teenage son with this, the car is not running as fast as it should with such an incorrect setup.

There are 3 issues that can actually prevent secondary opening:

1. Secondary lockout lever staying engaged. There is a lockout lever on the secondary throttle that will prevent the secondaries from opening before the choke is wide open. I.e., the secondaries are not allowed to open when the engine is cold, thus preventing engine damage from the engine being placed under excessive load before being properly warmed up. The secondary lockout lever can lock out the secondary throttle if the choke is not opening fully, or if the choke is incorrectly adjusted. It is very easy to observe if the lever is retracted and if the throttle is operating – simply look at the lever and verify that it is allowing movement of the secondary throttle shaft when the choke is open. Some carbs, such as Pontiacs and pre-’68 Chevys, use a lockout lever on the secondary airvalve rather than on the throttle shaft. Same principle applies – simply look at the lever and make sure it is retracted when the choke is open.
   

1. Mechanical jamming of the secondary airvalve. There are 2 things that can commonly cause this:
   

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   1. Distortion of the airhorn casting at the rear airhorn attached screws. It’s common to see these two, long, rear attach screws grossly over-tightened, and this will actually bend the airhorn around the screws. When the airhorn bends in this area, it can jam the secondary airvalves and prevent them from opening. If you see that the airvalves are touching & “catching” the casting in this area, you can do 2 things:
      

i. Take a fine file and gently re-shape the distorted area. You need to remove the carb to do this to avoid getting filings down into the intake manifold.