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Old 10-13-2012, 11:23 AM
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On multiple carb set ups, most often a progressive linkage will work better on the street than linkage set up to open everything at the same time. If the linkage isn't progressive, the driver needs to have a good feel for what the engine needs- much like using a double pumper on the street. There's nothing wrong in particular to that- but there is a learning curve involved; not a plug-and-play deal like a vacuum secondary or progressive linkage set up.

Pontiac (and possibly others) went so far as to hook the end carbs of some of their Tri Power induction systems to a vacuum motor that acted to bring the end carbs in like the secondaries of a vacuum secondary carb. Chevy used vacuum operated Holley 2-barrel end carbs on the BBC 'Vette tripower. In both cases this was to make the system more user friendly, requiring less thought on the part of the driver.

If I had a nickle for every Holley vacuum secondary carb I've seen w/a screw in the secondary throttle arm to make it into a "mechanical secondary" carb, I'd have a at least a couple bucks.

The problem w/this is there's no active accelerator pump for the added airflow, so what results is a huge bog when the secondaries are made to open before they would normally. The primary accelerator pump can be made to cover some/most of the hole in the air/fuel ratio that results when the secondaries are opened prematurely- but then the carb won't work worth a damn on the street because there will be WAY too much pump shot under anything but a WFO throttle. Ordinary around town driving will foul plugs, the off idle response will be the pits and economy will be out to lunch.

There are mods that can be made on the Q-jet to get the secondary throttle blades to open w/less primary throttle blade angle, but this will often bring in the secondaries much more often and sooner than needed on the street and drivability and economy will suffer as a result.

Holley has 1:1 primary:secondary throttle linkage that will open both the primary and secondary at the same time, but this is used on double pumper carbs. Again, not for the street.
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