STROKEXD said:
Used a screw to wedge open secondary swing arm where the rod fits in the slot so secondary butterfly's when primaries reached about 1/2 opening. Crude but effective.
There is a basic flaw with hard wiring the secondaries on a vacuum secondary carburetor. The "double pumper" is actually a "mechanical secondaries" carburetor. The reason (we all should know this) it is called a double pumper is because it uses a pair of accelerator pumps to richen the mixture as an effect of engaging the throttle at a position (progressive secondaries) whereby the secondaries open. The vacuum secondary carburetor is a "single pumper" in that it only has a primary accelerator pump installed and no secondary accelerator pump.
When manually converting a VS carb to open, a mixture is not adequately richened and the engine could enter a rather severe lean condition until engine speed goes up enough to flow a proper ratio of fuel through the secondary metering plate or block is so converted.
With available space, a 50cc primary accelerator pump can be used to reduce this condition, especially if proper attention is paid to the accelerator pump cam. Using a Holley "Trick Kit" is about the only way to effectively tune this parameter without buying an assortment of accelerator pump cams (cheaper than a trick kit, but can be harder to source in some locations).
The goal for accelerator pump cam tuning with a 50cc pump on a VS carb would be to select the pump with a steep initial ramp so that an immediate shot is squirted in combined with a trailing ramp that is relatively "flat" and consistent so that the squirt produced lasts a long time (all the way through the throttle opening). What you want to do is get the first 20-30cc in all at once and then spread the following 30-20cc in over the entire course of throttle position. This will greatly reduce the lean area.
Of course, modifying a VS carb to be a mechanically actuated secondary carb is not what I'd consider to be a fruitful idea. It is far better to make it work as intended and tune it properly within its designed functionality. I would modify it to remove the secondary metering plate on any engine where a solid lifter cam is used or where a cam with more than about 220* of .050" duration is implemented. I would also modify it for any engine where 6000+ RPM is expected. I'd caveat this by saying that a 302W or a 289 would be engines where I doubt that it is necessary for richness, but it may be required to properly lean it without spending forever and a day on "tuning" the spring rate to open slowly and consistently enough for these smaller displacement, but higher revving capable engines.
A 302C may also apply, but since a secondary metering block kit, trick kit and associated jet set (when the inital price of the 750 VS is included) is more costly than a 650 DP, I would probably just buy the DP. If I already had the 750, I would make it work, but I wouldn't "wire up" the secondaries or some other such thing to make it "mechanically actuated."
These are really simple carburetors. They are easy to tune. Of course, tuning them is never easy without the proper tools and experience and a reasonable method and location for tuning them. There are not very many good VS carb tuners, it seems. It isn't really very difficult at all, but it does require some patience and some iteration through the process in order to get it sorted. I recommend sending Holley's tech support people a message regarding it, and they will email a tuning recommendation back. That recommendation will be a very good starting point for fine tuning it, if necessary at all.
One thing to keep in mind whilst tuning a VS carburetor is that the goal of the secondaries should be to open smoothly and consistently over a noted period of time. It isn't proper for them to just flop open, even though it may provide substantively more performance than not opening at all. It doesn't do to have them open very late in RPM and "come in all at once" either. It may "feel" faster, but this is usually a result of the hesitation created in the acceleration curve that allows ones weight to transfer forward then suddenly backwards for that "neck snapping" sensation, that is really just an interruption of consistently good/strong acceleration.
There are documents online at Holley's web site that describe the basic tuning for each of their products. Following them is easy and very inexpensive. You really only need basic hand tools and a jet set, spring assortment and sometimes a couple of different power valves and squirter options. All of these parts are found in a "Trick Kit." For a VS carb engine that sees a lot of track duty, a quick change kit for the VS diaphram spring is a good idea. A well sorted racer's log will make it easy to know what spring is going to work well on a given day for the conditions.
A most stock 302C with a couple of bolt ons like extractors and inlet may be one of those situations where you can get away with hard wiring a VS carb and not running too lean. However, it won't be making as much power as it would if properly tuned or completely converted to mechanical secondaries operation including a proper secondary flow bowl with the accelerator pump circuit designed into it.
:davis: