Our needle and seat assemblies...
Our needle and seat assemblies were modified by enlarging the internal restriction from .097 inch to .120 inch. The increased cross-section allows faster bowl filling by making the fuel flow more responsive to changes in float position.
Also addressing the driveability and carburetor's low-speed transitions, the primary to secondary linkage was modified, replacing the long link positioned at the lower linkage hole to a shorter piece engaged in the upper hole. This will delay the rate of the secondary actuation. The idea here is to delay the point of secondary engagement and slow the action for improved signal to the secondary booster prior to the secondary coming into play. Bear in mind that this is a transitional change, and at wide-open throttle the secondary still comes to wide open the moment the throttle is at the floor. Similarly, we found that the pump shot was weak at the initial portion of throttle actuation, causing a lean stumble with slow throttle engagement at part throttle. The cure here was a swap in pump cams, replacing the pink cam with the more aggressive green cam. Either cam will give the full pump shot with the throttle stomped wide open, but the green cam will give more fuel earlier in the throttle.
Short jet extensions help...
Short jet extensions help keep the main metering circuit in the fuel under both acceleration and braking by placing the jet in the center of the bowl. The longer jet extension, top, is more typical of a drag race unit used in the secondary bowl to compensate for acceleration. Carb shop installed the shorter extensions in both primary and secondary bowls.
So what did this moderate level of carb modification do for our Laguna? Putting theory to the test on the open road, we found the changes delivered as intended. We had the carb making good on all counts, from cleaning up the idle, with improved transitions and low-speed, part-throttle manners, to effectively metering even when giving the chassis a real workout. A carb can be a very efficient and cost-effective way of delivering fuel, and with the right level of know-how, it can be calibrated to cope with varied driving conditions. Based on the way the car behaves on the street, The Carb Shop has the expertise to get it dialed in.
Where The Rubber Meets The Road
We've hit a number of autocross and road race events with the Laguna this year, and one of the biggest hurdles we've experienced is keeping the fire lit during hard braking and cornering. Enigmatically, faster road courses pose less of a problem than slower autocrossing, which is why the NASCAR guys never see this issue pop up. With our Carb Shop mods in place, we noticed improved street manners right away-this is due to the more aggressive accelerator pump cam, the plain throttle blades, the slower linkage, and a change from a 10.5 to a 7.5 power valve. These mods had the added benefit of improving street driving, particularly part-throttle response, which was an unexpected bonus. The following week we took the Laguna to the Costa Mesa Goodguys autocross, and noticed the improvement right away. Some stumble is still evident-you can't eliminate it 100 percent-but it is much more manageable. Braking is where we saw the most improvement, with the Laguna experiencing not a single stall in the stop box all weekend. Along with the improvement in street manners, we're now rethinking whether we need to step up to fuel injection, which we always had in the back of our mind. We like the carb's simplicity and cost-effectiveness, and now with a near perfect tweak, we can see years of trouble-free operation ahead. -Johnny Hunkins
Where The Money Went
Although an increase in diameter...
Although an increase in diameter of .023 inch does not sound like much, the cross-sectional area open to flow increases by 53 percent. A visual inspection makes the dramatically increased area readily apparent.
The Carb Shop stripped, cleaned, rebuilt, and modified our carb for a price of $320 including parts and labor. Some of the small parts used in modifying our carb, and their individual mail-order prices (via Summit Racing) are listed below.
|Holley pump cam assortment kit:
|Throttle plates (111/16 inches):
|Nytrophyl float kit (each):