Tuning a carburetor doesn't require a mechanical engineering degree by any means, which isn't to say that it doesn't take a bit of knowledge to properly set one up. If you're tuning for the street, then you'll need to take heed of idle quality, including off-idle throttle response, which is addressed this month in "Just the Right Size" on page 40. On the other end of the spectrum, tuning for all-out maximum performance at the dragstrip is a whole other ball game.
This month, we spent the afternoon at California Speedway in Fontana, California, dealing with less than ideal track conditions and sweltering temperatures well over 100 degrees F. Braving the heat with us was Matt Sendejas and his small-block-powered '71 Nova. What's interesting about the Nova is that Sendejas had literally finished piecing the car together the day of our testing. It had yet to make a single pass down the quarter-mile and while it was a slight gamble-OK, a big gamble-it was one that paid off, making everyone involved look like heroes.
To facilitate our testing, we enlisted the QMP crew out of Chatsworth, California, who own and operate one of the premier engine-building facilities in the San Fernando Valley. Considering these guys compete in the NHRA Comp class with a naturally aspirated Monte Carlo that dips into the mid-7-second range, we were confident they would be able to tune Sendejas' street/strip juggernaut.
Now it's important to note that the results you see here are very specific to our '71 Nova, and we can attest to the fact that every car will be different, depending on individual combinations including engine, suspension, and choice of transmission and gearing. Let's not forget that grand variable, Mother Nature. In our case, it was hot and miserable, with the density altitude ranging anywhere from as little as 3,500 feet to well over 4,300 feet. Nevertheless, we've provided all the details and results with the carbs' fresh-out-of-the-box and the numbers after being tuned by the professionals. All said and done, nothing blew up and we made rather remarkable gains. The only drawback was the nasty sunburn, but hey, it's all in a good day's fun and we've already set aside a date for the next outing.
Q&A with QMP
CHP: What's the biggest difference when it comes to tuning for the dragstrip?
Mike Consolo: I think it's much easier to tune at the dragstrip. You're just trying to get the right jetting and fuel flow, whereas on the street you're more concerned about off-idle qualities and having to deal with things like the accelerator-pump spring adjustments, squirters, power valves, and the accelerator pump cam. On the strip you're only worried about the initial leave and the performance at wide-open throttle.
CHP: There's a general tendency to block off the power valves in a drag-race application. Why is that?
MC: The power valves are instrumental to controlling daily drivability concerns. When you're at full throttle, the power valve isn't doing anything. By blocking them off, it gives you complete control of the fuel through the jets.
CHP: How do you adjust the mixture screws?
MC: I start by turning each screw all the way in and then backing them out by one and a half turns. I'll then turn a screw until the rpm fluctuates; nothing drastic, mind you, just until it wants to stumble, and I'll turn them out until the motor has a nice idle. Once I know the number of turns I'll set the other three to a similar setting. Also, my personal feeling is you should never have any of the screws vary by more than a half-turn.
CHP: We know every tuner has his own way of adjusting carburetors. How about you? What do you generally like to do?
MC: I like to open the butterfly a little bit before the squirter, but to do that I also run a larger squirter to eliminate potential stumbles. You may not see the hesitation by simply revving the engine, but when you leave off the line, it helps the initial hit of the throttle by supplying it with additional fuel. Again, for all of this to work, there is a fine balance of infinite adjustments that need to be addressed.
CHP: Have any additional tips to eliminate carb stumble?
MC: If you adjust everything and still have problems, check your timing. It's a real basic tip, but it's not uncommon to overlook how the timing is being ramped in. If you're still using the stock springs supplied with an aftermarket distributor, you may need to swap over to a lighter spring to ramp in the timing sooner. When you first roll into the throttle, if you only have 15 degrees of timing coming in at 3,500 rpm, it's going to cause a problem. The lighter spring will bring in the timing sooner, say, at 1,500 rpm, where you really need it.