10 Biggest Carb Mistakes

You may have decided that your buddy doesn’t know squat and you have a better chance of getting it right. Good for you! Now don’t blow it by thinking that jetting the carb the same as the guy in your car club who “runs real fast” will make your ride fly. Slow down and RTFI (that’s Read The Friendly Instructions.) It goes against car-guy mantra but instructions are not meant to be thrown away. They have (and we’ve read them all) very good details on how to get your carburetor running well. Once you’ve read them, go online and search the manufacturer’s websites for tech info and tuning instructions. Edelbrock.com and HolleyTV.com both have an amazing series of carburetor short movies showing everything from basic installation, to jetting, to advanced tuning techniques.

Often, factory engines are designed with what appears to be a spaghetti factory for vacuum lines. Some of those are critical, and some aren’t. Either way, mark every single one of them prior to removing your old carb, and look at their function. Keep an eye out for which vacuum lines use ported (to distributor) vacuum and full manifold vacuum. On the other end of the spectrum, if the car is destined for racing only, QFT’s Zach Baker says: “I see people run PCV valves on motors that have huge-by-large camshafts in them. They just don’t work that well. It’s an emissions piece and not a performance piece.” Look for an alternate means of removing crankcase pressure.

While not technically a carburetor problem, choosing the wrong camshaft, intake, compression, or exhaust will often result in the car owner pointing the finger at the new shiny thing on top that’s easy to get to and adjust. No amount of carb tuning can fix a poor engine combination. The best you can hope for is masking the problem slightly. The best fix is a preemptive one. Call the tech lines for the carb and cam companies to be sure the combination is copacetic.

Again on the edge of being called a “carb” problem, but what we’ve learned is that most carb problems aren’t really carb problems at all. If there is insufficient ignition advance, a carb will act lazy and rich, often leading to the tuner leaning out the idle mixture as a bandage when correcting the timing will result in a much better fuel and power curve.

Seat-of-the-pants tuning—and even dragstrip tuning—can only get you so close to the optimal power on a street car. If you want to get the most out of your street car and you don’t mind spending a few hundred dollars on dyno time, a chassis dyno (go eddy current, not inertial—we like SuperFlow or Mustang) will get you maxed out. Make sure they have a wideband air/fuel ratio data logger and someone with a good track record of dyno tuning carbs. In this Westech dyno session of the ’75 Laguna project car with the original solid-roller 408ci small-block, we gained 12 rear-wheel horsepower by checking and adjusting the throttle linkage for max opening, then leaning it out four secondary jet sizes to put us at a solid 12.8:1 air/fuel ratio.

Both major carburetor types are designed with idle mixture screws to set the fuel flow at idle. For the most part, these are right on the money. Once again, when running a big camshaft or some combination that is anything but normal, sometimes the idle speed and idle mixture need to be adjusted. For example, on a 4150-style carb, if the engine needs the idle speed screw adjusted more than about a turn, the rectangular transition slot will be exposed to the point where the main jets are providing the fuel and the idle mixture screws (exiting the round hole) won’t react to changes. You can verify the idle speed adjustment by this simple test: If you can screw the idle mixture screws all the way in and the engine keeps running, the transition slot is overexposed. The easy cure for this is to remove the carb and flip it upside down (after draining the fuel) and opening the secondaries slightly via the small Allen head set screw on the underside of the baseplate. Reinstall the carb and go through the normal idle setting procedure again.