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How to Avoid Common Carb Tuning Mistakes

Words And Photos: Jeff Smith

So often in racing, it can be as much about what you do correctly as about avoiding the costly mistakes that prevent finishing the race. Experience is a great teacher for preventing some of the near limitless ways there are to lose a race. Among the positive steps toward winning are simple Holley and Rochester Q-jet carburetor techniques that not only prevent problems, but also improve power and throttle response.

JET Performance has decades of experience building Holley and Rochester carburetors for circle track racing, yet this knowledge base is often overlooked. None of the ideas here are groundbreaking, but they all have evolved out of the crucible of competition.

You might even roll your eyes and think “Everybody knows these” – until you discover you’ve committed at least one of these miscues in the past. We won’t ask for a show of hands, out of consideration for the guilty. Instead, consider this as a gentle nudge to go out into the shop right now to ensure you’ve covered these five simple steps to getting close to that checkered flag.

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1. Check What?

Check WOT. Yeah, that’s right. When did you last have the driver mash the throttle pedal and physically check wide-open-throttle (WOT) at the carburetor. If the carb has been removed or the engine’s been out, or there’s now a spacer under the carburetor, don’t assume everything went back in the way it came out. It only takes a moment or two. It might be worth the effort.

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2. Richer is not Always Better

The old line used to be “You can’t be too rich or too thin.” But, when it comes to race engines, too rich isn’t where you want to be. If you don’t have a wide band oxygen sensor, how to you know what the air-fuel ratio really is? The typical response is to add fuel in response to issues with a hesitation or a misfire, but common problems like these can be just as often caused by an over-rich condition.

Unlike in drag racing, throttle transition tuning is essential to modulating power coming off the corners. Too much accelerator pump fuel can make an engine lazy yet seem lean. There are multiple paths to excessively rich mixtures that have nothing to do with atmospheric changes.

A dual band air-fuel ratio meter in the car can reveal bushel loads of information, especially if you are willing to step up and data log during testing. And this isn’t just for gasoline-fed race engines. Nearly all air-fuel ratio meters will also monitor ethanol, E85, and even methanol air-fuel ratios just as easily. Perfecting air-fuel ratios (rich or lean), especially for part-throttle corner exit, can effectively lengthen the straightaway for your driver. Who doesn’t want that?

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3. Undersized Fuel Filter

So, you have a killer fuel pump capable of 90 gallons an hour at 5 psi and the fuel lines are hand-bent to minimize flow restrictions. Why, then, are you running that cheesy, take-apart piece with the surface area barely larger than a dime?

It may not be a restriction when its clean, but imagine what happens to fuel flow the moment it begins to clog up from dirt. A larger filter with a cartridge area of 50 to 60 square inches allows for more than sufficient filter area without sacrificing fuel flow.

For example, JET’s cellulose version offers 10 micron filtering capability combined with a larger filter area to prevent a restriction. If you race with E85 fuel, be aware that stainless filters should be the only choice. JET offers this as an option with 100 micron filtering capability. E85 often reacts with cellulose style filters, which can quickly clog even these larger filters.

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4. Fuel Pressure

The first important question is do you know what your fuel pressure is under load? Many racers don’t even bother to check pressure, and if they do check pressure, it’s at idle on a dead-head system with no return system. This can lead to low fuel pressure at WOT when demand is high, especially if the pump is marginal.

Alternatively, there is evidence to suggest that fuel pressure exceeding 5 psi can cause aeration in the float bowls of both Holley and Rochester carburetors. This aeration will affect fuel metering and could also be the cause of air-fuel ratio related problems. That’s why JET Performance adds larger (0.135-inch) needle and seat options for their circle track carburetors, like the Circle Track Rochester Q-jet.

A larger needle and seat increases flow without resorting to high fuel pressures. Even stock Q-jets are sensitive to fuel pressure, due to their short float fulcrum length which reduces leverage and, therefore, sealing pressure on the needle and seat. There is growing evidence to support the notion that lower fuel pressure combined with sufficient fuel flow rates and larger needle and seat flow areas can produce a more stable air-fuel ratio over a large rpm band.

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5. Power Valve Tuning

Some poor ideas die hard, and among these is the misconception the primary power valve should be blocked off in favor of richer main jetting. The truth is that much of mid-corner performance is fed by deft application of part-throttle power.

The advantage of a tunable power valve is that leaner main jets can produce crisper throttle response for early onset throttle. This leaner air-fuel ratio is then augmented at the proper time with fuel delivered by the power valve.

Adding this fuel is controlled by two factors: when the fuel is delivered and how much fuel is added. The when is triggered by manifold vacuum so the tuner can choose either early fuel (such as 8.5 inches Hg) or later (4.5 inches Hg). The amount of additional fuel is controlled by the power valve channel restrictor (PVCR).

This can be a valuable tuning tool that can be easily custom-tuned to match driver preference. Sometimes, just the suggestion of more power with a crisper throttle is as good as a real power increase.
If nothing else, the tuner is a hero – with help from JET Performance.

 


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