Ask Away! with Jeff Smith: Bogged Down on a Blow-Through Carburetor

I have a 355 c.i.d. small-block Chevy to which I’ve added a small Vortech centrifugal supercharger. The supercharger blows into a hat over the carburetor. For now, the engine is carbureted so I am using a Quick Fuel blow-through carburetor. I’ve got the engine running good at idle and it hauls ass at full throttle, but when cruising at part throttle, the engine runs very rich. I’m wondering if I have the boost-reference hose to the fuel regulator in the right place. I have it hooked to manifold vacuum. Is this right? Do you have any other suggestions? — T.D.

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Jeff Smith: There are a couple of issues here that require some explanation. The direct answer is to move the boost reference line to the hat over the carburetor.

But there are other forces at work as well, so that’s why this long-winded answer is crowded with additional details that are important to understand.

My friend David Page at Fuel Air Spark Technology (FAST) once told me how to remember the proper way to reference a fuel pressure regulator.

His rule is in regard to EFI but it relates to your application as well. The rule is if your injectors are located below the throttle blades, then the regulator needs to be connected to manifold vacuum. If the injectors are located above the throttle blades like with a throttle body (or a blow-thru carburetor), just leave the regulator open to the atmosphere.

The reason for this is that with the injectors below the throttle blades, the manifold pressure at idle and part throttle is actually a vacuum that is well below atmospheric.

With low pressure in the manifold, the pressure regulator needs that reference to maintain a proper balance of fuel pressure across the fuel injectors. An example will help and trust us, this does relate to your carburetor question.

Engine vacuum is most often measured in inches of mercury (“Hg). One inch of mercury (1“Hg on a vacuum gauge) is roughly equal to 0.5 psi.

If the engine is running with 15 “Hg of manifold vacuum at part throttle this is the equivalent of negative 7.5 psi of pressure.

Let’s set the fuel pressure on our multi-point EFI engine at 43 psi with the engine not running. With the fuel pressure regulator referenced to atmospheric pressure, this negative pressure below the injectors would be the same as adding 7.5 psi to the fuel pressure — or escalating the pressure to 50.5 psi. By vacuum-referencing the pressure regulator, this negative pressure in the regulator reduces the line pressure by 7.5 psi, creating the effective pressure in the manifold at the injectors at 43 psi.

With any supercharger, you already know you need to boost-reference the fuel pressure regulator in order to match the fuel pressure 1:1 to the pressure created by the supercharger.

This is true for EFI engines and also necessary in your application because as the blower pressurizes the carburetor inlet, this simultaneously increases pressure in the float bowls. If we start with 5 psi of fuel pressure, when the blower reaches 5 psi of boost (with no boost reference to the regulator), boost pressure equals the fuel pressure and fuel flow will stop.

This is why the fuel pressure must be boost-referenced. This is accomplished by running a small hose from the blower hat to the top side of the pressure regulator.

With pressure on the top of the diaphragm assisting the pressure relief spring in the regulator, this increases the pressure in a 1:1 relationship with boost and maintains the 5 psi of fuel pressure entering the bowls. At 15 psi of blower boost, the regulator will push the pressure up to 20 psi but the effective fuel pressure will only be 5 psi (20-15= 5 psi).

Now let’s look at what happens when the boost reference line is connected to the intake manifold. At part throttle let’s estimate the engine is making 10 inches of vacuum (10” Hg). This is equivalent to negative 5 psi of pressure or 5 psi below atmospheric.

Let’s also say you’ve set your fuel pressure regulator to 5 psi on a return style regulator with the electric fuel pump running but with the engine off. With the engine running at part throttle cruise at 10 inches of manifold vacuum, the actual fuel pressure delivered to the carburetor is 10 psi!

Holley recommends no more than 7 psi fuel pressure because pressure above this level will push fuel past the needle and seat. When this happens, the float level rises which will instantly make the engine run very rich. Worst case, it will push excess fuel into the engine and perhaps even flood the engine. None of that is good.

The solution is to move the boost reference line to the blower hat. This will only deliver positive manifold pressure to the regulator to match the fuel pressure to the boost, which is what you want. But at idle and part throttle with high manifold vacuum, the fuel pressure will remain at the preset 5 psi limit.

Making this simple change may not solve all your part-throttle rich mixture problems, however. Our admittedly limited experience with blow-through carburetors has been similar to what you’ve experienced despite the fact that our boost reference line was correctly located in the carburetor hat.

You mentioned you’re using a Quick Fuel Technology’s (QFT) blow-through carburetor. We’ve used these carburetors in the past on the dyno and in all cases they worked exceptionally well. These carburetors use an annular booster because this style booster is very responsive and will flow enough fuel to prevent running lean under boost. Unfortunately, these boosters are also very responsive to even light throttle opening and as a result flow lots of fuel at part throttle. The challenge becomes how to lean the main metering circuit at part throttle without making it too lean at wide-open throttle (WOT).

There are a couple of professional carburetor tuners who have got this figured out and can modify the primary main metering block to use a boost-reference power valve that will supply additional fuel under boost.

You can do an internet search to find these carburetor modifiers should you want to go that route. Reports are these modifications work very well but they do require significant modifications to the carburetor and are not something the average enthusiast can perform.

I don’t pretend to be a carburetor expert when it comes to heavy modifications on the main metering circuit. But there is a way to lean out the part-throttle circuit yet still deliver enough fuel at WOT.

The short version of this process is to reduce the size of the primary main metering jets (6 to 8 sizes, for example) while increasing the size of the power valve channel restrictors (PVCR) in the power valve circuit to make up for the reduced fuel delivered by the smaller primary jets. You can also increase the secondary jetting by two to three jet sizes to help the air-fuel ratio at WOT.

I’ve had limited success with this attempt although the engine I was working on was a very small 4.8L LS engine with a centrifugal supercharger. My issue was leaning the air-fuel ratio on the Quick Fuel carburetor to produce a desired 13.5:1 air-fuel ratio at part throttle. I think this was because the carb was designed for a much larger displacement engine and all the circuits were intended to flow lots of fuel — even at part throttle.

This situation could probably have been successfully tuned but I ran out of time as my attentions were redirected toward other projects and I never was able to get the carburetor to work as desired. Of course the easiest, but expensive, solution would be to convert to multi-point EFI and the tuning process then is a cake walk.

Hopefully this discussion has offered some insight into potential solutions. The easy part for a blow-through carbureted engine is making power at WOT. The hard part is tuning that same carburetor to work well at part throttle with a decent, mileage-friendly air-fuel ratio that doesn’t drown the engine with fuel.

NOTE: 1” Hg = 0.491 psi