Ask Away! with Jeff Smith: How to Tune a Holley Street Avenger Carburetor for High Altitude Operation

My 670 cfm Holley Street Avenger carburetor on my 427ci Cobra engine (small block Stroker) has never worked properly since I have owned it. Just as your articles on tuning Holley carbs describe, it is very rich at idle. It’s like peeling onions and the exhaust makes my eyes water. It also has that infamous stumble when accelerating.

I tried to slightly open the vacuum secondaries on the carb, but that did not work, so I progressed to removing the carb and, sure enough, the transition slots on the primaries were way more than the 0.030 inch maximum you recommended. I then closed the primary throttle plates to expose 0.025 inch of the transition slots as your article described. This required basically backing off the idle screw completely which let the plates rest against the throttle bores.

I then drilled two 0.078 inch holes in the primary throttle plates (primaries only) and then re-installed the carb on the engine. This resulted in a high idle which I was able to bring down by turning the four corner air mixture screws in. This was all making sense—the transition slots were almost closed, but the only problem was nothing changed as far as rich smelling exhaust and it still has the off-idle stumble. The engine idles at around seven inches of vacuum at 950 rpm.

Also, I’m not sure if this makes any difference but I live at 9,200 feet above sea level. I know the altitude affects power but not sure if this is the same thing. Can you help? 

C.B.

The above letter and my answer are based on a combination of about five emails back and forth between this reader and the author. I condensed them to make the question and answer more concise. Luckily, he did mention his location at 9,200 feet which makes a huge difference in carburetor tuning, so let’s start there as this is a subject that gets very little attention.

Altitude & Its Impact on Your Fuel Mix

To begin, nearly all carburetors are tuned to operate between sea level and 2,500 to 3,000 feet, because that range represents the altitude where the majority of the population in this country live. As altitude increases, the air becomes thinner and contains less oxygen for a given volume. Engines operate by processing a volume of air often measured in cubic feet per minute or cfm. But if that same volume of air contains less oxygen, then it’s essential to reduce the amount of fuel mixed with that lower oxygen content to maintain a similar air-fuel ratio.

The beauty of electronic fuel injection is that sensors constantly measure the existing air pressure so when the engine is operated in higher altitudes, the system will (in theory) compensate. Carburetors, however, don’t enjoy the benefit of pressure sensors or digital control. Because a carburetor is a simple mechanical device, it must be modified when operating at higher elevations. This means that if you use your Holley Street Avenger at 9,000 feet, the air is radically thinner with less oxygen and all the metering circuits must be appropriately leaned.

Some Carburetor Science

We all know that air pressure at sea level is 14.7 psi(a). The little “a” refers to absolute pressure. A normal pressure gauge like a tire gauge will read zero in normal atmosphere so this value is referred to as psi(g) or gauge pressure.

What is sometimes called uncorrected air pressure at 9,000 feet of elevation drops from 14.7 psi(a) at sea level to barely 10.5 psi(a) at this higher altitude. That’s a radical 28.5 percent drop in air pressure. That means not only is there less pressure pushing air into the cylinders, but more importantly there are fewer oxygen molecules squeezed into a cubic foot of air because there’s less pressure forcing these oxygen molecules together. With less oxygen content, we must mix less fuel to the same volume of air in order to maintain the proper air-fuel ratio.

In this particular case, we have a carburetor that is tuned to operate at between sea level and 3,000 feet that is now operating at three times that elevation with significantly less oxygen available to combust the fuel. This means we need to radically lean out the carburetor. In reference to your experience, this means not only reducing the main jet size by roughly 9 to 10 jet sizes or one jet size per 1,000 foot of altitude increase, but we also need to decrease the feed jet size for the idle circuit and we will likely also need to increase the size of the idle air bleed at the top of each venturi.

Carburetor Tuning Basics

Now that we’ve covered the theory, let’s start with some basics regarding the carburetor itself. You need to make sure the float levels are correctly adjusted and that the power valve(s) are not leaking. A high float level will make an engine run rich and a blown power valve will also leak excessive unmetered fuel into the intake manifold. Assuming both these circuits are adjusted and working properly then we can move into the modification stage.

C.B. mentioned that he’s already drilled air bypass holes into the primary throttle blades and we’ll assume that the transfer slot is mostly covered up. Ideally, we want that slot closed off altogether as this will help reduce the amount of fuel introduced at idle which should eliminate that off-idle stumble.

You may also enjoy this article: How to Fix a Rich Idle on a Big Block Chevy

Adjusting a Carburetor to Compensate for Altitude

What we need to do now is drastically reduce the size of the idle feed restrictor in both the primary and secondary metering blocks. This jet establishes how much fuel is metered along with air from the idle air bleed at the top of the carburetor on its way to the idle mixture screws. The idle feed restrictor diameter will generally be around 0.032 to 0.035 inch

There will be two idle feed restrictors in each metering block with a restrictor for each of the four idle circuits. In most Holley carburetors, these idle feed restrictors are small brass cups with a fixed orifice. The quickest way to reduce the size of this restrictor is to place a length of small diameter wire inside these brass restrictors. This will reduce fuel flow to the idle circuit. In your case, I would use a single strand of multi-strand electrical wire that is roughly 0.017 to 0.020 inch in diameter that is about 3/8 inch in length.

Use a magnifying glass to make sure you place the wire inside the hole in the restrictor. It must decrease the area of the flow hole that is already pretty small—roughly 0.032 inch. The wire will drastically reduce the flow of the idle circuit. We’ve included a photo of two different Holley metering blocks and the two positions these idle feed restrictors are located.

Once these restrictors are in place, carefully reassemble your carburetor making sure the wires don’t fall out and then bolt the carb back on the engine. Before starting the engine, reset the idle mixture screws to roughly 3/4-turn out from fully seated and install a vacuum gauge to monitor manifold vacuum. Once the engine is warmed up, if the engine is idling too fast or too slow, adjust the idle rpm with the curb idle screw until you have the desired rpm.

We have found that very small adjustments to the four idle mixture screws can make a big difference. We prefer to use 1/32-turn adjustments—this is roughly the thickness of the screwdriver blade slot in terms of movement of the idle mixture screw. Note the effect of leaning all four idle mixture screws on idle speed and manifold vacuum. If this change results in a higher manifold vacuum and a higher idle rpm, then lower the rpm with the curb idle screw back to the desired rpm. If the idle speed drops, then you will need to go out past the original 3/4-turn setting and adjust richer and see what effect that has on the manifold vacuum and idle speed.

What you are shooting for is the highest manifold vacuum at the desired idle rpm—in this case, 950 rpm. Keep adjusting all four idle mixture screws and curb idle speed in small steps until you find the ideal combination. If the idle mixture screws are less than 3/4-turn out from fully seated, then the wire placed in the idle feed restrictors may need to be larger to restrict more fuel.

Altitude Considerations

As you can see, there’s quite a bit to modifying your Holley to run at higher altitudes. This also includes reducing the size of the main jets and perhaps even the power valve restrictors. We will assume you have already changed the main jets leaner. Main jet sizes do not affect the idle mixture since they operate on separate circuits.

These procedures should produce a significantly leaner idle circuit which will allow the engine to run at a proper 13.0 to 13.5:1 air-fuel ratio which is what you should shoot for. This is lean enough to minimize carbon buildup in the combustion space that is common with rich-running engines.