It's a big day. You've finally installed that lumpy cam in your big-block street-stormer, and you're ready for the testdrive. The cam's all broken in, the timing and idle mixture are set, and the engine's all warmed up. You drop it in gear, tip in the throttle, and the engine dies.
Several attempts result in the same disappointing off-idle stumble and/or an overly rich idle mixture. You can hit the throttle hard enough to pump fuel into the engine and launch with tires spinning and everything's great-but you can't seem to execute an easy, light-throttle leave from a stoplight.Arrrggghhh!
So what's the fix? Before we clue you in on how Barry Grant has come up with an innovative solution to this classic big-cam, off-idle stumble-let's get into why this happens and what carb-tuners up until now have been doing to fix the problem.
Idle BasicsAll carburetors come with a simple yet efficient idle circuit. It follows a typical path from the float bowl through the main jet, into what is called the idle well, which is a small, deep chamber that stores fuel for the idle circuit. Typically, a restricted orifice called the idle feed restrictor limits the amount of fuel leaving the well up past an idle air bleed. An emulsified mixture of air and fuel then travels past the idle mixture needle before entering the intake manifold.
In addition to this circuit, there is a tiny slot cut into the throttle opening on the primary side (and on all four barrels if the carburetor is equipped with four-corner idle mixture adjustment). This slot is exposed to manifold vacuum as soon as the throttle is opened. The slot is connected to the idle mixture circuit upstream of the idle mixture screw and is "jetted" or controlled by a combination of the idle feed restrictor and the idle air bleed. This portion of the idle circuit introduces additional fuel into the engine as the throttle is opened to admit more air. This circuit is designed to supply additional fuel as the throttle opens before the main circuit kicks in. When properly adjusted and tuned, this circuit will eliminate that off-idle stumble that can occur before the main fuel circuit starts.
Big Cam BluesSo why doesn't this circuit work on an engine with a big cam? The answer is amazingly simple. The first result of a long-duration camshaft with lots of overlap is drastically reduced idle vacuum. Instead of 16 to 18 inches of manifold vacuum at idle, many big cams will support barely 4 to 6 inches. This reduced vacuum requires more throttle opening to feed sufficient air to the engine since there is less of a differential between manifold vacuum and atmospheric pressure.
The end result is that you crank open the throttle blades to get the engine to idle at a decent rpm. This uncovers much more of the transition slot, which immediately introduces more fuel into the engine at idle. Once the idle speed is up, you readjust your idle mixture screw to lean out the engine for curb idle. But by doing so, you're now using fuel supplied by the transition slot. Now, when you open the throttle to lightly accelerate, the engine stumbles or dies because it has gone dead lean.
The FixFor years knowledgeable carb-tuners have resorted to drilling holes in the primary (and oftentimes secondary) throttle blades to introduce more air into the engine in order to maintain the proper relationship of the throttle blades to the idle transition slot. Bigger cams with less idle vacuum require larger holes. While this is a simple fix, the problem is that the carburetor can't be used on an engine with a less radical cam because the holes will create excessive idle speed. Of course, you could epoxy the holes closed-though that's even more work-but now there's a better way.