kyle86 said:
I thought about that. I have 72s in it right now which is the factory jet size. I had 74s in it before which turned my plugs a darkish gray color. I cruised for about 2 miles at 45mph (2200rpm) and then cut the ignition and pulled over and then pulled some plugs out to inspect them. The middle portion of the insulator is what I was looking at with a flashlight. I havn't pulled the plugs yet since I put the new jets in.
Dark gray is just fine, excessively rich would be black, to fluffy black, to wet black.
But given the range of different metering circuits, simple plug reading isn't a sure fire indicator till the signs get out on the edges of being black from rich or stark white, to white speckled on the lean side. Between these two extremes is a range of colors influenced by startup on choke which is quite rich, idle which tends to be toward the rich side, transfer which can be rich if the throttle plates are not aligned properly, main metering which only happens, or should only happen after the throttle blades move across the transfer slot, to the power jets which should come on at about 1/2 cruise vacuum and the jets are sized correctly (typical Holley, others use metering rods which are an exercise in and of themselves).
A carb and intake have problems with delivering fuel mixture of the same ratio to each cylinder. The overall mixture has to be rich enough to keep the leanest cylinder rich enough to stay out of detonation. This why fuel injection ALWAYS delivers better economy with more power for otherwise identical engines.
Mixture temperature and engine temperature have a big influence on how the plugs look and even how the mixture shows on a mixture ratio gauge. Cold manifolds where no exhaust heat is applied can cause the plugs and a gauge to read rich while the mixture that burns is actually lean. This is because where the intake is cold and the engine RPMS low, there is a tendency for the fuel to settle out. In that case raw fuel runs into the chamber and doesn't begin to burn till quite late and then incompletely. This will give a false plug reading as rich while what is burning is lean. The same thing happens when the engine is operating under 180 degrees or has a plug whose heat range is too cold. Cold plugs are a common problem with a high cruise vacuum. High cruise vacuum is a sign of a lightly cammed and or low compression engine at low RPMs; this configuration has a hard time keeping the plug tip hot enough to stay clean again leading to a false rich look on plug color.
The cruise detonation you're seeing is an excessively lean mixture, or a hot mixture, or hot engine, or too much timing lead. A common problem with too much vacuum advance as the centrifugal is coming in. High vacuum begets high amounts of vacuum advance. Vacuum advance is there for off idle (part throttle) at low RPMs and low load on the crankshaft. Centrifugal advance is there for high RPM operation at wide throttle opening; the latter causing a low manifold vacuum. Therefore, as the throttle is opened and the RPMs come up, the amount vacuum advance diminishes in proportion to the reducing manifold vacuum. But the centrifugal starts to pick up the advance. It is tricky to time them such that too much vacuum advance and too much centrifugal advance for the engine's operating condition aren't happening at the same time. This is why these systems are adjustable. However, in the case of vacuum, one usually needs to go to the aftermarket for an adjustable vacuum cannister. Certainly and more commonly the centrifugal is thought of as adjustable. But each needs to be played with along with mixture.
Typically the only way to read plug color is to warm the engine up on a set of plugs. Then replace them with new and make a banzai run from which the engine is shut off at red line and the car coasted down in neutral to prevent pulling mixture from an ignition dead but rotating engine as this will give a false rich. Once stopped the plugs are pulled for reading. Like I said before; just pulling plugs after driving around gives some sort of average reading based upon all the carb circuits that operated and how hot the engine/plugs get. A rich look can also be the result of excessive pump pressure or a high float setting or too small an air bleed in the emulsifier circuits.
A blown diaphragm in the vacuum advance would be a vacuum leak into the engine, resulting in a lean "firing" mixture which would make the engine ping prone on very little advance or load. One, also, needs to be careful with emissions capable carbs as some have taps for venturi vacuum. This is the reverse of manifold vacuum, where the higher the air flow thru the boosters and venturies the higher the vacuum on the port. Such a hook up on a conventional engine would apply way too much advance all the time above idle, so be double sure of your vacuum source. This is not to be confused with timed vacuum which is ported below the venturi and above the throttle blade such that blade position controls whether and when the vacuum port is active. This is something you have to tune for against full time manifold vacuum and will greatly affect idle and just off idle operating quality.
This is a lot more complex process than it looks to read plugs as trail to mixture ratios as a lots going on and quite a bit of that has nothing to do with mixture ratios.
Bogie