I publish this here for comment from our learned members, before submitting as a possible FAQ. I'm thinking it might go in the carb FAQ. Comments?
Reading Spark Plug Deposits for Mixture Readings -- Author: Lloyd Oliver
There are lots of internet sites that explain this in more detail than here, found by search engines. But, most are for liquid cooled engines whose cylinders are mostly held at a constant temperature by the coolant flow.
First a little background info:
Spark plug deposits are supposed to burn off with the heat generated in the combustion chamber. If the perfect ratio of fuel/air is achieved, no deposits are left behind in an ideal combustion chamber. The SOHC4 carburetors rarely achieve this as they have no means of compensating for air temperature, combustion temperature, or barometric pressure, which all effect the perfect pairing of fuel to the available oxygen in the ingested air. An engine running rich with more fuel than air, will deposit some of the extra unburned fuel into corners and crevices of the combustion chamber and the outer, cooler, walls, including recesses in the spark plug.
An air cooled engine has no means of fine temperature control over the cylinders. There must be enough fins on the motor to provide enough heat removal to keep the engine alive when the cylinders are producing maximum heat.
The engine's chambers produce maximum heat while under load at WOT (Wide Open Throttle). The engine's cooling fins are sized to remove this heat even when operating at altitude and hot weather.
All other throttle settings will generate less heat. But, the cooling fins still wick away heat like it would at max throttle, over cooling the combustion chamber walls and preventing full fuel combustion.
Idle speeds are where the engine temps are lowest (provided there is some airflow around the engine). Because the cooling fins are still sized to wick away heat at max power output. Therefore, checking plugs at that speed ought to show rich running conditions from low combustion heat. The outer ring of the plug will almost always be black under those run conditions, as the spark plug body is attached to the head and all those cooling fins.
Additionally, for SOHC4s without an accelerator pump,the idle mixtures are set to run rich so that throttle response will be good, even with 1/2 travel of the twist grip. If you set the idle mixture to be lean and give maximum heat at the plugs, engine pick up will be horrendous without some means of adding extra fuel to the engine (as in an accelerator pump).
What is too rich? Or, how to keep the plugs from fouling?
The center electrode insulator is the key. How clean and how much of it is clean is what to look at. To keep plugs from fouling, the deposits on the center electrode insulator must be absent for at least twice the spark gap distance from the center electrode at all throttle settings. Or, carbon build up will provide an alternate path for the current flow instead of jumping the gap, causing misfires.
Another variable is the plugs heat range. The length of the center electrode relates to how quickly the combustion heat is transferred from chamber to the engine fins. Combustion temps and resultant deposits are also effected by this selection.
Of course, fuel mixtures provided by the carbs will certainly factor into deposits of unburned fuel.
For an air cooled motor, the spark plug outer ring should always be black unless the engine is running at peak power output. The threads and body of the spark plug are at the highest temperature of combustion in this regime, but still lower than the core combustion temp. Deposits will still happen but their color will be slightly lighter than during other operational regimes.
The real intersting part of the spark plug, for reading-the-mixture purposes, is the center electrode insulator, and how far away from the center electrode you find deposits and its appearance.
As noted previously, the readings have to be balanced with the excess cooling effect the fins have on the engine.
Ideally, at loaded WOT, with the throttle slides all the way open, there should be little deposits fairly deep (maybe half way) into the spark plug, yet there should be no 'glass globules" found with a magnifying glass near the electrode tip. And, of course no melting of the electrodes. The latter two signs are of excessive heat. Either a richer mixture and/or colder plug will be desired. A richer mixture change can be effected by a larger orificed main jet. There is good rationale for setting these first as this determines the max fuel delivery required by the engine.
When we back off the throttle to 3/4, the mixture is determined by both the main jet and the throttle valve. However, the peak heat output of the engine is now lower due the fact that the engine is making less power. Yet those cooling fins are still pulling heat out of the motor lowering combustion temps and increasing spark plug deposits somewhat. The center electrode deposits should show a gradient color change where deposits show a bit closer to the electrode tip on the center insulator. A carburetor mixture adjustment at this throttle setting relates to the diameter of the slide or throttle valve needle inserted into the throttle valve jet. Since the needle is tapered, raising or lowering the needle in relation to the slide position can be effective, as can changing the taper angle of the needle or diameter of the needle at that point of it's insertion. This is significant to understand, since the throttle valve meters between 1/4 and 3/4 throttle position. Needle taper and inserted positions are interactive at mixture control.
In general, spark plug deposits should be light tan at 3/4 throttle position. And, the needle position/ and or taper are the primary adjustment means for mixture control. at this setting.
As the throttle is backed down, so do combustion temps, as the cooling fins still draw out the heat near the rate of an engine at max power. So, given the proper mixture delivery, less of the center electrode will be clean along the depth of insulator, and the deposits will darken towards brown, deep into the spark plug insulator recess.
As mentioned before, Idle to 1/8 throttle is where the least combustion heat and power output of the engine occurs. The big fins are still wicking away heat. Extended periods of operation in this range (assuming mandatory airflow around the engine) will show the darkest deposits extending up the center insulator towards the tip. This mixture setting ought to be on the rich side as mentioned previously. However, it must be lean enough so the combustion temps are reached that will allow deposit burn off at least twice the spark gap distance away from the center electrode on the insulator. This range of operational mixture control is from the slow circuits. Fine adjustments are done with the air bleed screws (or Idle Mixture Screws on some carbs). Larger adjustments are made with slow jet orifice size changes.
By now, you should realize that an engine driven in normal use operates in all these throttle setting and spark plug deposits get "averaged". Simply pulling a plug that has been used for a while, can only give general indications. For example, a plug that is completely covered in soot can tell you that one or more throttle positions is extremely rich operating. Further testing is required to determine root cause and whether one or more metering devices in the carbs needs adjustment or if some other factor such as float operation, choke malfunction, air filter restriction, air jets clogged, etc., need addressing.
Inspection process:
Reading the plugs is done after a "Plug chop". The engine is warmed to operating temperature, stopped, and clean plugs are installed. Then the engine (under load) is run for 15 seconds to 5 minutes at a specific throttle setting. (The variance is due to the heat generation during combustion and the number of firing cycles in a given period. WOT operation needs less time to collect deposits than idling.)
Simultaneously, the kill switch is engaged, and the clutch pulled in, to stop the engine. The plugs are then extracted and inspected with a magnifying glass for deposits and condition. If not within desired parameters, adjust carbs for compensation of that operating range, and do another plug chop. The process is reiterative for the various throttle settings.
