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Old 05-21-2014, 01:33 AM
J-Mech J-Mech is offline
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Default Technical Tuesday, May 20, 2014: Listen to the parts, Part 1

Ok, sorry it's Wednesday I know.... I was busy yesterday.

When working on your motor do you listen to what it is telling you? Think about that . There are so many tricks to reading or “listening” to an engine that it isn’t possible to teach or even discuss them all, so let’s just touch on a few. This will be the first part in this series.

Spark plugs:

It was mentioned in the last TT article about reading a spark plug. Here’s some stuff you may not know. It can tell you A LOT about how well, or not so well, your engines “health” is. Spark plugs can tell us more than you might think. Here are four of the most common conditions you may run into when working on your cub.

A lean condition:
"Bottom electrode" (curved) is white. Center insulation is white, no black anywhere on the plug.
A lean condition is caused by (maybe obviously) the engine not receiving enough fuel. This can be surprisingly caused by one of two things. Either the carb is not set right, or is plugged, or there is a vacuum leak in the intake section of the motor. A lean condition just means that the amount of fuel is not enough for the amount of air. So, checking for vacuum (suction leaks) around the carburetor is just as critical as checking the carb itself for problems. Too lean runs really close to another condition. Overheat. If an engine isn't receiving enough fuel, they run hotter. Too hot will look similar on the plug as a lean condition. On an air cooled engine, it is better to run rich than lean as it tends to overheat easier since there is no coolant.

A rich condition:
A rich condition is pretty easy to tell. Nice and black on the plug.
Usually accompanied with black smoke out the exhaust. Most generally this is a simple carb setting, or failure. Sometimes the float setting is too high. Or the float valve isn't shutting. But, just as in the lean condition, it can also be caused by not enough air. Could just as well be a plugged air filter, or even in extreme cases a plugged exhaust, or even a valve problem. Like mentioned above, it just means that the fuel going into the motor is too much for the amount of air that is also going in.

Burning oil:
oil fouled1.jpgOil fouled 2.jpg
Here's a common one. Burning oil. We've all seen it. The brown crusty build up. We know what causes it, worn cylinders, bad rings, plugged crankcase vents and the like. But, what you may not know is a plug that looks very similar to this may also indicate dirt entering the combustion chamber. If you pull out a plug that looks like the first pic, but don't seem to have an oil consumption problem, check to make sure the air filtration system is intact and sealed properly. Also, for those of you who run fuel additives like MMO, or Sea Foam, too much of a good thing is possible. If you run that stuff, you could possibly foul the plug. Use it sparingly, and I suggest not every time.

Some swear by it, but it can:
~Cause carbon build-up
~foul plugs
~plug exhaust

Like most things, sometimes is ok, but all the time can be bad. Personally, I don't use additives, unless I'm trying to diagnose a problem, or prolong a repair. Occasionally I use them to "clean".

Pre-ignition/detonation/spark knock:
This one usually doesn't make it this far on our single cylinder cubs. Motor quits running long before it gets this bad. But, pre-ignition/detonation is a problem and can be caused by different things. Here they are in the order I usually see them.
~Incorrect timing
~carbon build up in combination with incorrect fuel settings
~incorrect heat range spark plug
~Incorrect grade of fuel
~Internal engine damage from prior repairs/problems that didn't get corrected (Such as, dents/dings in piston top from say, a butterfly screw that got sucked into the engine.)
~Fuel additives

My suggestion if you see this type of wear on the plug. Replace it, and tune up your engine making sure that you get the timing and fuel set correctly. For added measure, it doesn't hurt to pull the head and just check the internals. Heads are easy to pull, and gaskets are cheap compared to engines. If this condition isn't corrected, it is the most damaging. Here is what happens when pre-ignition goes un-checked.....

damaged piston - timing.jpg

Lastly, here is about the most perfect "text book" picture I could find on Google for what a plug should look like. Slightly tan, not white on the tip of the lower electrode, no build-up, not black (at least in the center) gap still close to correct. Very nice example.

Spark plugs can tell us a lot more, but these are the most common things that in the “cub world” you will see.

Notes, tips and tricks:

I never throw out old parts until the job is done. Just never know when you might need to go back and look at them. Or when the new ones come, you might need to compare parts. I usually just get a box, and set it where I’m working and throw all the old parts in it until I’m done. When the job is complete and out the door, then toss them.

Next week part 2:
Head gaskets and valves.
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