reading spark plugs

[Bobs]

Al, I just ordered a TRM 231 Stage 1 for a Spectra G build but my question pertains to a new G260 (3 gallons of fuel through it) I'm running in a Bergen Intrepid. I'm running Coleman fuel with 5 ounces of Blue Marble to a gallon (per Bergens recommendation). My low speed needle is open one and a quarter turns and the high speed is open about one and three-eighths. While hovering (1650 head speed) I'm still getting four-cycling now and then but the performance seems pretty good: no bogging on full power climb-outs. Also, after landing when I check the engine temp right below the spark plug the hottest it has ever registered is 235 degrees, with it usually right around 220-225 degrees using a laser temp gauge. After the first two gallons of fuel I put a new RZ7C spark plug in. Now, after a gallon of fuel, the insulator still looks white but the metal around the base of the plug is black. Is this normal when running Coleman fuel that the plug does not show any tanish color on the insulator? Due to the engine temps and the intermittent four-cycling I know the motor is still running on the rich side. I just want to know if it's normal for the plug not to start showing some discoloring other than white. Even the original stock plug after two gallons of fuel showed no discoloring on the insulator but the outer metal base on the bottom of the plug was very black. Is what I'm explaining normal because of running the Coleman fuel? Appreciate any advice you can offer. On my next gallon of fuel I'm switching over to Amsoil Sabre using a mixture of 50:1. If I were using gasoline, would this discolor the plug more than the Coleman? Thanks.

[bigrcr]

The Coleman may cause a motor to run slightly hotter than on gasoline..no big deal.
The plug will mostly stay a caramel color if the mixtures are correct and yes, there will be a black part of the plug. This black color should be color only and not be oily or sooty. If your plug is all white you won't hurt it if you were to riched-up a bit. One and 1/8 seems a bit lean on the high end to me.
Make sure your G26 does not have a W677 carb on it..

Hope this helps!

[Bobs]

Thanks, John. My high-speed needle is set at one and three-eighths. While hovering I still get four-cycling every now and then and it's very noticeable when I back off the throttle while descending. My carb is a W643. I flew again today and the engine temps immediately after landing are right around 200 degrees right by the spark plug. My understanding is that these temps are very low for a gasser. In Idle up 1 (1750 head speed) I do not get any four-cycling. I don't understand why the insulator stays white when I know it's still running on the rich side in normal mode. And with the engine still being relatively new I have not been running in Idle up mode except to adjust the carb. How much fuel needs to be run through the engine before a new plug should start to show a change in color? I now have a full gallon of fuel through the motor with a new plug and the insulator is still a dull white color. It's kinda confusing because while the plug is saying it might be lean, the sound of the motor is saying it's rich and so is the temp. And I'm not really complaining because it starts very easily, idles smoothly and the performance is just great: no flat spots, bogging or any complaints. I just think it needs to be leaned some more to eliminate the four-cycling and the plug says no.

[bigrcr]

It will not hurt to see where the needles actually are and there is a simple technique. First take the high needle and rich-en it a small amount at a time and go fly around. When the performance falls off noticeably, stop richening and go back to the last good point. Do the same for the low needle keeping in mind that if the engine will not idle at first it is not a big deal as the correct mixture may require an adjustment of your throttle trim. The main idea is to ensure that the engine has enough fuel. Once you have completed the above routine, you will know for yourself where the needles are.

With that in mind, the engine may be running on the sweet spot of the low needle and be a bit rich on the high; hence the occasional four cycle you speak of. Of course the opposite may be true as well (sweet spot on high needle and lean on low).

Just because the porcelain is white does not mean that the motor is too lean. Its like this: One indicator of being lean is being white, the plug still being lightly colored does not mean in itself that the motor is too lean. At optimum circumstances, the plug would have none to nearly no color change meaning that the plug is hot enough to burn the deposits and the impurities in the fuel and products of combustion are not being deposited. Of course the fine line to operating at that condition may not be desirable and it would be far too easy to cross the line into lean territory!

A quick lap around the field on a leaned down needle should not hurt if you are paying attention, especially if you perform the above routine so that you know where your needles are before-hand.

No problem with that carb.

Hope this helps!

[Bobs]

Thanks, John. I'll do a little more experimenting but I think I'm close. I have tried richening the low speed needle from where it's set at now (one and a quarter turns open) but it four-cycles very noticeably then and the idle trim has to be raised to keep the motor idling. What about the engine temps? Good or bad indicator of how the motor is running? Do my temps sound normal, high or low? I'm just curious if I'm in the ball park. My nitro motors are running at 200 to 220 degrees and I've read that gas motors run a lot hotter, like 300 degrees and even hotter. All my readings have been taken right next to and below the spark plug.

[bigrcr]

I do not use engine temperature to adjust the engine as it really does not tell you much other than what the temperature is as opposed to another point in time and is only a relevant measurement. Exhaust gas temperature would be a much better indicator.

Don't get me wrong, you can get some very useful data reading the temperature and you are going about it correctly by reading it where you are and always reading it in the exact same spot. If in that same spot the engine always reads 265 degrees when you know it is running correctly and with good mixture settings but today it reads 300+, that is very useful in pointing out the motor is hotter than normal. Is the rise in temperature mixture related?; airflow related?; or something else?

Later,