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Orange Coloured Spark Plug.


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I've rejetted my Thor several times (float bowl carb), with corresponding adjustments to the needle and air screw and got it running 100 percent better than it used to be. However, with the final tweak which has made it run the best yet, I am getting an orange plug. I've never come across this colour before and wondered what others thought. My first thought was to do with the oil (I use Motul 800) perhaps having some dyes which leave a residue which were invisible as it was oiled up previously on factory jetting. I don't have a CHT gauge but the flight which produced this colour plug had the engine sounding and running perfectly. Any thoughts on an orange plug would be appreciated.

Edit: The porcelain is orange on 50 percent of it, the other half isn't.



Sorry about the blurred shot, iPhone refused to focus after the first shot.

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Personally, I'd get mix slightly rich in winter then its good for all year.

Unless you are chasing economy records!!

CHT gauges are pretty good for long term checking, but they are slow to respond.

EGT is faster to indicate 'issues'.

AFR is really the only way to go, but lambda sensors and two stroke emissions do not happy bedfellows make......

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Thanks for all that, certainly AFR and EGT just seems to impractical just for a bit of leisurely flying. CHT is easy and is on the shopping list, probably the Polini gauge. I will take a look at upping the main jet to be on the safe side, although I would still love to know what makes the plug porcelain orange.

Any thoughts on why half the porcelain stays relatively clean? When a new plug goes in after the first flight half of it (and the rest of the plug) are pretty spotless.

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Generally, when the AFR and plug heat range are correct, plugs are pretty much self cleaning.

It makes me cringe when people start spouting off about 2T's oiling plugs at low revs, it just means they cannot sort the low speed circuit of the carb - heathens!

Hence checking the base of the porcelain for correct mixture/jetting.

Cutting plugs does get expensive though!

Many factors can affect the colouring build up on the plug. The way the mix is introduced into the cylinder from the transfer ports, squish gap, piston/head shapes.....many variables!

When we run cylinder heads with changeable volume inserts (VHM are things of beauty), even the direction of the earthing strap facing the exhaust port comes into play!!

Another good check of mixtures is checking the flame/burn pattern on the piston crown (piston wash).

OK, I admit it, I do have a thing for 2T's.........sorry! :oops:

The Polini gauge looks very well made, but lots of wonga.

There was an article from an American car racing site I found recently. It reall did go into much greater detail about mixtures etc and reading what the plugs were telling you.

Even down to the different areas on the plug and what is affecting them, from timing, heat range, mixture.....geeky but very informative.

The plug is basically a window into the heart of the engine. You just have to know what you are looking at.

The good old days of leaded fuel were great for plug chops, this unleaded junk is a right PIA!

Also, the choice of 2T oil has some small effect on the readings.

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Most lambda sensors should work on a 2T, but not for longterm due to the hydrocarbons expelled through exhaust!

They coat the sensor and stop the residual Oxygen causing a chemical reaction between the different metals in the sensor. This generates an electrical current.

The later types use a heating element in the sensor to speed up the reaction times on first startup.

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