NGK SPARK PLUG ROTAX 125 MAX evo engine series

Spark Plug Fouling

The following tips will help you to avoid misfire because of wet fouling of your NGK
spark plug.

What is spark plug “fouling”?
A spark plug is “fouled” if there is no spark anymore between center electrode and the ground electrode. Possible causes can be either oil deposition due to bad combustion or excessive fuel in the combustion chamber because of a too rich mixture. The latter is called specifically
wet fouling.


Measures to prevent spark plug fouling:

• Start the engine as late as possible:
Long idling periods before leaving the starting grid will foul the spark plugs. Start the engine just before leaving the grid to grant a smooth running of the engine.

• Don’t blip the throttle on the pre-grid:
Try to maintain a steady load on the engine. This will avoid over fuelling and again reduce the risk of spark plug fouling.
Blipping the throttle will disable a clean combustion and thus pollute the spark plug by means of oil sedimentation.


• 2 strokes need to change spark plugs occasionally:
The spark plug for 2 strokes is a wearing part. Change them regularly. Especially in rainy and cold conditions a change to a new spark plug before the race reduces the risk of fouling to a minimum!

• Try to avoid repeated heavy acceleration and braking:
On the Warm up laps, try to avoid repeated heavy acceleration and braking at low engine speeds. When possible try to clear the engine out by revving up.

• Clear the engine:
Whenever you start the engine on the stand, try to ensure you clear the engine out before shutting down. This again will help avoid the chance of fouling the spark plug also if you maintain some load on the engine during this time. Repeated low rev restarts on the stand without clearing out will foul the spark plug.

• Spark plug gap:
A bigger spark plug gap will also increase the risk of spark plug fouling. The spark plug gap may not exceed the values stated in the latest RMC technical regulations.

Kind regards,

Your Rotax Team

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James, what do they mean by “clear the engine” as a pre shut down step?

Give it a nice rev so you aren’t letting it load up with fuel on the stand.

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A post was split to a new topic: Spark Plug Questions

Apologies for bringing up and old post, I have a question and need some clarity

What is the best spark plug to use in the FR125 Rotax Max?

I usually run the Denso Iridium IW27 mostly. Am I right in thinking, in colder weather I would maybe use the IW29 and in hotter weather use the colder plug like the IW24?

Thank you

Think you have it backwards. An IW24 is a hotter plug than the IW27 and you would use it on cooler days. The IW29’s and 31’s are cooler plugs to be used on hotter days.

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This is what I wasn’t completely sure of.

When we say hotter or cooler days… to what degrees of temperature rise or drop would I then decide to change the plug away from the IW27? What would be the common running temperatures? What negative effects would the wrong plug have?

Caveat: This was on a non-evo engine, road-raced in the Mid-West where there were long straights run at full throttle. Plug and jetting were done via the Jet-Tech program.
For ambient temperatures between 70-80F, we usually ran a IW29. In the mid to upper 70’s, would move to the W31. 80’s and above is where the W31 was always used. Below 65, the IW27 was used. Never got to run the IW24.
There is no absolute point on when to move up or down a heat range because there is a wide overlap between plugs. On short tracks, I think I would have used the IW27 instead of the W29.

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