Rotax Max Spark plug fouling

Has anyone got any input on my spark plug that keep fouling on rotax max 125 ?

I’m using denso 27 plugs and sometimes denso 24 ( I think this is the correct heat range for the temperature in the uk about now )
My fuel mix is 30.1
My carb is cleaned
Float level is 3mm
Needle position is 3
I use the rotax app to help with the main jet selection on the day ( normally between 162 and 168 )
The 2 stroke oil I’m using is Shell Advance Racing M 2 Stroke Kart Oil
I’m not idling for long and I don’t think I’m driving to slow :smirk:

Should I change the brand of spark plug ??

I have a new fully charged YUASA battery YT7B-BS 12V 6.5AH

Iv just purchased rotax own brand of 2 stroke oil to see if this helps . ( not tried it yet )

Can anyone tell me if I’m doing something wrong or if I should be doing something eles.
Fouling spark plugs are making my karting outings very expensive with £12 ish a plug .

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This is my first thought. You should find it helps

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Have you set the oil ratio in your jetting app. I usually run 40:1 with rotax oil. The denso plugs should be fine.

Iv set the oil ratio on the app.
I’ll try the 40.1 ratio next time out see if it helps :+1:

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Is it fouling to the point that it’s shutting down? Or is it running OK but always shows black/wet plugs?

Shell M is a great castor based oil, but it’s really not suited for Rotax. If you haven’t been cleaning the power valve it’s worth inspecting it now as an oil like shell M is more likely to gum it up.

Rotax oil at the correct ratio should help with the oiling of the plug.

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Oh, you can clean a fouled plug with a blow torch. Once a plug gets into its heat range it’s self-cleaning. Failing that, a good spray of ether (easy start) should get it clean enough to go again.

Do not use a wire brush

The kart runs fine when the plug is new , then once iv had 10 to 15 mins round the track and go in for a rest 20 / 30 mins, the next time I go out on the track it wouldn’t start or only make it half way round the track before it shuts down. Then the only way to start it is fresh plug then the circle starts again
Thanks for the tip on cleaning the spark plug that will definitely save me some penny.
The plugs don’t look to bad when they foul that’s why I’m confused about why it’s happening

Sounds good. Let us know how it goes. What track in the UK?

I’m surprised it’s shutting down based on what you e said.

The track yesterday was lockwell hill. Not a very big track, Iv had the same problem at pfi international and whilton mill tracks.
I’ll be going out on the kart again next week with the new rotax brand 2 stroke oil in the fuel mixture. So maybe that will stop the plugs fouling

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I would say that you are usingvwrong oil with erong ratio. Why are you using 1:30? Rotax is made for 1:50. You are running too much oil and why racing m? This oil is not 100 % syntetic and is made for engines which rum high rpm. Like 16000. So change the oil and mixture and your problems will be solved. Rotax rpm range is very low and the oil you use isnt burned properly.

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Have you come across folks that are racing at 50:1?

Everybody here in europe is racin 1:50. Some teams use 1:40 but not with every oil. Castrol xr77 is suitable for that for example. There is no benfit of going lower than 1:40. If you use 1:30 you will have problems past 12500 rpm. The ignition changes in timing and alot of oil will be left unburnt. In this range you want to have the engine as lean as possible. I run my engines 1:50 and didnt have any problems.

I do not know how you gasoline is, but here we have a pretty good standard of pumpgas so there is also no need of racing fuels

I run my denso spark plugs for hours without any problems. Only the cap has the tendency to go loose. Check it regulary


It seems that experimenting with the oil amount might be more pervasive in the US maybe.

Unless you’re chasing a national championship, run 40:1 and a synthetic oil.

You can use castor in a Rotax however only if you’re chasing every last tenth and are prepared to strip your engine down with a builder very frequently. And in no circumstance would it be Shell M for Rotax.

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Why should you run castor even if you are searching the last tenth? It gives you no benefit in rotax, only disadvantages.

1:30 only if you’re running the engine in after a rebuild. Otherwise 1:50

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Thanks for the input guys .
Sounds like I need to change the oil Iv been using and the mix ratio and hopefully that should stop my plugs fouling.
I don’t know what I’d do with out this forum. Iv always had my question / problems solved by people who take time to help me
Thanks again :+1::+1::+1::+1::+1::+1: