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Castor oil based 2 stroke premix oil


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I've just done a top end decoke on a friend's (low hours) 2022 TRS 300 and was astounded at how tenacious, dry and thick the carbon deposits were that completely covered both the piston crown and the inside of the head. It ranged from 0.25 to 0.5mm thick. The tenacity and hardness of the carbon in this TRS reminded me of the carbon that formed in my 1970s air-cooled two stroke enduro bikes running on 20:1 castor oil premix.

Sure enough, when I then asked the owner what premix oil they use, they told me it is castor oil based.

I'm accustomed to seeing soft, wet smudges of carbon that cover about 40 % of the piston crown on my 2 stroke trials bike pistons.

Is using castor oil premix in modern trials bikes common practice?

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Based on smell rather than an accurate survey I don't know of anyone using castor in anything other than a few old bikes like BSA.  Our last trial was 80 riders and not a whiff of castor exhaust fumes.   I can't think of any reason to use rubbish old technology other than nostalgia for the distinctive smell. 

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In John Robinson's 2T tuning book, he classified an oil's load-bearing capability based on the results of Falex and ZN wear tests.  In addition to two modern oils (Motul 800 and Silkolene Pro 2) the highest-rated oils were castor based.


So for engines having the very highest specific power outputs (e.g. road racing) using a castor-based oil is not completely foolish.  But a trials bike does not fall into that category.  And castor's drawbacks are, as mentioned, quite a nuisance.


At one point in my life I was very interested in the chemistry of combustion.  I reasoned that castor oil could also act as a chemical supercharger since it is an oxygen-bearing fuel.

Edited by konrad
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