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(for some reason, this excellent site is down at this time. Here is the archived copy of this essay from Google)
Subject: Piston Ring Sticking
Problem: The chemical composition of gasoline has changed significantly during the past ten years. Most notably, the level of tetraethyl lead has been reduced from approximately 3 grams per gallon to 1/10 gram per gallon or eliminated completely.
During the same period, oxygenates and ethers were added to enhance octane and extend supplies of hydrocarbon fuels. Also. beneficial components were removed from some gasoline's because of blending and economic reasons.
The result of this evolution is that some contemporary gasoline's deposit significant amounts of carbon during the combustion process.
If these deposits are not addressed, they can reduce engine performance, foul spark plugs, cause piston ring sticking, and contribute to powerhead failures.
Solution: There are four things that you and your customers can do to minimize the negative effects of carbon deposits:
Note by Had Robinson – I am not sure these additives are the best for paramotor engine. I would recommend (and use) Chevron's Techron fuel additive to clean things up. I have had good luck with it.