Q: I own a 1978 BMW R100S. What’s your take on gasoline additive and octane rating? Personally I have been feeding her SP98 (best quality available here) and no additive whatsoever. What do you think CMM?
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A: We have three separate issues here. They are octane rating, lead in fuel and the addition of ethanol. I know that the US models of the late seventies did have some problems using low-octane fuels, but their idea of low-octane is very low compared with Western Europe, so I think that ordinary 95 grade should be adequate. However, we must now consider the question of lead and your bike would have been designed to use leaded fuel. It would probably be safer to use the lead substitute additive you are referring to, but you might feel brave enough to try an experiment. If your use is occasional and you don’t ride too hard and fast, then you will probably find that you will see no ill effects from using unleaded fuel. I see, by the way, that as you refer to your best quality fuel as SP98, you must be in France as SP stands for sans plomb (without lead), and thus you will not be able easily to buy leaded fuel. Your best approach for this experiment is to check the valve clearances on a more frequent basis than the service schedule demands. This way, you will be able to check if the clearances (particularly the exhausts) are becoming smaller, which would indicate valve seat recession (i.e. the valves are gradually sinking into the valve seats). If you find that this is the case, then use the additive.
Finally, I would recommend that you use ordinary SP95 as opposed to SP95 E10. E10, as you probably know, stands for 10% ethanol (rather than 5% which is standard now in France I believe), and I think it would be better to avoid it if possible as you could find problems with corrosion in the fuel tank and carburettors, and damage to fuel lines and other rubber parts which are exposed to fuel in normal use.Enjoy more Classic Motorcycle Mechanics reading in the monthly magazine. Click here to subscribe.