Do I need to adjust the timing on my Isuzu KB 260 when I change from leaded to unleaded fuel?

First of all, your question is posted under motorcycles instead of automobiles. The Isuzu KB series was manufactured between 1980 and 1988 and sold under several different names. Yours could have had one of 4 different gasoline engines depending on model year, your location, and ordered options. In any case regular leaded gasoline is rated at 89 octane while regular unleaded is rated at 87 octane. Octane is a measure of the resistance to burning, and a lower octane rated fuel will ignite at a lower temperature. What this means in a gasoline engine is that a lower rated fuel than what it was manufactured for will ignite before the piston reaches the top of its stroke which results in spark knock. If you do notice any spark knock after the switch to regular unleaded then try using mid-grade gasoline, which depending on refinery runs between 89-91 octane. The knock should go away. If you insist on running regular unleaded, then you will need to retard the timing 2-3 degrees from what the factory specification says on the sticker under the hood and/or owners manual. For example if it calls for 8 degrees BTDC (before top dead center) then reduce it to 5 degrees BTDC and the knock should go away. Keep in mind this will reduce the power output of your engine since the fuel and ignition are not what the engine was designed for.



http://en.wikipedia.org/wiki/Isuzu_Faster

You should not have to adjust the timing. The timing dictates when the cylinder will receive its spark to ignite the fuel in the chamber.

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