[beatr911]

Well, let it be said that "It all depends". How far one can advance or retard cam timing depends on the engine configuration.

My experience (not just what is stated on this forum) leads me to believe that there is some room to play with cam timing on nearly every OEM engine design. Adjusting the existing cam closer toward your specific goals is entirely possible, usually with minimal cost. Remember that power output will be negatively affected at some point in the operating range. You decide what you can live with.

We are not after maximum power output here, but reduced fuel use under daily riding conditons.

Sure if one wants to spend more money, have a custom cam made, fit high compression pistons, fill and reform the ports for high velocity, etc. It's a personal choice for the goals of each person and disposable cash availability. Also, to reach the goals you'll need knowledge of how to make it all work together for best BSFC at daily riding power outputs. Most engine builders focus on maximum output which is not the same thing.

For many motorcycles, the duration of the intake cam approaches the duration an Atkinson Prius engine. I think that retarding the intake cam only, on a carbureted motorcycle engine is not likely to work due to carburetor pressure confusion. Others should try this to verify or contradict my experience to add to our collective knowledge. It costs nearly nothing and is very unlikely to harm the engine.

Retarding ONLY the intake cam reduces overlap with the exhaust, allows reversion back to the intake at low speeds, reduces dynamic (engine running) compression, allows more spark advance, increases intake valve distance to piston at TDC. Retarding both keeps overlap constant, allows reversion back to the intake at low speeds, reduces dynamic (engine running) compression, reduces exhaust cycle blow down, allows more spark advance, decreases exhaust valve distance to piston at TDC, increases intake valve distance to piston at TDC.

Advancing ONLY the intake cam increases overlap, increases dynamic compression, may require ignition retard, decreases intake valve distance to piston at TDC. Advancing both keeps overlap constant, increases dynamic compression, may require ignition retard, increases exhaust blow down, decreases intake valve distance to piston at TDC, reduces exhaust valve distance to piston at TDC. This may or may not be a big deal on your engine especially if it is operated slower which is what advancing cams optimizes.

I think I got that right.

There are negatives to each but there are also positives. Only testing will determine which solution is best for your engine configuration. Think it through, adjust your cams, turn the engine over by hand to verify you do not have piston/valve contact. Try it out!