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Message: <blockquote data-quote="MSGT-R" data-source="post: 24877" data-attributes="member: 229">Yep, everyone is on the right track and adding good points along the way.Hight velocity is needed at low RPM to keep the fuel droplets in suspension from the carb to the combustion chamber. Choke is needed while the engine is cold to compensate for the wetting down of the passage walls like fog on a cold mirror; heat aids vaporization (that can be good and bad depending upon how much and where). Intake passages must never be polished to a shiny finish because of this. Go crazy on the exhaust, but this will eventually carbon up again.Most airflow from carb to chamber will travel the fastest around the outsides of each bend, the most critical being in the "pocket" just above the valve seat. The tightest turns and the worst obstructions (valve stem) are here, and grinding can either improve or destroy the performance of a cylinder head. Shaping the valve seat angles further aids in air flow past the valve.Can I complicate things further? The airflow past the valve depends upon the amount of lift, the shape of the back side of the valve, and its closeness to the chamber wall. Hemispherical chambers are the best shape.Most head castings on motorcycles are quite good and often only a port-match to the mating surfaces and/or gaskets is needed. Occasionally you will find a really crappy one and you have to be careful on how much and where you can grind without busting through to the cooling jacket. Bad juju. Here's a qucky in-bike porting (exhaust) that I did last weekend on my KLR250. The port was way smaller than the gasket and tail pipe diameter. It was like a child with tonsilitis it was so small. I took almost a handful of aluminum out of it. I'll refine it later when I have the head out of it.<img src="https://p1.bikepics.com/pics/2011/04/11/bikepics-2186001-full.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></blockquote>

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