Port Timing

justlearnin · Aug 10, 2004

OK so the closes I've come to tearing down a chainsaw is taking the air filter out. But I am curious. I'm seeing these questions about port timing numbers. And I don't know what port timing is. Anyone care to enlighten me? I not afraid to admit I'm justlearnin.

glens · Aug 10, 2004

Do you know how a two-stroke-cycle engine works?  In all of them (the ones "we" hand-hold while running), there are transfer passages which conduct the fuel-oil-air mixture to the area above the piston from the crankcase below; and there is the exhaust, too.  Both those systems use openings in the cylinder wall which get covered/uncovered as the piston passes by.  The intake is usually handled by either a reed valve or by another piston-actuated port (some use a crank-driven rotary valve for the intake, but I don't think those get used on chainsaws which aren't solely racers).  The cylinder-wall "valves" open and close at discreet positions of the crankshaft, and those positions are the port timing figures you're hearing about.  Other considerations are the shapes and locations of the ports, but you didn't ask about that...  The whole mechanism is, quite simply, a bit complicated.

Glen

Chris J. · Aug 10, 2004

For general chainsaw information

try howthingswork.com (I might have the name wrong). They breakdown a chainsaw, & also provide more detailed info on two cycle engines, clutches, etc. Might be kind of basic for the experienced saw guys, but there seems some useful info there.
Hope this helps.

Chris

justlearnin · Aug 11, 2004

So when they chage port timing. Are they changing the port size. Or just changing when the plug fires? And thanks for the web sites I'll have to check them out a time or two.... Wayne

glens · Aug 11, 2004

The plug firing has nothing to do with port timing, and yes they will be changing the port size (unless they're changing the timing another way, in which case the size won't change).  This is almost exactly the equivalent of altering the cam profiles/positions in a 4-piston-strokes-per-operating-cycle engine.  If you don't understand that you'll have a hard time coming to grips with this.  I'm not trying to be harsh; this is basic design stuff but the whole picture has to be in focus and this is just one segment.

Glen