Ignition System Conversion for a 277° Firing Order 650 Twin:
A simple ignition system conversion for a 277 degree or 270 degree firing order twin cylinder engine.
The stock XS 650 Yamaha twin cylinder engine ignition system and most aftermarket ignition systems have a waste spark that fires on the cylinder that is not on the power stroke at the same time the power stroke cylinder actually fires. That waste spark alternates from cylinder to cylinder and fires when the engine is in a position called overlap. That is when the exhaust valve is closing, and the intake valve on the same cylinder is opening as the transition from the exhaust stroke becomes the next intake stroke. The two phases overlap, hence the name. Now remember through the explanation below that what is often overlooked is that when the 360° twin has both pistons at top dead center and the motor is on overlap on one cylinder that the actual ignition firing time is 40° before this minds eye TDC position, so the motor is actually on overlap with the intake valve open more than the exhaust valve. So it does not fire the waste spark when the pistons are exactly at TDC, but 40 degrees before top dead center in this twin cylinder design.
Now, in a 277° crank and valve train rephrased twin cylinder engine, when the original number one cylinder (left) is at Top Dead Center (TDC) on the compression stroke, the number two cylinder is exactly at maximum lift on the intake cam with the intake valve wide open at exactly 83 degrees after TDC and the piston is very near the maximum velocity point about halfway down the cylinder. The exhaust valve is getting close to shut but still open. OK, since the timing is firing at 40° before number one is at TDC on compression, on the re-phased number two cylinder the intake valve is open and approaching maximum lift and the exhaust has not closed yet so both valves are actually still open, even though the center point of overlap has passed. So now you back up 40° for the waste spark firing and the intake is open much more than the exhaust valve is, but the motor will fire the waste spark and work just fine like this on the 277° rephrased motor. It fires just like it did with the original design, at a different transition point in the overlap event, but there is no compressed mixture cylinder pressure to ignite.
In a performance motor, fresh fuel and air mixture is ram tuning in on overlap, the exhaust valves get red hot in a race motor, (brand new steel valves instantly turn blue), and the exhaust pipe at the cylinder head port outlet is glowing orange at times, and nothing ignites on overlap on these motors either. This is because there is no compression of the mixture, no matter what position of overlap the cam timing is in.
The 277° rephrased crank motor ignition change is really simple and can be done with one stock Boyer ignition system by simply moving one of the hall effect trigger magnets.
The ignition magnet trigger assembly goes on the left hand side of the engine on the end of the camshaft. So, I set up an exact magnet relocation method with a degree wheel on the crank that is at the arbitrary figure of 40° btdc on number 1, then I rotate it over to 40° btdc at the same place on number two cylinder and dial in the magnet at zero on the indicator again from the top face of it. The slack in the hole is enough for minor adjustment movements after I end mill (file) it with a new slot for the trigger magnet. That is, after you get out the white epoxy locktite stuff that they safety glued it in with originally. This ignition system change is surprisingly easy.
Top dead center for #1 cylinder is at 0° on the degree wheel above, TDC for #2 is at 83° on the degree wheel above, but is at 277 degrees after number one fired at 40° before TDC.
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