Harley Ignition Systems

Twin Cam engines fire both plugs at the same time but only one plug is in a position to fire a fuel charge and make power with it as opposed to a single fire system where each plug is fired independently of the other at the proper time in the compression stroke.

Single fire systems use one coil to fire both plugs as compared to older engines that had two coils and each coil would fire one spark plug.

When the wasted spark in the single fire system is on the rear cylinder it's just at the start of the intake stroke. The intake valve is wide open and if you ignited on that spark you'd get a backfire through the carb. Also the piston on the rear cylinder is descending so you would not get a kick "back" but a kick "forward".

When the wasted spark is on the front cylinder, it's during the exhaust stroke. Again... a valve is wide open but in this case it's the exhaust. You'd get a backfire out the exhaust if the spark ignited anything.

nomercymike · Jun 30th, 2009, 10:56 PM

What tells the coil to fire: the crank or the cam? If the trigger were on the crank wouldn't it always fire every 360 degrees and always on the rear or always on the front cylinder?
I'm just trying to figure this out and your diagram and explaination was nice but I don't understand how it knows to fire at 30 degrees TDC for each cylinder, even though I understand the other cylinder is a wasted spark whether it's at 75 TDC (exhaust stroke)or 15 ATDC (at the very beginning of the intake stroke) 30 TDC for #1 cylinder is 75 degrees away from 30 degrees TDC #2 cylinder. Please tell me it's triggered by the CAM!!

glider · Jun 30th, 2009, 10:59 PM

It depends on the bike and year how things happen.

The older bikes had a cam sensor in the nose cone to fire things off but the newer bikes use a crank sensor up by the oil filter to fire things off now.

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Jun 30th, 2009, 10:56 PM	#1
nomercymike New Family Ride: 87 FXSTC Join Date: Jun 30th, 2009 Posts: 1	Re: Harley Ignition Systems What tells the coil to fire: the crank or the cam? If the trigger were on the crank wouldn't it always fire every 360 degrees and always on the rear or always on the front cylinder? I'm just trying to figure this out and your diagram and explaination was nice but I don't understand how it knows to fire at 30 degrees TDC for each cylinder, even though I understand the other cylinder is a wasted spark whether it's at 75 TDC (exhaust stroke)or 15 ATDC (at the very beginning of the intake stroke) 30 TDC for #1 cylinder is 75 degrees away from 30 degrees TDC #2 cylinder. Please tell me it's triggered by the CAM!! Last edited by nomercymike; Jun 30th, 2009 at 10:59 PM.

Jun 30th, 2009, 10:59 PM	#2
glider Community Liaison Ride: 07 Road King Classic Join Date: Jun 21st, 2007 Posts: 21,044 My Mood:	Re: Harley Ignition Systems It depends on the bike and year how things happen. The older bikes had a cam sensor in the nose cone to fire things off but the newer bikes use a crank sensor up by the oil filter to fire things off now.

Harley Ignition Systems

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