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slow450

Four stroke single cylinder motorcycle spark plug firing

17 posts in this topic

On a four stroke single cylinder motorcycle does the spark plug fire on each revolution? Since the coil is triggered off the flywheel it would seem that the plug fires on every rev not like an automobile engine that fires on every other rev. The real question is if you use a portable tachometer that is triggered off the spark plug wire is the indicated RPM (meter reading) the actual engine RPM or is the indicated RPM half of the actual engine RPM?

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Yes it does.

On the portable tach, or an hour meter, you would tell it if it is 2 or 4 stroke and how many cylinders.

Ride safe

Ian

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the vast majority of the 4 stroke engines on the market well only fire on the power stroke. All modern high perfornace engines use a ingition modual that controls the spark. However, there are a few smog motors out there that will fire on both power and exhaust stroke. THis helps burn any unburnt fuel in the motor. More than likly your bike is only power stroke so you are good to use a induction rpm tester to measure your rpms.

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one more note. Most dirk bike engines do not time the spark plug of the coil. The coil is a charging sytem that provides the energy for the coil to fire. They are not a magnetic pick up like your timpical briggs or 5hp honda motor.

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Does a four stroke KTM only fire on the power stroke? How does that work if is triggered off the flywheel.

Slow

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Gotta disagree zuch. How does the ignition module know which stroke the crank is on.

If it has a crank trigger it's going to fire every stroke unless it has some sort of sensor on the cam. Some bikes, like XR200's have the ignition trigger on the cam and only fire on the power stroke.

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the vast majority of the 4 stroke engines on the market well only fire on the power stroke. All modern high perfornace engines use a ingition modual that controls the spark. However, there are a few smog motors out there that will fire on both power and exhaust stroke. THis helps burn any unburnt fuel in the motor. More than likly your bike is only power stroke so you are good to use a induction rpm tester to measure your rpms.

You are incorrect. The motor does fire 2 times. It fires once on compression and once on exhaust. The pick up for triggering the spark is on the flywheel and triggers a spark. The ignition does not care if the motor is on intake, exhaust, power or compression stroke. Infact, to prove this you could put the bike on top dead center, remove the cam chain and turn the crank over 180 degrees, put it all back together. The motor will fire right up with out problem. :thumbsup:

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Gotta disagree zuch. How does the ignition module know which stroke the crank is on.

If it has a crank trigger it's going to fire every stroke unless it has some sort of sensor on the cam. Some bikes, like XR200's have the ignition trigger on the cam and only fire on the power stroke.

you are absolutely correct. The pick-up is mounted righ on the head behind that cover that is marked CDI on a XR200 and many more honda motors.:thumbsup:

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Here's where I want to go. It's 2 1/2 hours from me. Looks awesome. This forest service road travels along a ridge on one side of the Linville gorge.

 

https://youtu.be/Vh5yCsS8n7I

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On a four stroke single cylinder motorcycle does the spark plug fire on each revolution? Since the coil is triggered off the flywheel it would seem that the plug fires on every rev not like an automobile engine that fires on every other rev. The real question is if you use a portable tachometer that is triggered off the spark plug wire is the indicated RPM (meter reading) the actual engine RPM or is the indicated RPM half of the actual engine RPM?

four cycle engine spark plugs fire every other revolution a two stroke fires every time

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Nearly all 4S bikes fire every revolution (known as a wasted spark).

Not worth the effort to make them fire only on the power stroke. To do so would requite a separate sensor on the cams. More parts, more complexity more weight, no advantage. The reason the trigger is not on the cam is due to inaccuracies in the cam drive mechanism that would cause the spark timing to be erratic.

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mkporn, on 10 Jul 2008 - 4:26 PM, said:

...you could put the bike on top dead center, remove the cam chain and turn the crank over 180 degrees, put it all back together. The motor will fire right up with out problem. :thumbsup:

 

I don't think so.

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As has been said: Wasted Spark!!!

 

1 fire per revolution, once on the power stroke and once on the exhaust stroke (the firing on the exhaust stroke does nothing and hurts nothing).

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one more note. Most dirk bike engines do not time the spark plug of the coil. The coil is a charging sytem that provides the energy for the coil to fire. They are not a magnetic pick up like your timpical briggs or 5hp honda motor.

 

 They fire on every stroke.

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Infact, to prove this you could put the bike on top dead center, remove the cam chain and turn the crank over 180 degrees, put it all back together. The motor will fire right up with out problem. :thumbsup:

 

Please do this and provide a video.  I'll prepare the popcorn.

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 Infact, to prove this you could put the bike on top dead center, remove the cam chain and turn the crank over 180 degrees, put it all back together. The motor will fire right up with out problem. :thumbsup:

 

Please do this and provide a video.  I'll prepare the popcorn.

 

I think mk meant 360 degrees. I don't think there's a possible way to even start it 180 degrees out of phase, and if you did, it would tear itself apart really fast.

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Since the cam is 1/2 speed, 180 at the cam is 360 at the crank.

Wasted spark makes great fireballs out the exhaust. Just hit the kill switch while riding and hold the throttle open. Then release. Boom.

Edited by slotracer577

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