# Tachometer firing revolution setting?



## wantboost (Aug 5, 2014)

i just ordered a tach. it has 3 modes for firing pattern. not sure which to choose....
1- fires twice per revolution
2-once per revolution
3-every other revolution


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## Normex (Feb 21, 2014)

I would be inclined to say once per revolution. Good Luck


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## classiccat (Mar 1, 2014)

This can be confusing. 

A 4 stroke crankshaft makes 2 revolutions per cycle. So you'd think that it fires ever-other revolution.

My understanding is that with most of our snowblower 4-stroke engines, there's a "wasted spark" at the top of the exhaust stroke; you're getting a firing on every revolution.

Start with "2" as Normex suggested.


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## scrappy (Dec 7, 2014)

Another vote for door number 2


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## Normex (Feb 21, 2014)

classiccat said:


> This can be confusing.
> 
> A 4 stroke crankshaft makes 2 revolutions per cycle. So you'd think that it fires ever-other revolution.
> 
> ...


A Tach measures rpm so it doesn't matter if you had a six stroke one still have to measure the # of rotation per minute of the crankshaft.
Take Care


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## classiccat (Mar 1, 2014)

Normex said:


> A Tach measures rpm so it doesn't matter if you had a six stroke one still have to measure the # of rotation per minute of the crankshaft.
> Take Care


 The engine wasn't provided nor was the type of tachometer being used.

If we're splitting hairs here, a tachometer doesn't measure RPM's, it's measuring electrical impulses. 

For example, the cheap hourmeter tachs are inductively coupled to the ignition system by wrapping the wire around the spark plug lead. If there isn't a "wasted" spark & there's only a spark at the top of the compression stroke, then there is no signal however there was a rotation. Therefore, it does "matter".


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## Normex (Feb 21, 2014)

classiccat said:


> The engine wasn't provided nor was the type of tachometer being used.
> 
> If we're splitting hairs here, a tachometer doesn't measure RPM's, it's measuring electrical impulses.
> 
> For example, the cheap hourmeter tachs are inductively coupled to the ignition system by wrapping the wire around the spark plug lead. If there isn't a "wasted" spark & there's only a spark at the top of the compression stroke, then there is no signal however there was a rotation. Therefore, it does "matter".


I beg to differ since tachs that measure rpm by electrical impulses takes into account with transistors to give the number of revolution a crankshafts does in a minute. So it doesn't matter after all, look it up.
Most small engines have a wasted spark.


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## classiccat (Mar 1, 2014)

Normex said:


> I beg to differ since tachs that measure rpm by electrical impulses takes into account with transistors to give the number of revolution a crankshafts does in a minute. So it doesn't matter after all, look it up.
> Most *4-stroke* small engines have a wasted spark.


 ^^^ there, fixed it for ya.

I'd say we agree to disagree...but we agree that the setting should be "2" so... 

Firing frequency seems to be a common confusion that I'm attempting to provide some clarity. Someone accustomed to working on distributor and/or computer controlled ignitions might be inclined to use the "3" settings on one of our small engines and worse...try to compensate a perfectly good engine


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## scrappy (Dec 7, 2014)

where's the 'like' button


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## Normex (Feb 21, 2014)

classiccat said:


> ^^^ there, fixed it for ya.
> 
> I'd say we agree to disagree...but we agree that the setting should be "2" so...


 Agreed! since there are so many variations of engines and tachs hence the multi setting one the OP has.
I have to admit I'm slow today with a cold but what do you mean with your first statement? I know I'll probably slap my head (lightly for my headache) once you tell me.


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## classiccat (Mar 1, 2014)

Normex said:


> Agreed! since there are so many variations of engines and tachs hence the multi setting one the OP has.
> I have to admit I'm slow today with a cold but what do you mean with your first statement? I know I'll probably slap my head (lightly for my headache) once you tell me.


 "Most *4-stroke* small engines have a wasted spark." <-- I added something 

Hope you feel better Normex!


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## countryboymo (Oct 13, 2014)

Now going back to the points era the points would not allow for a wasted spark would they on a four stroke cycle? On electronic ignition you would have a spark every time the magnet on the flywheel excites the coil which is once per revolution.


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## classiccat (Mar 1, 2014)

countryboymo said:


> Now going back to the points era the points would not allow for a wasted spark would they on a four stroke cycle? On electronic ignition you would have a spark every time the magnet on the flywheel excites the coil which is once per revolution.


 The points are controlled by a cam on your crankshaft...so every revolution the points close triggering a spark.


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## scrappy (Dec 7, 2014)

classiccat said:


> The points are controlled by a cam on your crankshaft...so every revolution the points close triggering a spark.


Crank trigger, no distributor.. will make a spark on every complete revolution.

How hard is this to understand.


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## scrappy (Dec 7, 2014)

classiccat said:


> The points are controlled by a cam on your crankshaft...so every revolution the points close triggering a spark.


Points open make the spark...


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## countryboymo (Oct 13, 2014)

I had a braincramp thinking about regular engines with the points operated off the camshaft. So yes either way spark. Two stroke or four. The difference is on a two stroke each spark is used but it is still only one Revolution.


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## classiccat (Mar 1, 2014)

scrappy said:


> Points open make the spark...


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## scrappy (Dec 7, 2014)

too funny


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## wantboost (Aug 5, 2014)

8hp tecumseh heres the tach

http://www.ebay.com/itm/221572067450?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT


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