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another small engine question


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#1 thom OFFLINE  

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Posted April 29, 2016 - 03:27 PM

I remember an argument/discussion among coworkers from a few years ago. I say that a Briggs and Stratton, and others, fires on the exhaust stroke as well as the compression stroke. Since the timing is controlled by the crankshaft, and not the camshaft , how can it not fire on every rotation of the crankshaft? Which brings up another question ... would a tachometer that requires no connection show the correct RPMs or show twice the actual speed? Inquiring minds gotta know.


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#2 grattone OFFLINE  

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Posted April 29, 2016 - 03:44 PM

It fires on the power stroke

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#3 larrybl ONLINE  

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Posted April 29, 2016 - 04:09 PM

OK I'll bite. 4 Stroke Single Cylinder will fire on the power stroke, and Exhaust stroke. Reason being the Flywheel magnet and Coil pickup sensor pass each other every (one) revolution. Two revolutions are required to complete all 4 cycles. This is also true for twin cylinder engines.

 

Ignore this if it is a Points and Condenser setup.   


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#4 DougT OFFLINE  

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Posted April 29, 2016 - 04:16 PM

 

Ignore this if it is a Points and Condenser setup.   

Depends if the points are operated off the crankshaft or the cam. Briggs made engines both ways. If the points are under the flywheel they operate off the crank so those would fire every time over.


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#5 James Bosma OFFLINE  

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Posted April 29, 2016 - 04:35 PM

It is called wasted spark when engine fires every revolution.

There are also many 2 cylinders that use the wasted spark system.

1 cylinder on power and other cylinder on exhaust


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#6 James Bosma OFFLINE  

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Posted April 29, 2016 - 04:38 PM

 

 

Ignore this if it is a Points and Condenser setup.   

 

 

Depends if the points are operated off the crankshaft or the cam. Briggs made engines both ways. If the points are under the flywheel they operate off the crank so those would fire every time over

 

I have a Honda 305 Dream motorcycle, not a lawnmower but it has points and condenser, but is a 2 cylinder and uses the wasted spark system, It has 1 set of points and condenser, 1 coil with 2 spark plug leads


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#7 Bruce Dorsi OFFLINE  

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Posted April 29, 2016 - 06:30 PM

Doug T  has the correct answer ! 


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#8 grattone OFFLINE  

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Posted April 29, 2016 - 07:25 PM

Gentlemen, I'm not sure but I was under the impression that there is a timing relationship between the magnets and when the points open that keeps it firing at the top of the compression stroke with both valves closed. And that is why we make sure that the valves are closed and the piston is all the way up when we set the points. I will check the first chance I get and see how many times the plug fires when I crank my 10 hp Briggs through by hand. 

You got me.

Grat



#9 Bruce Dorsi OFFLINE  

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Posted April 29, 2016 - 08:20 PM

Gentlemen, I'm not sure but I was under the impression that there is a timing relationship between the magnets and when the points open that keeps it firing at the top of the compression stroke with both valves closed. Correct. .....That is why a partially-sheared flywheel can throw off the ignition timing on some engines. 

 

  And that is why we make sure that the valves are closed and the piston is all the way up when we set the points. Some engines require the ignition to fire before the piston reaches Top Dead Center. 

 

I will check the first chance I get and see how many times the plug fires when I crank my 10 hp Briggs through by hand. 

You got me.

Grat


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#10 ducky OFFLINE  

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Posted April 29, 2016 - 09:09 PM

Thanks Bruse, I think you have this zeroed in.  Points open =coil fires be it a single cylinder or a twin where we fire both cylinders one with waste spark.



#11 MiCarl ONLINE  

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Posted April 29, 2016 - 09:23 PM

As far as the tachometer goes:

 

Many inductive tachometers have a setting for how many times the plug fires/revolution.  I've got one I use in the shop.  I keep it set to once per revolution which works on wasted spark motorcycles (most of them) and 2-strokes.  I have to remember to switch it for some of the old Hondas that have points driven by the camshaft.

 

It makes no difference if it's a points/condenser, CDI, transistor or computerized ignition system.  If the timing input is on the crankshaft it will fire the plug every revolution.


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#12 ol' stonebreaker OFFLINE  

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Posted April 29, 2016 - 10:14 PM

  As far as tachs go the best $$ I ever spent was for my HF phototach. Just a little piece of reflective tape on the crankshaft, pulley or flywheel cover screen. It's accurate checked against a 1750 RPM electric motor.

                                      Mike


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#13 speedbird OFFLINE  

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Posted April 30, 2016 - 04:58 PM

I keep learning I'm never to old to learn something new.   Glad to hear there is another tractor guy with a 305 dream. Here in Ontario too!


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