Small Engine Gurus: Spark question...

[Analog]

On a lawnmower type engine, does it spark on every revolution? i.e. being a 4 stroke engine, do you get spark during exhaust, or does it spark every other revolution?

 

[geno]

They're 2 stroke motors. Every revolution includes a compression/combustion

 

[woodie1]

Think all the B & S 4-stroke air cooled engines spark every revolution.

 

[geno]

Originally posted by: woodie1
Think all the B & S 4-stroke air cooled engines spark every revolution.

They make 4 stroke lawnmower motors? Eh, I guess I'm out of the loop when it comes to small engines I guess

 

[notfred]

Like others have said, lawnmowers should be 2-stroke motors, hence they spark every revolution. 4-stroke motors only spark ever other revolution.

 

[woodie1]

Originally posted by: notfred
Like others have said, lawnmowers should be 2-stroke motors, hence they spark every revolution. 4-stroke motors only spark ever other revolution.

Why does the B & S engine have a camshaft and valves if it is 2 stroke? Also, you do not mix oil with the gas and it has a crankcase with oil that should be changed at least every season.

 

[teddymines]

Originally posted by: notfred
Like others have said, lawnmowers should be 2-stroke motors, hence they spark every revolution. 4-stroke motors only spark ever other revolution.

I don't think that's right. The lobe on the crankshaft closes the points every revolution, which discharges the capacitor. So you'd get a spark just before TDC regardless of the valves' positions.

 

[kranky]

I think Eli is a small engine guru.

 

B&S engines are 4 stroke, these engines fire every other crankshaft revolution, the points open via a lobe found under the flywheel.

 

[Eli]

Originally posted by: Roger
B&S engines are 4 stroke, these engines fire every other crankshaft revolution, the points open via a lobe found under the flywheel.

Sorry Roger.

They don't really make 2-stroke lawnmower engines anymore, although I'm sure you could find one. They pollute too much and are generally an inconvienence due to having to mix gas and oil. And they're unreliable.. at least compared to a trusty 4-stroke.

Since the points are actuated off a flat spot on the crankshaft, the points close and open every revolution. That means they create spark every revolution, since the flywheel magnets also go around every revolution.

Even with the modern Magnetron(electronic) ignitions, since the device is actuated by the flywheel magnets, and.. the flywheel magnets pass the magneto every revolution, you will get a spark every revolution.

There are only a few small engines that employ systems where you get a spark every 2 revolutions.

Certain older Briggs and Strattons, like the model 9.. which is long extinct..

Kohler engines employ a system where they spark only at the proper time also... and I believe some older? Honda's.

Why? Are you looking to get a tachometer/maintenance/hour meter for your mower?

 

DOH !

I don't know what I was thinking

You are correct Eli, the points do run off the crankshaft on B&S engines

 

[Bulk Beef]

Originally posted by: notfred
Like others have said, lawnmowers should be 2-stroke motors, hence they spark every revolution. 4-stroke motors only spark ever other revolution.

Wrong on both counts.

 

[Eli]

Geeez, no wonder nobodys lawnmowers last very long, if you're all putting the oil into the gas instead of the crankcase..

 

[Analog]

Originally posted by: Eli

Originally posted by: Roger
B&S engines are 4 stroke, these engines fire every other crankshaft revolution, the points open via a lobe found under the flywheel.

Sorry Roger.

They don't really make 2-stroke lawnmower engines anymore, although I'm sure you could find one. They pollute too much and are generally an inconvienence due to having to mix gas and oil. And they're unreliable.. at least compared to a trusty 4-stroke.

Since the points are actuated off a flat spot on the crankshaft, the points close and open every revolution. That means they create spark every revolution, since the flywheel magnets also go around every revolution.

Even with the modern Magnetron(electronic) ignitions, since the device is actuated by the flywheel magnets, and.. the flywheel magnets pass the magneto every revolution, you will get a spark every revolution.

There are only a few small engines that employ systems where you get a spark every 2 revolutions.

Certain older Briggs and Strattons, like the model 9.. which is long extinct..

Kohler engines employ a system where they spark only at the proper time also... and I believe some older? Honda's.

Why? Are you looking to get a tachometer/maintenance/hour meter for your mower?

I am looking to measure the RPM of the motor via the spark using a CT. Sooo, by knowing if it sparks every rev. vs. every other rev., I could accurately clock it. Thanks for the nice post Eli.

 

There is no need to do that, go out and buy a tach for small engine use.

Tachometer

 

[Bulk Beef]

Originally posted by: Roger
There is no need to do that, go out and buy a tach for small engine use.

Tachometer

I had one of those on my race bike - twas crap. Didn't react fast enough, not that it mattered, since you shouldn't really be looking at it anyway.

 

[Analog]

Originally posted by: Roger
There is no need to do that, go out and buy a tach for small engine use.

Tachometer

Interesting, looks like by wrapping the small wire around the ignition wire, you get a small pulse that registers and is counted for RPM. Thanks Roger.

 

[Eli]

Originally posted by: yellowfiero

Originally posted by: Roger
There is no need to do that, go out and buy a tach for small engine use.

Tachometer

Interesting, looks like by wrapping the small wire around the ignition wire, you get a small pulse that registers and is counted for RPM. Thanks Roger.

TinyTachs suck.

http://home.comcast.net/~epruett3/Ebay/Engines/tachsampleauction1.html

YGPM.