RE--Small Engine Tach.

[Jiles]

I got a lot of information from you guys in my original post.
Now I have a question--I am a mechanic but I cannot figure out why some manufacturers state that a particular tachometer will work with single cylinder but not two cylinders.
This is the type tachometer that has a wire wrapped around a sparkplug wire.
I may just be having a senior moment but could someone explain?

 

[Brad Gyde]

[Only thing I can come up with would be like on the Onans that both cylinders fire at the same time. (I think onans are the one that do this, as well as some others)

Seems like if that were the case, you could use a form of mathematics to figure out the actual rpm though..

Brad

 

[JDseller]

The ones that are just for single cylinder engine will read half RPM on a two cylinder engine

 

[rlp in Co.]

A one cylinder engine fires once on every revolution. One spark is wasted on a four cycle engine. So does a four cylinder engine. That why my tach from a subaru reads the correct RPM. Not sure about two cylinders. It might only read half the RPM. Maybe if you put the pickup on both plug wires it would read the correct RPM.

 

[guido]

Hello Jiles,
Engine R.P.M.'s refers to the cranckshaft speed
The pulses that the engine gives to the R.P.M's gauge are as many as the cylinders.
Some older tach and duell had a switch for each cylinder amount. remenber a 2 cycle engine fires every time the piston comes up.
Four cycle engine every other time.
For a 2 cycle engine you will have to read 1/2 the reading on the tack, and also the tach will have to have the function for the amout of the engine cylinders you are reading from.
Guido.

 

[Dusty MI]

On a 4 cycle engine no matter how many pistons/cylinders each piston fires once for every 2 revolutions of the crank shaft.

Dusty

 

[CJ3b_jeep]

Every four cycle, 1 cylinder engine fires once every two revolutions. Only a two stroke fires on every revolution. I can't answer why some tachs won't read a one cylinder though.

 

[JDemaris]

They are used on electtric motors also, not just four-stroke-cycle and two-stroke-cycle engines. They usually can read RPMs from 2000 up to 30,000 RPMs. Different makes have different numbers written on them, but they all do the same thing. They work on electtric motors, lawn mowers, chain saws, 1,2,3,4,5,6,8, and 10 cylinder engines, two-stroke, four-stroke, gss, diesel, etc.

If you buy one from a small engine company, they will sell you a version with numbers on it for a particular engine type.

An example of a more general version. Vibra-Tak instructions:

2 stroke engines - divide reading by number of cylinders

4 stroke engines - divide reading by half the number of cylinders

For electric motors - read directly with no conversion

They will read RPMs on anything as long as vibrations can be sensed, but you can't always go by the RPM scale numbers written on it. That scale converts cycles to RPMs for small engines. It measures harmonics. You extend the wire unti it vibrates the most.

I've been using vibration tachs for over 40 years. They are also used for testing electric starter motors that don't have ANY pistons firing, and work fine.

Funny thing is, they used to cost $3 apiece and all came from Germany. Seems most still come from Germany, but the prices have gone up.

 

[Kentb of SWMO]

The tach's that connect to the spark plug wire sense the field of the spark not the firing of fuel in the cylinder. Most single cylinder engine built today fire the spark plug EVERY time the piston comes to TDC whether the engine is a 2 or 4 stroke cycle. Also FLAT TWIN engine also fire BOTH PLUGS EVERY TIME THE PISTONS come to TDC. Vertical twins like the THD Wisconsin also fire this way. Most V-Twin engine have ignition system that are separate for each cylinder that are basically like the single cylinder engines. The only engine you will get in trouble with are engine that have the points driven from the camshaft like the K-series Kohler and some of the early cast iron Briggs engines. These engine will only show a reading of 1/2 the RPM they are turning.
When I am checking the RPM of Battery ignition K-series Kohler's, I use a Sears Bach/Dwell meter set in the four cylinder/low RPM. In this setting a K-series Kohler turning 3600 RPM will read 900 RPM.

Kent

 

[rlp in Co.]

I meant to say the spark plug fires every revolution on a one cylinder engine. One spark is wasted. So it is the same as a four cylinder engine.