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hour meter/tach question


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so i bought an hour meter for my 04 crf450r today and in the instructions it asks how many times the engine sparks for every revolution of the motor so i can set the tach up

so my question is how many times does the engine spark for every revolution of the motor?

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One time every revolution.

It sparks at BTDC on the compression stroke, and then again at BTDC on the scavenge stroke, doing nothing.

so its 2 sparks for every RPM? correct never set one of these up and i waana get it right the first time so i can set my idle correctly as the last time i got the valves shimmed they set the idle way up like idiots:foul: needless to say last time she will see a stealership

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No I don't think that's what's being said. It sparks once per "RPM" (revolution), but there are two full rotations between each power stroke. Intake - Compression - Power - Exhaust. The piston is approaching TDC on the Compression and Exhaust strokes. So it's sparking once per revolution, but only every other spark is "important".

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One, I'm pretty sure.

I'm trying to think of a case where you would want more than one.

I remember back in the days of gapping points I had a meter where you had to specify the number of cylnders, but now I can't even remember why. Thank goodness for electronic ignition and timing!

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so when setting up the hour/tach meter i should tell it 1 spark = 1 Rpm? or 2sparks = 1 Rpm?

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I don't think you can "tell" it anything!

It is a tricky question. From how you asked it it would be 1. If you had 2 cylinders that fire at different times it would be 2.

The answer is 1.

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Not to confuse you but for the hour/tach meter I researched last night the setting for a CRF450 with FI should be set to 0.5! FYI - Spark per revolution doesn't alsways translate into what the meter should be set to. Depends on the unit.

Good luck.

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One time every revolution.

It sparks at BTDC on the compression stroke, and then again at BTDC on the scavenge stroke, doing nothing.

?? "4-stroke"..

1st revolution is intake,

2nd is compression

3rd is "boom"

4th is exhaust.

Assuming there is a scaventing spark thing.. it would be 1 spark per every 2 revolutions or am i missing something??

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Lots of confusion over here. A single cylinder four stroke engine has 4 strokes for a complete combustion event. A stroke is a movement of the piston in a direction. For every two strokes of the piston (down, then up) the crankshaft will revolve once. So for a complete combustion cycle in a four stroke engine, the crankshaft will spin twice.

Here is a little chart explaining things:

Piston Movement - - Stroke - - Crankshaft Revolutions

Down ---------------Intake--------------.5

Up----------------Compression-----------1

Down----------------Power--------------1.5

Up------------------Exhaust--------------2

The CRF's spark once per revolution, so they will spark near the top of the compression stroke, and spark near the top of the exhaust stroke. The spark at the exhaust stroke is wasted, so it is called a wasted spark ignition system. Some four strokes only spark once every two revolutions, but not the CRFs.

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got it.. thanks! taking 1/2 a second to think about it I have no idea why i thought it was one complete revolution per "stroke" of "4-stroke" . Going down "pulls" the a.f. mix in, and going up "pushes" spent gas out.

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The ignition doesn't "know" What "stroke" or revolution it's on. It sends a signal to the coil every time the trigger on the flywheel passes the pickup.

A four stroke makes two revolutions per power "stroke".

A two stroke makes a power stroke every revolution.

The ignitions work similarly, in that they both spark every revolution, near TDC, but on the four stroke every other spark is wasted.

It just happens at a time in the combustion "cycle" when there is no compression and very little fuel present, so no "bang".

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Question:

so i bought an hour meter for my 04 crf450r today and in the instructions it asks how many times the engine sparks for every revolution of the motor so i can set the tach up

so my question is how many times does the engine spark for every revolution of the motor?

Answer:

The ignition doesn't "know" What "stroke" or revolution it's on. It sends a signal to the coil every time the trigger on the flywheel passes the pickup.

A four stroke makes two revolutions per power "stroke".

A two stroke makes a power stroke every revolution.

The ignitions work similarly, in that they both spark every revolution, near TDC, but on the four stroke every other spark is wasted.

It just happens at a time in the combustion "cycle" when there is no compression and very little fuel present, so no "bang".

If a four stroke makes two revolutions to get through all four strokes and sparks twice to do it but you only want to count one of the two sparks then the number of sparks per revolution is actually 0.5. Unless my math is wrong!

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ok so all i want to know is how many times the engine sparks to count 1700 rpm is it really 3400 sparks to each 1700 rpm if so i need to set the tach up to read 2 sparks for every rpm so it knows that the engine will spark twice to every counted rpm or am i just confused at this point

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I'm thinking you should set it for 1 spark for 1 revolution. The unit is counting sparks so that it can calculate RPM. 1 RPM means 1 revolution of the crank shaft regardless of number of strokes or cylinders.

Set it at 1 and try it out. If it tells you that your idle is 3400 RPM then you know you set it wrong.

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Here's a twist...

Let's assume the tach you have is specifically designed for 4-strokes and what they are are really asking is if the spark plugs lights up once or twice per complete cycle. Based on what I'm hearing the bike you have actually ignites the plug twice per complete cycle vs other bikes that might take readings off the cam or use a computer to decipher the exact time to spark.

Common sense says it should be set at "1", but... Why don't you get a single cylinder bike that's known to idle correctly and compare the sound. My bike is supposed to idle around 2,000 RPM. It would fall flat on it's face at 1,000 and it would sound obnoxious idling at 4,000. It would be pretty simple to figure out which one it was by sound.

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ok so all i want to know is how many times the engine sparks to count 1700 rpm is it really 3400 sparks to each 1700 rpm if so i need to set the tach up to read 2 sparks for every rpm so it knows that the engine will spark twice to every counted rpm or am i just confused at this point

Once again, it depends on the unit. Here's a popular one that should be set to 0.5 for most EFI engines.

http://worksconnection.com/images/instructions/tach_hour_meter_web_instruction_text.pdf

Read your instructions or call the manufacturer.

Good luck.

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