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Tachometer on a single cylinder engine (pitbike)

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tzrtom
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Joined: 08 Jun 2011
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PostPosted: 19:21 - 27 Jun 2011    Post subject: Tachometer on a single cylinder engine (pitbike) Reply with quote
these look quite cool and you can pick them up for about £20 on ebay...

however i have a problem, all the affordable tacho's only work on 4-6-8 cylinder engines, this means that if i connect it to my single cylinder engine, when the engine is revving to 2000rpm, i will get an indicated 500rpm (assuming its set in 4 cylinder mode)

so im hoping someone can point me in the right direction, there is a video on youtube, where someone seems to have the tacho in tune with the engine revving! can be seen here...

https://www.youtube.com/watch?v=u_6wHu87k7Y


thanks alot for any help! Very Happy
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Pete.
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PostPosted: 19:35 - 27 Jun 2011    Post subject: Reply with quote
Single cylinder bikes fire once per revolution, and four cylinder bikes also fire one coil once per revolution.
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The Shaggy D.A.
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PostPosted: 19:36 - 27 Jun 2011    Post subject: Reply with quote
From the sound of the bike, that tacho is registering double the revs; that didn't sound like a 2k idle.
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P.
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PostPosted: 20:02 - 27 Jun 2011    Post subject: Reply with quote
It sounds like 2k to me... too fast for the 1200rpm tickover of my masai.. definitely sounds fast.

Standard idle speed will have the exhaust doing that annoying bob bob bob sound Laughing
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finpos
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PostPosted: 20:08 - 27 Jun 2011    Post subject: Reply with quote
This is my pet gripe - you have to pay over the odds for a rev counter that works on a single cylinder bike with wasted spark ignition, and for the sake of a 2p component.

The solution is to to be be able to take the rev counter apart, which isn't always possible without destroying the case. Then, if you are lucky, you will find inside a circuit board on which you can follow the traces from the 2-4-8 switch. Then if you are more lucky still you will find that each trace goes through a separate resistor which you can read the value of. If there's no value to read you can use a multimeter to measure them. Usually these are pretty obvious because they are calibrated resistors and may even be marked as such.

Next - maths! If the luck is holding out the resistors you measured will have some linear relation e.g. 2cyl=20kohm, 4cyl=40kohm, 8cyl=80kohm. That way you can figure out what value you need to half or double the range as required.

Then, you need to replace one of those resistors with one of the value you worked out and it's a really good day then that will be within the acceptable range of the frequency to voltage converter and it'll work.

f.
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tzrtom
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Joined: 08 Jun 2011
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PostPosted: 20:12 - 27 Jun 2011    Post subject: Reply with quote
finpos wrote:
This is my pet gripe - you have to pay over the odds for a rev counter that works on a single cylinder bike with wasted spark ignition, and for the sake of a 2p component.


f.



cheers fin! thats just what i wanted to hear, i dont suppose you could link me to one you know that is simple to open up and replace the resistor, whilst also keeping cost below £30?

thanks again! Very Happy
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finpos
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PostPosted: 20:15 - 27 Jun 2011    Post subject: Reply with quote
Eh - unfortunately not! If you find one tell me Smile

f.
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Teflon-Mike
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PostPosted: 21:30 - 27 Jun 2011    Post subject: Reply with quote
Pete. wrote:
Single cylinder bikes fire once per revolution, and four cylinder bikes also fire one coil once per revolution.


Err... a four stroke engine, takes two crankshaft revolutions to make one 'cycle'... so you 'want' a spark every OTHER rev.

A Two stroke engine, takes one crankshaft revolution to complete once cycle, so you 'want' a spark every rev.

So, a two stroke single, one spark per rev, a two stroke twin, two sparks per rev. A four stroke twin, one spark per rev, a four stroke 'four' two sparks per rev, etc....

four stroke engine has four 'strokes' perfoming the four 'phases' of the 'Otto cycle', suck, Squash, Bang, Blow......

1/ Piston falls, half a crank rev, sucks in charge
2/ Piston rises, half a crabk rev, compresses charge
BANG... we want a spark to set fire to the charge!
3/ Piston falls, half a crank rev, Combustion....
4/ Piston rises, ghalf a crank rev, Exhaust, gasses pushed out the pot ready to do it again.

We want spark at a few degrees before Top Dead Centre on the compression stroke, to set fire to the charge and start the burn of the power stroke......

So, for a four stroke single cylinder engine, one spark every other revolution, and we need some 'means' of triggering that spark at the right time.

Mechanically, used to be done with a switch, called points, opened by a cam, to trigger the coil.

So, four stroke engine, we need a cam, that opens the points once every other revolution...... if we had a cam on the crank-shaft, that would open the points every revolution, so we'd get two sparks, one when we want it, and one, somewhere we possibly dont....

However, we have a cam shaft, rotating at half crank speed opening teh valves once per rev, so, stick an extra love on it to open points, job done.....

BUT, back to the idea of having the cam on teh crank... if we made a spark every rev, where would the 'unwanted' spark happen?

Well, spark we want occurs at a few degrees before top dead centre on compression.... so rotate the crank, when we get back round to a few degrees before Top Dead Centre again, we find we are on the exhaust stroke..... nothing in the pot to set fire to, so if the spark plug was triggered then, as WELL as on the compression stroke, all we'd do is spark in some exhaust fumes.... no great loss, not going to do any harm, so why not?

And many small bike engines do exactly that, and fire the plug once per rev, once on compression, to light the fire for the power stroke, once on exhaust, 'loosing' the 'unwanted' spark.

Make a twin cylinder engine, and have both pistons rise and fall together, you can take advantage of this, and use a 'duel' coil, two spark plugs on one coil, with common trigger.... as engine goes through the cycle, crank triggers spark on BOTH cylinders similteniousely, at a few degres before TDC, on one crank revolution, one cylinder will be on the compression stroke, and teh spark in that pot should light the fire; meanwhile other pot is on exhaust, and spark should do nothing. On the next rev, its the other way around, and teh cylinder that was on exhaust is now on compression, and that spark lights a fire and the other one looses its spark in exhaust gasses.

RIGHT..... four cylinder car engine.... conventionally cam shaft triggered. Cam shaft rotates half crank speed, and completes one revolution per engine cycle, during which time all four cylinders will have each completed a cycle. normally four lobes, on the cam, opening the points, at 90 degree intervals, making one spark every 180 degrees of crank revolution... a distributor then diorects the spark from a single coil to whichever cylinder is on the compression stroke at the time....

BUT, four Cylinder BIKE engine, liklihood is its ignition is arranged as two 360 degree twins, with TWO twin coils, 'lost spark' coils, fired by two triggers on the crank shaft...

On to the important bit......

After-Market Tacho..... I presume its ignition triggered, and as such its actually counting signal pulses on the ignition feed, rather than actual revs.....

Four cylinder four stroke, 'distributor' car ignition, the single coil is getting two pulses per revolution..... so in four cylinder 'setting' tacho displays one rev per two coil pulses.

Single cylinder, 'lost spark' ignition, gets ONE ignition pulse per revolution... so in four cylinder setting it would read HALF the actual rpm.

Twin cylinder bike engine, with 360 crank and common coil, that one coil would, just like a single, be getting one pulse per rev, so likewise would read half speed....

Four Cylinder bike engine, with twin coils, you can only take the sensor off ONE coil, so, again, would be getting one pulse a rev, and would give half rev reading.

SO, if its an ignition triggered Tacho...... really it doesn't matter whether it's a four stroke, a two stroke, a single, a twin, tripple or four...... using 'lost spark' principle ignition, and multiple coils, rather than a distributor...... you pick up off one coil feed and regardless, of engine type, get ONE signal pulse per rev.....

Only if you have a car type distributor ignition, and pick up off a single COMMON coil, do you have to recalibrate for the number of sparks that coil is making per revolution, depending on the engine configuration.....

AND, if the tacho, IS coil triggered unit, and designed for a distributed system, almost certain, it wont calibrate for a motorbike ignition system..... UNLESS it has a 'motorbike' or two cyl setting as well as 4, 6 & 8 cyl settings.....

So.... you need a motorbike tacho...... but good news is, almost ANY coil triggered motorbike tacho is likely to calibrate correctly...

Make sense?

If you REALLY want to make 4 cyl ignition trigered tacho work on a bike ignition, you need to get a bit 'clever', and its not impossible..... you simply DONT use the ignition trigger.....

You need two pulses per revoilution to make it calibrate, SO, you make a 'dummy' ignition trigger, that gives two pulses per rev....

BUT we are presuming ignition triggered tacho....

Many modern tacho's take thier trigger from modern car engines ECU, or use the same crank sensor.... though more common, the are triggered by the alternator, taking a 'pulse' from the un rectified voltage on the alternator windings which will change in frequency according to engine revs.....
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Vincent This post is not being displayed because the poster is banned. Unhide this post / all posts.
Old Thread Alert!

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oldbeaver
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Joined: 21 Jul 2018
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PostPosted: 15:11 - 24 Jul 2018    Post subject: The single cylinder motorbike tacho problem: unsolved Reply with quote
Pete. wrote:
Single cylinder bikes fire once per revolution, and four cylinder bikes also fire one coil once per revolution.


True only for two stroke engines.

The vast majority of bikes have a four stroke engine, this means they fire once each other rev.
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MCN
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PostPosted: 17:15 - 24 Jul 2018    Post subject: Re: The single cylinder motorbike tacho problem: unsolved Reply with quote
oldbeaver wrote:
Pete. wrote:
Single cylinder bikes fire once per revolution, and four cylinder bikes also fire one coil once per revolution.


True only for two stroke engines.

The vast majority of bikes have a four stroke engine, this means they fire once each other rev.


No wurries big yin. Tefers totally covered it.
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Old Thread Alert!

The last post was made 7 years, 30 days ago. Instead of replying here, would creating a new thread be more useful?
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