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Measuring idle rpm


Cloudinspector

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How can I measure engine revs on a 2LW at idle?

I have an all singing all dancing snap on timing light but it has an inductor pick up that clamps on an HT lead so it won't work on a diesel.

 

On a slow revving 180 deg twin like yours, count the number of beats (two power strokes blending into one) over 10 seconds then multiply by 12. Its surprisingly accurate.

 

Alternative is one of these at £7.63 - click for link.

 

$_35.JPG

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How can I measure engine revs on a 2LW at idle?

 

Interesting that you raise this question, as I was wondering about the same thing. I think that my 2LW is ticking over a bit too quickly. What is your target idling rpm? I have heard that 420 is about right.

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Interesting that you raise this question, as I was wondering about the same thing. I think that my 2LW is ticking over a bit too quickly. What is your target idling rpm? I have heard that 420 is about right.

That's correct. According to my manual it should be set at 420 once the engine has warmed up.

I think mine's idling a bit fast also. There's a main idle set screw which should be set around 410 rpm then a fine adjustment screw to set at 420.

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That's correct. According to my manual it should be set at 420 once the engine has warmed up.

I think mine's idling a bit fast also. There's a main idle set screw which should be set around 410 rpm then a fine adjustment screw to set at 420.

I have only found one such screw, at the back of the engine - a kindly RCR engineer pointed it out to me! You twiddle the screw, which bears upon the throttle control, then lock-nut it into place when it sounds right.

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On a slow revving 180 deg twin like yours, count the number of beats (two power strokes blending into one) over 10 seconds then multiply by 12. Its surprisingly accurate.

 

Alternative is one of these at £7.63 - click for link.

 

$_35.JPG

That's the one I have. Works very well. Handy for checking alternator speeds too

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Get a cheap cycle computer from Aldi or Lidl or somesuch. Follow these instructions: http://www.instructables.com/id/Tachometer-made-from-a-bicycle-speedometer-cycloc/ . Then you will have a permanent rev counter. I have done this, and have checked the output with one of the laser handheld ones recommended above, and it is accurate.

 

If you use a wireless cycle computer, don't expect to read it from the steering position! My sender is glued to the crankshaft pulley, and the head unit is mounted near the thermostat housing. Any further away and it is out of range.

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When I needed to do this to calibrate a rev-counter, I just recorded the sound of the engine ticking over on the laptop. Viewing the waveform in Audacity, the firing events were blindingly obvious, and I just measured the time interval between them and converted that to rpm.

 

MP.

  • Greenie 2
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If you like old school solutions, as I do, try one of these - click image

 

$_57.JPG

 

Bought similar one on eBay some time ago for very little. It samples revs over a fixed few seconds whilst dial moves around scale, after which it freezes to display actually revs. Used on the end of injection pump camshaft then doubled reading.

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When I needed to do this to calibrate a rev-counter, I just recorded the sound of the engine ticking over on the laptop. Viewing the waveform in Audacity, the firing events were blindingly obvious, and I just measured the time interval between them and converted that to rpm.

 

MP.

Now I like that MP - 10 out of 10 for ingenuity....

 

and a greenie clapping.gif

 

Andy

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I did come up with the idea of painting a mark on the flywheel with tippex then filming with my iPhone for one minute and replaying in slow motion whilst counting the revolutions. Would this work or is it a crazy idea?

It might lead to suggestions that you should get out more.

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Get a cheap cycle computer from Aldi or Lidl or somesuch. Follow these instructions: http://www.instructables.com/id/Tachometer-made-from-a-bicycle-speedometer-cycloc/ . Then you will have a permanent rev counter. I have done this, and have checked the output with one of the laser handheld ones recommended above, and it is accurate.

 

If you use a wireless cycle computer, don't expect to read it from the steering position! My sender is glued to the crankshaft pulley, and the head unit is mounted near the thermostat housing. Any further away and it is out of range.

Wouldn't like to be in the engine room if that magnet comes off the pulley. Would be like a bullet!!!!

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I did come up with the idea of painting a mark on the flywheel with tippex then filming with my iPhone for one minute and replaying in slow motion whilst counting the revolutions. Would this work or is it a crazy idea?

Sounds feasible to me. Hope it works.

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I did come up with the idea of painting a mark on the flywheel with tippex then filming with my iPhone for one minute and replaying in slow motion whilst counting the revolutions. Would this work or is it a crazy idea?

 

Why bother when you really can just count the beats against a clock?

 

Tim

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Unless the OP has really good hearing. The number of combustion strokes per minute on a two cylinder four stroke IS the RPM!

 

Yes, but two combustion strokes are close together and generally sound like one 'beat'. If you get it running really slowly you can distinguish the two.

 

Tim

Edited by Timleech
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Yes, but two combustion strokes are close together and generally sound like one 'beat'. If you get it running really slowly you can distinguish the two.

 

Tim

 

 

True,

 

But on a nice vintage twin the two adjacent firing stokes can usually be clearly heard separately at tickover speed, which is what we've turned to discussing!

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True,

 

But on a nice vintage twin the two adjacent firing stokes can usually be clearly heard separately at tickover speed, which is what we've turned to discussing!

 

It would be bonkers to try to count those separate firing strokes. At 400-ish rpm, they do meld pretty well together even though you can distinguish them if you really feel the need.

 

Tim

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