4RT Wasted spark ignition or not?

[aussiechris]

Just a quick update to my original post...

I've purchased a more modern 4 channel oscilloscope to replace my old 2 channel unit.  This allows me to simultaneously display the relationship between crank sensor, fuel injection, and ignition.

The traces are labelled on the left...

Ch 1 (Yellow) is the crank sensor or "Pulser" coil.

Ch 2 (Pink) is the fuel injector signal.

Ch 3 (Blue) is the ignition coil signal.

Ch 4 (Green) is the spark plug inductive pickup.

The Pulser coil detects a row of holes in the flywheel.  There appears to be a slot followed by 5 holes followed by another slot, then 2 more holes.  These bursts occur every revolution of the crankshaft.  In this shot, the engine is idling at 1720rpm.

The fuel injector fires on the negative going pulse and stays open for the duration of the negative pulse (there is +12v on one side of the injector and the ECU pulls the other side to ground).  The pulse width determines the amount of fuel injected.  This pulse occurs every 2nd revolution of the crank at the start of the intake stroke.

The ignition coil also has +12v on one side and the ECU pulls the other side to ground to create the magnetic field in the coil.  The spark occurs when that ground is removed by the ECU and the field collapses.  So the duration of the negative pulse is effectively the dwell angle (in old lingo).  The spark trace is hard to see because it lines up with the trailing edge of the coil input.  In any case, the spark occurs every second revolution in between the fuel injection pulses - no wasted spark.

 

[lemur]

 The spark occurs every second revolution in between the fuel injection pulses - no wasted spark because it is timed off the crankshaft and not off the camshaft 😐. 

👍 nice scope traces, are you sure the C1 yellow trace is the timing trigger and does not represent the alternator output?

Edited February 27, 2025 by lemur

[lemur]

18 field coils, 18 blips on the C1 oscilloscope trace.

[aussiechris]

Not really.  Those stator windings feed three phase AC to the rectifier/regulator to provide 12v DC to power everything.  Those are connected via the three yellow wires on the 3 pin plug.  There are multiple magnets in the flywheel passing those coils.

Ch1 on the scope is connected to the Pulser or pick-up coil shown top right of your photo (with the two mounting holes).  It sits external to the flywheel and detects holes and slots machined into the outside of the flywheel.  That is connected to the white and green wires in the 2 pin connector.

Most bikes have only one slot in the flywheel for timing but the 4RT has a row of nine then a gap.

Edit: The Pulser wires are actually white/yellow and green/yellow.

Edited February 27, 2025 by aussiechris

[konrad]

14 hours ago, aussiechris said:

Ch 4 (Green) is the spark plug inductive pickup.

What are you using to monitor that signal?  I have a Hantek HT-25 "capacitive" probe, but never been too impressed with it.  Supposedly 10,000:1, but I have doubts about the accuracy of that number.

[aussiechris]

2 hours ago, konrad said:

What are you using to monitor that signal?  I have a Hantek HT-25 "capacitive" probe, but never been too impressed with it.  Supposedly 10,000:1, but I have doubts about the accuracy of that number.

Hi Konrad.  A proper high voltage probe would have been nice but I just used the clip-on inductive pick up from an old tach/dwell meter - remember those?  Also used on timing lights.

The red thing below...

I just attached a BNC connector to make an easy connection to the scope.  I thought it might be better than winding wire around the plug lead.

The duration of the spark is extremely short.  Unfortunately, at the relatively slow timebase needed to show the other signals, the spark impulse is just one pixel wide.  It's hard to see but I just wanted to verify the spark occurs when the ground is removed from the coil - same as a traditional "Kettering" points ignition (and unlike CDI used on other bikes).

 

[honda_freak]

Hi aussiechris

Interesting topic regarding wasted spark ignition on Honda Montesa 4RT. A few years ago I recorded some oscilloscope captures of the secondary ignition system versus compression pressure on a Honda Montesa 4RT model year 2007.

It should be said that these captures were taken during kick start of the engine, which is in contrast to your measurements.

But looking at this oscilloscope capture where I measure the secondary voltage on the spark plug wire versus compression pressure

over two revolutions of the engine - it is clear to see the extra ignition event characterizing a wasted spark system.

 

 

 

This was my small contribution to the discussion about wasted spark systems.

 

[aussiechris]

Hi honda_freak and greetings from downunder.

Thank you for taking an interest, and for your excellent measurements.  Your findings are certainly contrary to what I found.  Two possibilities come to mind...

1. Maybe the early models did indeed use wasted spark ignition.  But then what about the fuel injection?

or

2. Maybe all of them start with wasted spark but then change to only sparking on compression after the engine is running (and it somehow figures what stroke it's on).  That sounds pretty bizarre too but you've inspired me to find out.  Furthermore, does it inject fuel each revolution as well.

I still have all my test leads and the setup is stored in my scope.  What I propose is to run a much expanded timebase (say covering 20 to 30 seconds) and then kick starting the engine and triggering the scope on the very first spark.  My scope should have enough memory depth to be able to examine the first few cycles looking for a trace like yours and then search for a time were it changes to non-wasted spark.  It will be an interesting exercise but I probably won't get around to it for a month or two because gets to 40°C in my shed right now.  I'll definitely do more testing and report back.

As a side issue, I've occasionally had the situation where the bike won't start from a hot start no matter how many times I kick it.  Following advice from this forum, I open the throttle full and give one kick (to supposedly reset the ECU) and sure enough the bike starts on the very next kick.  I never really believed the reset ECU story but now I'm wondering... Could the main capacitor be keeping the ECU running long enough to maintain a non-wasted spark condition but have ended up on the wrong stroke because of poor kicking or a spit back.  A reset might get it back into wasted spark condition so it can start again.  It would be interesting to get to the bottom of that.  If testing were to show the bike runs wasted spark initially from cold before changing over, it would be interesting to turn off the bike and immediately restart it and run the same test over to see what those first few cycles look like.  I don't know - it  could be a load of rubbish but we'll see.

Thank you again for your input.

Chris.

ps. I'm keen to know about your cylinder pressure measurement/transducer.

 

[lemur]

If I was using a wasted spark ignition (ignition pick-up located on the crankshaft and not on the valve drive which rotates at half crank speed) and needed to apply a fuel injection cycle only on the intake stroke 🤔 I think I would make the fuel circuit dependent on high vacuum pressure being present at the throttle body.  

Poor starting was always the result of improper valve clearance on all five of the 4RT's I owned.