Merc 40 2 stroke - misfire at lower revs

LLRoag

Cadet
Joined
Oct 9, 2009
Messages
7
1990 4cyl 40 Classic 2 Stroke

I’m unsure if this is the case but it sure feels to me that when I take off from Idle the engine sounds lumpy like it’s misfiring until I get to 2500 ish? RPM, it then sounds fine. This engine hasn’t been in the water for 2+ years and I’ve only just ironed out unrelated issues by fitting a new gearbox. Hence back in the water.

I had so many issues 2 years ago that I’m only now remembering when it was being used last a couple of years ago my voltage regulator/rectifier was putting out less voltage than it had been. I fitted a 2nd hand rectifier/regulator from eBay as a replacement for the rectifier.

I have a dual display digital voltmeter which for the seasons prior to this had been showing a reassuring 14.2 volts going to both batteries at WOT but I’m now only getting 13.6 - 13.7 volts never the healthy 14.2 V I’d been used to?

Could this be a failing rectifier/regulator and could it be at all related to my misfiring at lower revs. Or at least a starting point to consider? Or could it be the stator?

Appreciate all input - not sure where to start?
 

LLRoag

Cadet
Joined
Oct 9, 2009
Messages
7
Here’s an update

The source of my misfiring and likely low output from V Regulator played its hand today.

Started the motor as normal for an afternoons fishing and within a minute of idling it just cut out suddenly.
Wouldn't start and even the sound of it turning over was different - No spark

Went ashore got the shop manual spark tester and my multimeter to confirm - not a single spark
Disconnected black yellow wire from switchbox to eliminate it - still no spark
Disconnected 2 yellow wires from rectifier/regulator
still no spark
tested coil resistances - good
tested trigger resistances - good
tested stator adapter resistance - good
tested stator green/white white/green resistance - good
tested stator yellow > Yellow resistance - High!
Book states 0.1-0.2 ohms I’m between 0.3-0.4 ohms
i don’t have a DVA to properly test either the switch-box or the stator so I’m not sure which it is?
Can’t seem to find a DVA anywhere but from the USA which will take a while

I’m wondering if I’m just gonna bite the bullet and buy a red stator and flywheel puller and hopefully I’m right?
 
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