V7 Stone, spark plug gaps, and engine issues

[sib]

Last weekend I did the 10,000 km/6200 mi service on my 2013 V7 Stone.  This  work included oil and oil filter change, air filter change, valve clearance check/adjust, and spark plug change.  After the service, something wasn’t quite right with the engine, not exactly missing, but what might be described as a grumbling stumbling sensation.  Also, the startup idle was rougher than before the service.  For the record, since it might matter, I've been using 89 octane gas consistently for the past few months.  I thought about everything that could possibly have changed because of the service.  At first I suspected that I might have adjusted the valve clearance incorrectly, but in fact they hardly needed any adjusting at all and only two were actually adjusted, only slightly.  Then I thought about the spark plugs.  Both the old plugs and the new ones were/are spec NGK CPR8EB-9.  I retrieved the old plugs and measured their gaps.  They were 0.038 in = 0.97 mm.  When I installed the new plugs, I set their gaps to spec:  0.025 in = 0.635 mm (spec. is 0.6-0.7 mm).  Could the rough running have been caused by the smaller spark plug gaps?  As an experiment, I removed the new plugs, set their gaps to 0.035 in = 0.89 mm and re-installed them.  Problems gone.  The engine now runs as smoothly as butter, no stumbling or grumbling, and has a smooth startup idle.

Now, if I were a scientist (which I was for 40+ years before retiring), I would try to replicate the experiment.  But, instead, I’ll rashly jump to the conclusion that the published spark plug gap setting for this bike (0.6-0.7 mm) is incorrect and that it should be 0.9-1.0 mm instead.  I will also recklessly speculate that the gaps on these plugs are not set when they are installed at the factory, and the plugs are simply installed the way they come in the box from NGK, with gaps of 0.032 in = 0.81 mm.  I’ll further speculate that some of the improvement in the running of these bikes that owners experience during the first few thousand miles is due to the opening of the spark plug gaps as the plugs wear.

I’ll leave it to the combustion engineers to explain these observations or tell me I'm all wet, but the bottom line lesson for me is that if I want my engine to run smoothly, I’ll keep my spark plug gaps in the 0.9-1.0 mm range and ignore what the manuals tell me.

[rodekyll]

A wider gap forces a stronger spark, which helps burn a lean mix.  So your wider gap could indeed be helping.  I've ranged mine from 0.032 to 0.022 and found that 0.026 (bb hydro) is about right for my engine.

You're also probably right about the 'pregapped' plug thing.  Pregapping means nothing.  When I worked the counter, I'd often get into discussions of pre-gapped plugs.  During the discussion I'd lean over the counter and very studiously drop the plug, in its little box, end-on to the floor.  Now what's it gapped to?

[clubman]

A wider gap forces a stronger spark, which helps burn a lean mix.  So your wider gap could indeed be helping.  I've ranged mine from 0.032 to 0.022 and found that 0.026 (bb hydro) is about right for my engine.

You're also probably right about the 'pregapped' plug thing.  Pregapping means nothing.  When I worked the counter, I'd often get into discussions of pre-gapped plugs.  During the discussion I'd lean over the counter and very studiously drop the plug, in its little box, end-on to the floor.  Now what's it gapped to?

If it's an NGK, same as it left the factory. Cardboard tube to prevent that little mishap.

[rodekyll]

That's not an assumption I'm willing to make.  For the potential grief down-the-road, imo not checking gaps is a silly corner to cut.

$0.02

[sonicboom]

I always leave the gap from inside the box, because who would know better what the best gap is for that plug then the plug maker themselves.

Only time I check the plug gap is if I drop the sucker. I'll check them out of the box making sure they all have the same gap.

I don't trust the owners manual on this V7, total garbage.

[mymymy]

I very much appreciate your speculations. I have just changed the spark plugs in my V7 Cafe Classic 750 2009. I noticed the gap on the old NGK BR8ES plugs I removed was wider than 0.7mm spec and assumed it was due to 5000+km running. I also noticed the gap on the new plugs was wider. I didn't actually measure the new plug gap but I could see that a 0.7mm feeler gauge ran through it with perhaps 0.1-0.2mm to spare.

Once installed at 0.7mm, I experienced hard starting and rough idle with frequent stalling - and more so, at 4000RPM, a loss of power. I've been using 95 octane gas. So, thanks, if I assume the NGK CPR8EB-9 is a match for the NGK BR8ES, I'll follow your advice and widen the spark plug gaps to 0.035 in = 0.89 mm or thereabouts.