nc750s randomly spluttering at low revs

[roddy 845]

Can anyone shed any light on a random problem that has appeared during the last couple of days of wet weather riding please? My NC750S splutters at low revs and sometimes at idle. I have taken it into my local dealer to have a look at.They tested it, there was a broken plug cap which was replaced and a couple of error messages on the computer which they cleared and they said they have been out on a test ride and everything seemed okay. I been out tonight after collecting the bike after a short run in very heavy rain again and it started again after 20 minutes. Really at a loss of what it could be any ideas? Thanks

[Grumpy old man 4,682]

Hi

I'm no expert but first thing I would do would be clean and dry the plug leads and caps and spray with WD40.

 

Good luck 

[trisaki 2,029]

Fit a fender extender or mudflap to the front  mudguard  to try and eliminate water getting to the plug caps - smear a thin layer of silicone grease over the caps 

[gonzo 2,211]

The plugs are nice and easy to get at, but catch the lot when its pissing down . honda must be confident about them being in the firing line with water ! After they looked at my transalp which had a similar problem as you , they found a loose fitting plug cap. their computer said no fault found . I would take it back again.

[roddy 845]

Thanks for the tips.

[ontwowheels 19]

Just a thought from a non-NC owner, but I've had very similar issues on a previous bike from water ingress into the fuel tank during  a period of heavy rain. When not running, the water settled in the tank below the fuel pick-up level, so the bike started and ran fine. After 20 mins or so of running, the water got mixed in with the fuel and caused spluttering and occasionally cutting out. No error codes, just luck/intuition found it. May be less likely on an NC with a covered fuel filler cap, but worth eliminating perhaps?

[wendeg 95]

Similar issues on previous bikes (and cars) were resolved by changing the HT leads. Alas, on bikes, this means you have to change the ignition coil too since lead is permanently fixed at the coil end.

[Andy m 23,589]

In addition to the above, consider running an extra wire from the battery negative to the frame or engine (or use a jump lead if it'll repeat the problem in the garage). The main negative connection tends to be in a stupid place at the back of the engine where it can collect crud. The spark goes first, then the starter motor.

 

Andy

[Reckless 945]

Twice this year I've had bikes with a Miss fire and less than perfect running, I suspected fuelling issues, blocked jets etc, one bike I even removed the carbs for cleaning with no effect, when I fitted a new set of plugs the running returned to normal even though the old plugs looked perfect and sparked great when previously tested. The second bike, a pan European had the same issue, a Miss fire and again I thought carbs, with it not being a easy job to get to the carbs I though I'd try the plugs first they again tested ok but when I fitted new plugs it went back to perfect running, don't dismiss the plugs, not too dear and easily changed certainly worth trying

[embee 7,288]

.... and don't be tempted to buy your plugs from cheap auction site sellers. There are loads of fake plugs around, folk cashing in on relatively expensive precious metal spark plug designs. They'll fake anything if they can make money on it.

FWIW I usually use either http://www.gsparkplug.com/ or https://www.sparkplugs.co.uk/ , no connection with either but both have been good suppliers in  my experience.

[roddy 845]

Once again,lots of insights to what it may be, going to get the plugs changed first then go from there

 

[embee 7,288]

If it only happens in rain then it's far more likely to be plug caps/HT leads than spark plugs. You could try running the bike in the dark and look at the plug caps/leads. If you see any sparking then that's your problem. You can try spraying it with some water on the caps/HT leads while it's running to see if you can provoke the problem.

Just because they've changed one item doesn't mean they've eliminated it from the list of possibilities.

Water dispersant like WD40 or silicone grease can protect the HT system from water issues to some extent. The NC isn't particularly prone to these issues but a number of cases have been reported, so as others say, because of where the plugs are situated in the front of the engine it can be a problem.

The early SV650 suffered from a similar problem to a much greater extent, most folk fit a fender extender and use silicone grease on the HT lead and cap, plus there's a water drain hole in the plug recess which can get blocked (you get a rusty tide mark on the spark plug body). I'm not sure off hand whether the NC has similar drain holes, I don't think so because they are almost horizontal.

Check the caps are actually on the plugs properly and fully snapped home in the cyl head, it's possible to get them not quite engaged (I know this , don't ask  ).

[Trumpet 216]

One thing all the modern Electronics and diagnostics always fail to pick up on... HT problems, either plug caps/leads tracking or dodgy plugs. More common than you think. A friend of mine had a very jerky from tickover GSXR1000, he invited me to ride it. It was running on three cylinders upto to about 2500 revs, then the fourth cylinder chimed in violently. Sheesh. What a kick.

 

The centre ceramic insulator inside the plug had broken and dropped down over the centre electrode and was only stopped dropping into the engine by the outer electrode. . No amount of fancy diagnositcs helped find that. Back to basics.

Edited December 18, 2017 by Trumpet

[SteveThackery 3,090]

2 hours ago, Trumpet said:

One thing all the modern Electronics and diagnostics always fail to pick up on... HT problems, either plug caps/leads tracking or dodgy plugs.

 

To be fair, it should be possible to pick up misfires, at least in theory.  The OBD II on my Honda car can report "random" or "persistent" misfires on any of its cylinders, although I can't say for sure whether the misfire detection has actually been implemented.

 

By way of explanation: if the insulation is bad on the secondary side, it alters the waveform on the ignition primary side, allowing for the possibility, at least, of it being picked up.

[embee 7,288]

Misfire detection came in with OBD2 on cars, many years ago. Usually done by looking at the crank sensor signal and spotting regular speed variations to identify the cylinder (with cam sensor for cycle), then that cylinder is disabled (fuel cut) and revert to open loop (typically) so it doesn't overheat the cat with burning the fuel in it. Narrowing the cause down to different system components can be more tricky and is usually a manufacturer's defined fault code (I believe) rather than regulation defined. Certainly simple things like circuit continuity checks are done routinely. I'm rather out of date on today's systems.

Something like the NC might well not have misfire detection as such, not sure. It doesn't have a full crank sensor wheel to use the speed fluctuation, and there's no cat performance monitor (sensor after the cat). It could check for the feedback hitting "clamp", i.e. full rich correction but still showing excess oxygen due to unused air. Guessing but that's most likely.

[SteveThackery 3,090]

On 19/12/2017 at 01:24, embee said:

It doesn't have a full crank sensor wheel to use the speed fluctuation....

 

Murray, can you take a look at p11-3 of the Shop Manual?  It appears to have a 15-2 tooth sensor wheel.  If so, that's easily fine enough to pick up crank speed variations.  At uni we played with a CB500 engine with a similar wheel and the speed variations were clear.  Of have I misunderstood what you were saying?

[embee 7,288]

You might well be right. This is the crank sensor wheel on my 700. Depending on where the gap comes relative to the firing points it might well be enough to detect which cylinder has a misfire.

[Tex 36,817]

Misfire detection is more useful on a V6 than a parallel twin..  

[embee 7,288]

24 minutes ago, Tex said:

Misfire detection is more useful on a V6 than a parallel twin..  

It's just to protect the integrity of the emission control system and minimise emissions under fault conditions. The owner doesn't matter.

[SteveThackery 3,090]

Murray (or anyone): do you know if any systems monitor the back EMF on the coil primary in order to detect misfires?  I ask because we played about with shorting and extending the spark plug gap and the effect on the primary voltage waveform was easy to see and very interesting.

 

Whether it's practical to do anything useful with it I don't know.  It will certainly show whether the plug actually sparked or not, but detecting whether the fuel/air actually ignited is probably much harder.

[embee 7,288]

I don't know. It would vary with load too (spark voltage requirement increases with engine load). I'm not sure what you'd get from it that other things wouldn't show more reliably.

[roddy 845]

Same problem again happened yesterday, after riding to work in the  rain then leaving it all day in the rain, The bike stalled once and the revs were up and down and then once I got going it was fine. They have replaced one plug cap which was cracked and left the other one as it looked okay.I am going to take it back to the garage and get the other plug cap changed. But I can't believe how expensive they are £82 ! Thanks for all the tips as well.

[Reckless 945]

I remember that changing plug caps was top of the to do list on mid to late 70's bikes, NGK of course, back then only a couple of quid each, and a large selection to choose from 

[TheEnglishman 401]

Mine did that.

 

New plug tops sorted it.

 

 

 

[Grumpy old man 4,682]

Do they change the coil as well for that price.?