No spark back cylinder

[bedford3374]

350 sport - i’ve got ongoing rear cylinder issue. Back cylinder will only run at 2000 revs. Have had new transponders and ignition pick up. Anyone else had this?

[MickeyMoto]

Plugs?
Swapped carbs?

[72degrees]

I had an issue with one NLM module which had to be replaced but it's not clear from the OP if this machine is running the OEM setup or not.

I'd first try swapping the plugs (or try a new one on the rear). If on OEM transducers then I'd next try swapping them and see if that switches the bad running to the front cylinder. Also check all the connections - particularly the earth of the rear transducer to the frame.

[3potjohn]

It may a blocked carb-does it come in at 2000rpm? One way on older bikes was to open it up and put your hand over the carb end to suck the muck out.If not as said could be a dodgy ht lead. JM

[julianharty]

I've just posted a suggestion to use inexpensive spark testers to help someone diagnosing problems on a 501. The tool would work equally well to help you check your spark (albeit it without load). Here's the link, viewtopic.php?f=7&t=5428#p29244 You can buy the tester for less than £5 especially if you're willing to wait several days for it to arrive.
I hope it'll help you work out the symptoms better and perhaps even the cause?

[bedford3374]

Thanks everyone for suggestions. Will let you know how I get on. Very much appreciate your help.

[julianharty]

And how have you got on? Here's hoping you've found a conclusive cause and resolved it

[bedford3374]

No unfortunately drawn a blank with everything. Not sure where to go from here. Back to drawing board!

[julianharty]

As a suggestion that might help provide some extra suggestions, how about describing what you've checked and how you've done so?

e.g. does the bike run on both cylinders BELOW 2000 rpm?
How have you checked the spark on each cylinder? And have you tried checking it AT/ABOVE 2000 RPM?
- If your bike will start and run on only the front cylinder you could check the spark for the rear plug by removing the plug from the head (this may also make it a bit easier to start and run on the front one). If you've yet to try out the spark testers I've suggested, they are an excellent way to check the spark.
- Also if you've got a HT strobe light then you can check the spark indirectly (at least that one's generated and making it through the HT system) by using one on each HT lead in turn.
- If you're willing to spend about £40 to £50 then a pair of "Inductive Tachometers" such as https://www.amazon.co.uk/gp/product/B07 ... UTF8&psc=1 I bought 2 and made a bracket to hold them on my top yoke for my 3 1/2 so I could try and work out what's causing the sporadic misfire. These meters don't seem to read perfectly and often show a higher RPM than the bike is likely to be doing in reality. However they do at least give me a clue of what's happening, and FWIW they behave similarly to the fancier and more expensive Vapor combined speedo and tachometer that was on my Kanguro I had a couple of years ago that also tended to overread the RPM at lower engine speeds.
- I've mentioned somewhere on the forum that I use an infrared camera to measure the temperature of the bikes - this might help if and when you get the bike to run on both cylinders for a few minutes and to check the effectiveness of each cylinder after taking the bike for a ride.

I'm assuming your bike's unridable, however if you can bring it to me I'd be happy to help you diagnose things using the tools I have. I live in Hazlemere, South Bucks, near High Wycombe and J2 to J4 of the M40.

[bedford3374]

This has been ongoing problem for 18 months now. It’s been back to NLM twice and has new ignition pick up and NLM transducers but still problem persists. If I swap transducers back to front it won’t go at all. I will have to put the bike to one side now because of other commitments but I will return to it.

Julian, thanks for offer of help but unfortunately we are quite a distance from you.

[MickeyMoto]

Hello,

Where in the country are you? There may be somebody closer able to help.

[pedro]

I posted on the 500 recently because of very ongoing ignition problems, swapped the NLM set-up for original parts and the bike is transformed.

[Al B]

I'm experienceing a similar issue. It's always started easily (at least by comparison with other's horror stories!) and idled smoothly but just lately there's some occasional exhaust popping at idle. Plus a distinctly flat feel to the throttle response until 2500 rpm or so and then everything seems fine.

I checked the timing and there is very little advance happening, maybe 4 or 5 degrees between idle and 6000 revs so it's too far advanced at lower speeds. So I'll try adding a diode in the red wire to the transducer and re-checking the timing to see whether it increases the amount of advance as suggested in another thread recently.

On checking the rear cylinder the spark is intermittent at idle - that would explain the popping and lazy low throttle response, but...

...Why would it spark more consistently as the revs rise? Can pickups partially fail or are they all or nothing?


I've tried alternative transducers and it appears the same. Plugs are a year or so old as are the caps so i believe they're OK. The only bits that I haven't changed are the pickup and HT leads.

[julianharty]

Al B,
I'd check the ignition winding, both static to measure the resistance, and dynamic - when the engine's running - to check how many volts it generates.

The resistance provides some idea whether the windings are in good condition, and dynamic checks whether the combination of the magnets in the rotor and the ignition winding, etc. are performing as designed and intended. My understanding is that as the engine's RPM increases the effect of the magnets increases which increases the voltage being generated which then leads to better sparking (assuming the rest of the ignition system is in good health).

As neither of my Moto Morini's have standard ignition I don't have standard values to compare your readings against. Others here will do.
Good luck, please keep us posted on your progress and results.

[mbmm350s]

Hi AlB


Please can you tell us the model and age of the bike and the ignition set up you have as it will help us with diagnosis.
Also:
The pickup type (red or black) whether marked as Ducati or not.
The transducer type (flat black, grey or grey with bulge or even blue, or black with bulge ) and note the part number 323...
The type of generator rotor flat sided or domed three slots with air scoop fins to its rear or e-start

However we always refer to Paul Compton's advice that ignition faults often turn out to be carburetion and vice-versa.

So just thinking about the ignition checks and bearing in mind it might be nothing to do with ignition!
These checks can for the most part be done simply and at no cost so its far better to make sure before spending money or alterations.

As Julian says the rotor magnetism and generator (green wire) winding resistance should be checked.

There is no hard and fast rule for the generator winding resistance test, however resistance lower than 220 ohms is often though not always
accompanied by difficult starting, especially kick starting. The manual gives a resistance specification of 300 ohms.
However experience of rewinds is that 270-280 ohms is achieved. Anything less than 230 ohms is candidate for rewind.

The resistance test is done with the green wire from the generator completely disconnected and measure the resistance to earth, on 2K (or similar scale)

Meaningful dynamic in circuit measurement of the generator voltage isn't really possible without an oscilloscope.
The waveform is complex and especially if you are debugging intermittent or missing spark only an oscilloscope will help.
The kick-over/cranking peak voltage should be sufficient to ignite a 90V spark gap (arrester gas discharge tube) connected from green wire to earth with nothing else connected.
Its is important to check that the spark gap is ignited for every passing of the pick up rotor and the first pole piece of BOTH pickup coils.
The A.c rms voltage will be around 30V fast tick over but its meaningless really unless its is obviously fluctuating,i.e unable to reach a steady value for constant rpm.
I can send you a video of the spark gap test. A multi-meter with peak hold could be used but an osciloscope is best.
WARNING: These voltages are considered high voltage in automotive context.

Some points to check:
Is the idling better with the green wire to the ignition switch disconnected.
Are all the transducer earths secure/clean corrosion free at the headstock earths?. Is the engine earth secure?
Are the HT leads and caps in good condition, corrosion in the front lead is common.
A disconnected and bared HT lead end should be able to jump at least 5mm in air
You should of course carefully check the insulation of the green wire from the generator, via fuse box (if present) to transducers.
When was the cambelt last changed is there too much slack perhaps causing instability in low rpm ignition timing?
The pickup rotor is secure, they do fail.

27 degrees (fast idle 2000rpm ) to 34 degrees (6000) advance is not uncommon - depends on pickup/transducer combinations.
Not sure I would think that popping or banging in the exhaust , i.e ignition of unburnt fuel in the exhaust to be attributable to advanced ignition more likely retarded?
air leaks (inlets) and compression should be checked for as well.
Others will think of other checks and carburetion checks to be done.

Mark