Engine cuts in and out.

[72degrees]

Have you tested the resistance of the ignition winding on the stator when it is hot (insulation on the winding possibly breaking down)? I assume it has plenty of ohms when cold as starting isn't an issue.

The transducer green wires are the feeds from that ignition winding via the fuse board.

[Sicxtyone]

All suggestions and help gratefully received, I have not checked this as I'm electrically backwards!
I will have a look in the manual for readings, ohms are about all I'm capable of. Just.
Humour me for a moment, do I take the reading from the green stator wire to er earth?

Graham

[72degrees]

All suggestions and help gratefully received, I have not checked this as I'm electrically backwards!
I will have a look in the manual for readings, ohms are about all I'm capable of. Just.
Humour me for a moment, do I take the reading from the green stator wire to er earth?

Graham

Yes. Opinions differ on absolute value but over 250 is good. A significant difference between cold and hot might be informative.
Plenty of info on other threads on the forum.
Will it run OK on one cylinder but not the other when hot - best done not under load back at home. I've never had a heat related transducer issue but you never know.

[mbmm350s]

Hi Graham,

First we assume that you have eliminated all fuel related problems, such as starvation.
Sometimes Some Morinis have a carburettion hole so bad that they cant be driven through it.

We also assume that you have checked the integrity of all the earths, particularly around the headstock and the earth from the engine to the frame.
It's also worth putting in new plugs such as NGK BP7ES. I say this because there are fake plugs about, and check all leads.

So yesterday had a couple of hours so decided that put the series 2 pick up and rotor from my 500 onto the 350.
Initially the bike started and was noticeably smoother running, even at tick over. I decided to go for a very gentle low rev trundle around and for a couple of miles, the engine was a lovely thing. But with some warmth in the engine the missing and erratic running returned. Can an on the blink transducer cause these symptoms?

It would help if you can report what the tacho is doing as the electronic tacho reports the rate at which one of the transducers is triggering and can be swappped between cylinders to see if it is one cylinder cutting out die to a triggering fault. Though it cant tell you if the transducers secondary coil winding is generating enough volts to fire the plug.

Do you know if it is running on one cylinder, or none. Can you check the spark plugs when it starts to misfire.
Does swapping transducers from front to rear move the fault.
Some faults in the transducer can affect both cylinders. For example if the thyristor in one transducer remains in conduction it will effectively short the generator winding thus preventing any sparks.However I have never heard of this happening only when hot, when they fail like that its usually terminal.

However I tend to agree with previous posters that the generator winding insulation is breaking down when it gets hot
could be a cause. Just pull the green wire out of the fuse panel and measure the resistance to ground. However this doesn't test the insulation resistance for that you would need a specialist piece of kit. However the basic resistance test could tell you if there are serious problems with the winding when hot. If you measure both hot and cold. Its obvious, at least to those who did O level/GCSE science that the hot measurement MUST be higher than the cold measurement.
The 500 stator could be tried , though don't connect the regulator as its a different type to the 500 and not directly compatible, just insulate the two yellow wires from each other and anything else. The white is an earth you can leave unconnected.

Also the 1976 bike would as standard have had the early rotor, flat sided, shiny with two holes, these do lose magnetism but normally loss of magnetism would be indicated by failure to pull higher revs or bad starting.
Hope this helps
Mark

[Sicxtyone]

Wow thanks Mark there is a lot of stuff there id not thought of will get on this evening.
I be done the obvious leads plugs caps. The rotor seems good for magnetism as can hang a 19 mm spanner from it!
I must admit the carbs could do with a strip and clean but when it was going it was going well and plug colours were good
Tachometer is working so will swap that as you suggest
The main engine earth I’ve not looked at so I’ll start there
Thanks again
Graham

[mbmm350s]

Hi Graham,

so the the plugs were not noticeably different? As you know the electrode colour ideally should be light tan like a biscuit, or perhaps a mid brown but don't take that as gospel, but the key point is both both cylinders should be the same if one is really sooty/wet then that's likely the faulty one if its a fault with one cylinder. Otherwise you a probably looking at a common fault such as the generator winding.
Anyway you can ride a twin morini for many miles on one cylinder, have done so many times!

Mark

[Sicxtyone]

Do 1976 bikes have the engine earth lead? This one doesn't. Maybe fit one and see what happens. Engine lug between oil filler and breather pipe.

[mbmm350s]

Do 1976 bikes have the engine earth lead? This one doesn't.

Possibly not, from about 1977/8 certainly did. It not shown in 1974 i don't have the later diagram to hand
but there is massa motore on the later diagrams for 6 pole switches. I don't think its your problem though,
but it cant do any harm.

Once I had a very nice 1977 strada in for service
that wouldn't rev past 5000 rpm. It didn't have an engine earth.
Giving it a proper earth solved the issue.
All a bit strange as I would have thought somewhere it would have earthed.
I suspect that everything had been so well painted and greased that there wasn't a decent earth.

I also had a 1300 Astra once that intermittently wouldn't start.
When I poked the earth strap it crumbled away. A new earth strap and all was well.

I have been in electrical engineering for enough years to know that earthing is a mysterious thing.

Mark

[Sicxtyone]

Green wire from ignition winding 210 ohms so not particularly good?

[EVguru]

Soren and I have measured many coils, including NOS ones, at 220 ohm. I have only occasionally seen the 300 ohm variety.

The resistance is a function of the number of turns (the total length of the winding) and the resistance per unit length (based on the cross sectional area of the wire used).

It's the voltage the coil produces that's important, which is dependent on the strength of the magnetic field, the rate of change of that field (engine rpm) and the number of turns. If you know what the coil resistance was when it was wound, then a drop in resistance will indicate some short circuited turns. The actual resistance is of little importance otherwise.

I should have my alternator/ignition test rig, along with y flywheel remagnetiser on the club stand at Stafford.

https://photos.app.goo.gl/6Ad87TSuKCy2s46o8

[mbmm350s]

Paul is right it matters not the resistance now but how it has changed from it's initial value.
The following taken from bikes than started and ran fine
and have original stator windings.
229 212 263 230 219 212 190 180
I had one at 160 and that was hard to start.
Yes I am mad enough to measure them because I always believed that 300 ohms was not the norm.
The 263 ohm one was purchased new in 1985 it presently measures 233 but it runs the later bulged and more powerful transducers.

The point about the resistance measure is too see if it goes down when hot. It should increase by 4% every 10 degrees of temperature rise.
Unfortunately it's difficult to tell if there is a fault from simple resistance measurement alone. Substitution with a known good stator is probably the only way to proceed.
Mark

[Sicxtyone]

Thank you Mark and Paul for your insight, plenty for me to look into later this week.
Is it possible to measure the ignition coil voltage on a cold, then hot engine.
Graham.

[mbmm350s]

HI Graham.

To avoid confusing you. The winding resistance is perhaps lower than ideal but not so low as an absolute value because the bike starts fine.
Measurement of the voltage can be done with an a.c. meter but better to use an oscilloscope. The value you read should go up with rpm and not collapse when the engine gets hot. I can't give you an exact meter value it should be as it depends how you measure it as it's not a nice sine wave. If the voltage is collapsing its likely that the bike will creep to higher revs but if you put a high demand on it will misfire. At tickover the peaks are around 100V. We are making the measurement with everything connected engine running green wire to earth.
I think substitution of stator and transducers is probably your best move.
Mark

[mbmm350s]

Also you can use the 500 transducers to try as a pair.
Just be aware that the ignition will be a little advanced by about 4 degrees so you might want to retard it a little on the pick up. Or maybe someone more local can lend you one.
Mark

[EVguru]

Also you can use the 500 transducers to try as a pair.
Just be aware that the ignition will be a little advanced by about 4 degrees so you might want to retard it a little on the pick up.

Other way round I think.

The 500 engine needs less total advance, so the trigger point was changed slightly in the ignition boxes. That way the static timing mark remained the same.

There's an article on the Dutch Morini club's site; http://www.motomoriniclub.nl/ignitioncurve.pdf