74 SF2 - Ignition help needed

Dellortoman

Hero member
So, what is the common link between the two cylinders?

Not much is common to both cylinders. I'm not at all familiar with the Laverda 750 twins, but I presume the ignition circuits for each side are pretty much independent of one another. The only common parts would be the 12V feed from the ignition switch, up to the point where it divides to feed each coil. Then perhaps the coil mountings if they are on the same bracket, and that's only relevant if they're the type of coil that needs to be earthed to the frame.

Of course, you could have one or more faults on each of the two ignition circuits, and they don't necessarily have to be the same. For example, you might have a dud condenser on one side and a dud spark plug on the other.

You just need to go through the system logically and check each component. You'll need a multi meter.

1. Check that you have 12V at each coil when the ignition is on and the kill switch is in the "run" position.
2. Check the coils with your meter. Disconnect the wires to them and measure resistances with your meter. The resistance between the two primary connector pins should be around 3-5 Ohms. Resistance through the secondary circuit will be around 6,000 to 10,000 Ohms. Bear in mind all the previous discussion about earthing them or not, depending on the type of coils you're using. If you have ballast (series) resistors in the coil circuit, you'll need to check them too. They'll be around 1-5 Ohms.
3. Check that the points are functional. You can do that with your multi meter on a volts scale. First switch the ignition on. If the coils are OK there should be 12V across the points when they're open and zero when they're closed. You can also use a 12V globe as an indicator. When connected across the points, the globe will light when the points open and go off when they're closed. This can be a handy device when setting the timing.
4. Check the condensers. There is a process to do that with a multi meter, but it's not for beginners. There's a bit of theory and calculation involved. Easiest way ensure the condensers are good is to fit new ones. They're cheap to buy.
5. Check plug leads (if you can disconnect them from the coil). If they're the copper core type, you can simply test for continuity with your meter. If they're the suppressor type (some type of carbon impregnated core) test for resistance. It could be several thousand Ohms. I'm not a fan of suppressor leads. Different lengths of lead will have different resistances, and breaks in the conductor are not uncommon. I prefer copper core leads and resistor type plug caps.
6. Plug caps. Measure the resistance. Suppressor (resistor) type are normally 5,000 Ohms. Non suppressor will be zero ohms.
7. Make sure you have good spark plugs. If you have resistor type plug caps, use non-resistor plugs. NGK plugs will have an "R" in the designation if they're resistor type - BR8ES for example. A non resistor equivalent would be B8ES. I dunno about other brands of plugs. If you're using another brand you'll have to look up their ID markings.

I think that's pretty much it unless I've forgotten something (I'm sure someone will say if I have). If all that checks out you should have sparks.
 

Casor

New member
Location
United States
@Dellortoman thank you for the methodical to do list. I will go down the list....
1) Done and yes
2) Done and ohmed within your spec noted. I will make sure that the casing is grounded
3) Have to do - perhaps again I forget!
4) I have new ones coming but may have some floating around I can use as a test
5) New plug leads coming - the ones on the bike had continuity but they were old and hard
6) I ordered non resistor NGK caps. There were resistor types on the bike as I got it and was doing my prev tests with these. Even the manual says "non resistor" caps
7) I have a pile of plugs and used new NGKs with same result. Will see if I have NR plugs

Thank you again
 

Ernesto

Hero member
Location
Hamburg, Germany
I will make sure that the casing is grounded
No, the coils on the 750s are the type that must'nt be grounded. Don't worry, the
coil's housing is isolated against the circuits inside.
See here why:
The coil is a so called economic coil, i.e. primary and secondary circuit use
terminal 1 as common earth.
If you connect the secondary circuit to earth, you „switch off“ the primary circuit.

Source is an engineer of electrics, a friend of mine.

Ernesto
 

Paul Marx

Hero member
Location
France
One coil feeds the other in fact.
I use a voltmeter to time the bike. Between point and earth, turn engine over, when volts appear, it sparks.

Is the engine earthed?


Paul
 

Tippie

Hero member
Location
Dr?bak, Norway
No, the coils on the 750s are the type that must'nt be grounded. Don't worry, the
coil's housing is isolated against the circuits inside.
See here why:
The coil is a so called economic coil, i.e. primary and secondary circuit use
terminal 1 as common earth.
If you connect the secondary circuit to earth, you „switch off“ the primary circuit.

Source is an engineer of electrics, a friend of mine.

Ernesto
Correction "No, the coils on the 750s are the type that (must'nt) do not need to be grounded." To be precise, even though Ernesto said the housing is isolated, it won't matter if they are.
The #3 checking that the points are correctly assembled so that they do not ground would have been a priority. I am pretty sure the original Bosch plug caps were resistor type, the bike is from a time before resistor plugs or leads came on the market. Use whatever you want but don't double up.
 

Casor

New member
Location
United States
Have used NGK resistor plug caps for decades with zero issues and never ever fouled a plug with B9ES - always use copper leads. Too easy.
Roger that thank you. I have non resistors on order and copper leads. I think I noted above that the manual says non resistor caps and plugs. After reading all the great commentary, I think my points are somehow grounded but will methodically go through the list.
 

bazzee

Hero member
Location
Sydney
Here is a document that I scanned from a Bosch automotive handbook about 25 years ago. Might not interest everybody!

cheers

bazzee
 

Attachments

  • Bosch ignition systems overview.pdf
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Paul Marx

Hero member
Location
France
Once again, is your engine grounded?
Take a jump lead from engine to battery - and check again.

Paul
 
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natm

Full member
Location
Bushmills
Here's what I did. Scrapped the points and fitted Vape electronic ignition and a single double output coil. Starts on the button now.
 

Laverda SF

Hero member
Bosch Duel points with 2 Bosch Coils will operate up to over 8000rpm on a Breganzie Twin - Take a deep breath and relax and really look at the system.

She has to shake hands with you.
 

Casor

New member
Location
United States
Here's what I did. Scrapped the points and fitted Vape electronic ignition and a single double output coil. Starts on the button now.
While I have been thinking about that, I feel that I have to get the bike running on what it has and fix anything that is bad. I have also read some controversy on this topic - points vs elec - on this site as well. Some of my old bikes have it, but most of them don't and run fine.
 
Paul, have you had a bad experience with Electronic ignition? I have removed loads of Boyer ignitions over the years that were awful cheap nasty things- but lots of people swear by them. I have, however swapped factory points on a new ( bought by me ) Guzzi for a Dyna system. There is such a noticable difference in the way it runs, no overrun popping , no hesitancy with large throttle openings-remember this is the only thing I changed so is down solely to the ignition. That was thirty years ago, never even taken the cover off. Good electronic ignition is very good ,bad electronic leaves you stranded & points are points. I do actually carry the original points backplate if I go touring, along with an inner tube and Christ knows what else, that’s just me. Can I ask, how often have you had to clean points at side of the road, if ever?
 

natm

Full member
Location
Bushmills
I've had bad experience with points, namely a partial seizure from a combination of old worn threads in the backplate and that 40 degrees of advance. The Wassell Vape has 20 degrees, I set max advance at 34° and the bike pulls noticeably better now. Today's electronic ignition has improved from the unreliable stuff we were using 20+ years ago. I also have a 1982 BMW and a 1975 Beamish Suzuki which have electronic as standard and never need looked at. Some people like spannering their bikes, I prefer riding them. I have a 1975 Montesa which is a total pain in the ass to adjust the points, covers off and flywheel puller job. I'm considering converting it too.
 

Paul Marx

Hero member
Location
France
I've haven't had a bad experience with electronic ignition, nor with points.
I don't go touring with a spare electronic ignition on my bikes that are so equipped, just a Visa card and a smartphone.

I ride my 750s around 10 000 yearly.

Paul
 
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