74 SF2 - Ignition help needed

C

Casor

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United States
Hey everybody, I bought an SF2 in October and in the interim sourced some needed cables and rebuilt the Dellortos. Attempted to start it today and got no spark on either side. Engine turns over freely and all the electrics are old but stock - points, coils, wires, handlebar switches. The bike does not appear to have been molested to any great extent. It's just old.

The points looked a bit shaky, but good enough to work. We cleaned them since they had some dirt etc on the contacts. We checked all the critical power spots - getting power to the coils, the starter works fine, and have power to the points. The coils were disconnected and ohmed and found to in spec. We reset the points to the spec .5 mm or whatever it was. The batt was fine at just over 12v. Plugs were filthy so I changed them and the spark plug caps were resistor type which they should not be - OK but I should get something. Even if the timing was off, I should at least be getting spark at the points and plugs. Nothing.

The only monkey in the wrench I could not check were the condensers and I have had issues with other bikes if these are old. I have some on order but I feel that I should at least have gotten something other than no spark at all on either cylinder.

I am new to these things but have the shop manual and went through it to make sure we covered the bases.

What am I missing here guys?
 
Points wiring is pretty simple. Power goes to one of the coil terminals, other coil terminal goes to points. There's a condenser wired in parallel with the points. That's pretty much it. Connect a battery and you should get a spark as the points open. Make sure the moving arm of the points isn't shorting against the static part (the pivot has to have an insulating bush).

The other thing to check is that the coil secondary (HV) circuit is earthed. That's normally done through the coil mounting. Paint needs to be scraped off the frame where the coil mounting bracket is bolted on. Likewise there needs to be a good connection between frame and engine.
 
Dellortoman said:
Points wiring is pretty simple. Power goes to one of the coil terminals, other coil terminal goes to points. There's a condenser wired in parallel with the points. That's pretty much it. Connect a battery and you should get a spark as the points open. Make sure the moving arm of the points isn't shorting against the static part (the pivot has to have an insulating bush).

The other thing to check is that the coil secondary (HV) circuit is earthed. That's normally done through the coil mounting. Paint needs to be scraped off the frame where the coil mounting bracket is bolted on. Likewise there needs to be a good connection between frame and engine.
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Thank you, I will double check these items. The coil mounts look quite shaky with corrosion as does the engine. No real rust but ugly. I can run a ground right to the batt from the coil body. Any comment on the condensers? I still think that's the monkey here.
 
The other thing to check is that the coil secondary (HV) circuit is earthed.
Click to expand...
The points are the earth.
Did you check if 12V from the ignition lock arrive at the coils?
Even if the condensers are shot, you'll get sparks.

Ernesto
 

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Ernesto said:
The points are the earth.
Did you check if 12V from the ignition lock arrive at the coils?
Even if the condensers are shot, you'll get sparks.

Ernesto
Click to expand...
Ernesto - Thank you. We are getting 12v at the coils but we did no checks of the ignition switch. We didn't feel checking the switch was necessary since the coils appeared to have what they needed.
 
I think that the points are being grounded somehow since I feel that it is odd that neither cylinder is getting spark. Also odd that both condensers would be bad. You would think that one of them would be working.
 
Do the 12V at least arrive at the points?
If not I guess the coils are shot. At least the primary circuit is interrupted.
I think that the points are being grounded somehow
Click to expand...
All the little insulating parts there? Washer, enough rectangular isolator plates?
 
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Ernesto said:
Do the 12V at least arrive at the points?
If not I guess the coils are shot. At least the primary circuit is interrupted.

All the little insulating parts there? Washer, enough rectangular isolator plates?
Click to expand...
The coils look ugly but they ohmed ok and they are getting power.....
 
Someone wrote about missing an iso washer on the points and them earthing, been there. You must be getting close, there isn't much left.
 
Vince said:
Someone wrote about missing an iso washer on the points and them earthing, been there. You must be getting close, there isn't much left.
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Thanks Vince - that's kinda what I thought. There are just only so many parts to this system. I really want to change the condensers before I have another go and maybe check the parts book for the exploded view to make sure the points are not grounded. Gotta be one of these, but I hate to assume!
 
If there's feed to coils and points it's obviously not ignition or kill switches.

Any idea of the history of the bike. Was it running when it was laid up or did somebody try to repair it?

Points gap is 0.35 to 0.45mm.

Did you or not get a spark from the bare HT lead to earth?

Paul
 
If that's an isolation problem, you would have 12V at the ground plate, too.
And concerning condensers, why not disconnect them temporarily, just for checking?

my 2 cents,
Ernesto
 
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