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70 Bonny - electrical #773066 05/08/19 9:41 pm
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Hugh Jorgen Offline OP
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Hi folks,

Been running fine close to daily for a month since rebuilding the shifter bits.
Went out earlier, started first kick, letting it idle for a minute to oil the uppers and it just quietly died.
Never been happy working on electrics, but have a basic VOM handy.

Is there a single point of failure I should be looking at first?
Thanks.


1970 T120R
1970 Commando
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Re: 70 Bonny - electrical [Re: Hugh Jorgen] #773069 05/08/19 9:59 pm
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JubeePrince Offline
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Huge Organ -

Need more info.....

1) points or EI?

2) Battery voltage?

3) Verified fuel flow?

4) Verified no spark? Lights? Horn?

Etcetera, etcetera......

Steve


'77 T140J
"Vintage Bike". What's in your garage?

"The paying customer is always right."

Fitting round pegs into square holes since 1961...
Re: 70 Bonny - electrical [Re: Hugh Jorgen] #773074 05/08/19 10:41 pm
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Hugh Jorgen Offline OP
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Hi JubeePrince,

I will certainly gather all that information early tomorrow.
I was just wondering out loud if there was a single point of failure that would account for the sudden stop.


1970 T120R
1970 Commando
Re: 70 Bonny - electrical [Re: Hugh Jorgen] #773089 05/09/19 12:18 am
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Jon W. Whitley Offline
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Originally Posted by Hugh Jorgen

I was just wondering out loud if there was a single point of failure that would account for the sudden stop.



Assuming plenty of gas in tank and petcock turned on ? Is idle set high enough ? Do you use a choke ? Bunch of questions and answers associated with this. However, your problem is probably a real simple one.


Jon W.


1957 6T Thunderbird 650
1968 T100R Daytona 500
1971 TR6R Tiger 650
1970 BSA A65F 650
1955 Tiger 100 - Project
1971 BSA A65 650 - Project
1972 Norton Commando 750 "Combat"

Re: 70 Bonny - electrical [Re: Hugh Jorgen] #773098 05/09/19 1:38 am
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henryanthony Offline
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You are not supposed to let it idle for a minute. You are supposed to kick it, and if it starts, you are supposed to hit the road. These bikes have no computer. They have a rider. When you stop, you blip the throttle. That’s what you did before there were Internet forums. Now, riders on new bikes blip the throttle as a matter of fashion. Maybe your expectations are different. I am old school. I grew up with these bikes.

Maybe you should trouble shoot carbs, electrical, fuel, timing, etc. Wires can crack inside their insulation and cause issues similar to those you report. Tough to diagnose.

My opinion only but, these old bikes won’t run like a new BMW. That said, there is no reason you can’t get your bike to run better than it does. Thing is, the symptoms you report have no clear solutions. Maybe your baby is running as it should. I don’t know.

Re: 70 Bonny - electrical [Re: Hugh Jorgen] #773100 05/09/19 1:55 am
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leon bee Offline
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When thinking of a single point of failure........no there are about 8 likely spots.

Re: 70 Bonny - electrical [Re: leon bee] #773109 05/09/19 4:18 am
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tridentt150v Offline
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Originally Posted by leon bee
When thinking of a single point of failure........no there are about 8 likely spots.

I disagree.....the owner.....lol. Eventually everything comes back to the owner.

BTW I mean all owners/us in general, not just the OP.

Re: 70 Bonny - electrical [Re: Hugh Jorgen] #773126 05/09/19 7:52 am
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kommando Online Content
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Looking at the fuse would be a good start.

Re: 70 Bonny - electrical [Re: kommando] #773133 05/09/19 1:03 pm
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Hugh Jorgen Offline OP
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Originally Posted by kommando
Looking at the fuse would be a good start.


Owe you a pint.
Rebuilt a Trident in 77, too long ago to remember there is a fuse in circuit.
Now of course the question of cause is begged.
Will replace and see what happens.



1970 T120R
1970 Commando
Re: 70 Bonny - electrical [Re: Hugh Jorgen] #773143 05/09/19 2:05 pm
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Hugh Jorgen Offline OP
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OK, new SFE20 in place.
Connect battery, good,
Key to On, ammeter pegs negative for half second, fuse blows.

Direct short to ground somewhere?
Any tricks to help isolate?

Cheers.


1970 T120R
1970 Commando
Re: 70 Bonny - electrical [Re: Hugh Jorgen] #773147 05/09/19 2:19 pm
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John Healy Offline
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replace fuse with a bulb. Turn key on and keep removing circuits until bulb goes out. If your bike has one start with the Zener diode.


Re: 70 Bonny - electrical [Re: John Healy] #773159 05/09/19 4:52 pm
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quinten Online Happy
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..., using Johns light bulb trick ...,( as a visual connection and current limiter)

You will probably find
a wire has rubbed it-self bare ... pinched in contact with the frame ,
seat pan or gas tank ... or even the finned rectifier (if still running one of these )

.

Re: 70 Bonny - electrical [Re: henryanthony] #773162 05/09/19 5:32 pm
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bodine031 Offline
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I love the ones who blip the throttle on bikes with EFI or carbs with no pumps before starting!!!

Re: 70 Bonny - electrical [Re: Hugh Jorgen] #773164 05/09/19 5:39 pm
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Irish Swede Offline
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Just a note: SFE20 is a 30-amp fuse, by British standards.

"USA ratings for fuses is for how much current the fuse will carry without blowing.
UK rating is for the amount of current required to blow the fuse WITHIN A CERTAIN TIME.
English standards differ from US standards. This accounts for the difference in ampere ratings."

(This information is from a 1974 edition of the Buss Fuse Car and Truck List,"
back page, "Foreign Car Fuse Replacement Data."

English Type...........Buss Replacement
50 Amp....................... AGC 30
35 Amp....................... AGC 25
30 Amp....................... AGC 20 (SFE20)
25 Amp....................... AGC 15
20 Amp....................... AGC 10
10 Amp....................... AGC 7-1/2
5 Amp........................ AGC 3

Re: 70 Bonny - electrical [Re: Irish Swede] #773168 05/09/19 6:11 pm
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Hugh Jorgen Offline OP
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Originally Posted by Irish Swede
Just a note: SFE20 is a 30-amp fuse, by British standards.

"USA ratings for fuses is for how much current the fuse will carry without blowing.
UK rating is for the amount of current required to blow the fuse WITHIN A CERTAIN TIME.
English standards differ from US standards. This accounts for the difference in ampere ratings."



Thanks Irish.

I did see in the manual that a 35 amp fuse is specified and seeing as an SFE20 came out of it did a bit of hunting and found the UK/USA differential detailed in another thread.
Would be a nasty mistake to make though...



1970 T120R
1970 Commando
Re: 70 Bonny - electrical [Re: Hugh Jorgen] #773169 05/09/19 6:12 pm
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Hugh Jorgen Offline OP
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Disconnected the Zener but no luck, blew the fuse.
Will pull up a chair and start the real hunting.
Thanks for the replies, think it's just grunt work now.


1970 T120R
1970 Commando
Re: 70 Bonny - electrical [Re: Hugh Jorgen] #773177 05/09/19 7:41 pm
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John Healy Offline
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The bulb in place of the fuse will save you a lot of time,frustration and blown fuses.


Re: 70 Bonny - electrical [Re: John Healy] #773180 05/09/19 7:56 pm
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Hugh Jorgen Offline OP
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Originally Posted by John Healy
The bulb in place of the fuse will save you a lot of time,frustration and blown fuses.


Will use that method.
Just wanted to test the Zener suggestion and had a 5 pack of SFE20's to burn.
Now I know it's something obscure.


1970 T120R
1970 Commando
Re: 70 Bonny - electrical [Re: Hugh Jorgen] #773188 05/09/19 9:43 pm
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koan58 Offline
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At least you have learned that the fuse only blows when the ignition switch is turned on.
Presumably the light switch was off when this happened?
So the short can only be an unlikely (but possible) fault in the ignition switch itself, or more likely a short in the ignition circuit supply.

I haven't seen any info on whether you have EI or breakers, or what ignition switch and light switch, but as John suggested, with the bulb as fuse, disconnect circuits connected to the white wires one at a time, and the culprit will be revealed.

Re: 70 Bonny - electrical [Re: Hugh Jorgen] #774629 05/24/19 8:13 pm
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Hugh Jorgen Offline OP
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And the winner is...
Headlight switch.


1970 T120R
1970 Commando
Re: 70 Bonny - electrical [Re: Hugh Jorgen] #776238 06/12/19 1:24 pm
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Hugh Jorgen Offline OP
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Just a follow up for anyone who stumbles across this thread in the future.

After a bit of hunting I laid hands on an NOS 31788 headlight switch, which is assumed to be identical to the 35710 in the manual.

Wired it 1 at a time and tested all pin voltages at each step, all good.
Buttoned it up, key on, started, fuse blows.

Open it up, replace fuse, leave shell open, runs fine.
Notice tiny mark on rear edge of main bulb holder.
Hmm. Aligns with #6 pin (red/black to the pilot lamp) on switch.
Bend pin up to left to give more spacing, button it up, week later still all good.

So, before you replace the hard to find and expensive switch, check that vibration is not bringing the #4 or #6 pin into contact with the bulb holder.

Cheers.


1970 T120R
1970 Commando

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