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Offline Vbled1

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Lose power when turn key
« on: January 28, 2012, 10:13:38 PM »
Been trying to iron out my wiring on my build (cb750) and am at a stand still. Bike had power and would turn over with my new battery about a week ago. I come back and suddenly have a problem. Battery is holding ~12v but when I turn the key to first position I lose all power, next position power comes back but bike won't turn over, and turn key off meter reads full again. I cant remember for the life of me which position the key was in for starting but I think it was first position. Any help is appreciated thanks.
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Offline HondaMan

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Re: Lose power when turn key
« Reply #1 on: January 28, 2012, 10:32:58 PM »
Which model 750 is this?
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Offline Vbled1

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Re: Lose power when turn key
« Reply #2 on: January 29, 2012, 04:08:38 AM »
1975 cb750k
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Offline my78k

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Re: Lose power when turn key
« Reply #3 on: January 29, 2012, 05:18:57 AM »
I believe far left should be fork lock. next should be off. first right should be on and should start. far right should be park where lights come on but will not fire.

keep in mind when starting all lights go out to divert power to starter (at least on my 78). so if the start is not turning it may appear to be loss of power.

Dennis
« Last Edit: January 29, 2012, 05:22:11 AM by my78k »
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Offline MoMo

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Re: Lose power when turn key
« Reply #4 on: January 29, 2012, 06:15:16 AM »
Seems like a bad switch, check for 12v out on the black wire when the key is in the on position...Larry
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Online Johnie

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Re: Lose power when turn key
« Reply #5 on: January 29, 2012, 06:54:22 AM »
I take it this is an OEM switch? Sure sounds like the contacts are geting lose or dirty. Off is down, then next click is on and last click is parking light for rear only. Sometimes you can jiggle the key with the ignition on and headlight on and if you get the flicker you know it is in the switch.
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Offline Vbled1

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Re: Lose power when turn key
« Reply #6 on: January 29, 2012, 11:54:16 AM »
Thanks I will look into the switch and get back.
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Offline Spanner 1

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Re: Lose power when turn key
« Reply #7 on: January 29, 2012, 12:13:15 PM »
Crappy fuseholder IMO. Do a search about fuseholder or fuseblock  ;)
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Offline BobbyR

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Re: Lose power when turn key
« Reply #8 on: January 29, 2012, 03:16:40 PM »
Quote from: Johnie on January 29, 2012, 06:54:22 AM
I take it this is an OEM switch? Sure sounds like the contacts are geting lose or dirty. Off is down, then next click is on and last click is parking light for rear only. Sometimes you can jiggle the key with the ignition on and headlight on and if you get the flicker you know it is in the switch.
There is a thread for rebuilding ignition switches by Two Tired has a lot of photos and procedures.
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Offline Spanner 1

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Re: Lose power when turn key
« Reply #9 on: January 29, 2012, 05:33:23 PM »
You can rebuild the switch for grins.. but that's not the problem  :o...
The 'giveaway' is in the first post where the OP says' and turn key off meter reads full again'. This says 2 things , first he has connected the meter before the ign. switch ( 'prolly Red wire@ switch and ground ), and switch is good as voltage drops to 0 when the switch is turned 'on' showing that the load AFTER the switch is causing the voltage to drop to zilch. Either there is a 'hard' short to ground after the ign. switch ( was A.O.K. before ) unlikely, or crap main fuseholder unable to pass the 4 or 5 amps required for the ign. coils ( common ).... ;)
« Last Edit: January 29, 2012, 05:36:21 PM by Spanner 1 »
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If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline Tews19

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Re: Lose power when turn key
« Reply #10 on: January 29, 2012, 05:52:27 PM »
Quote from: Spanner 1 on January 29, 2012, 05:33:23 PM
You can rebuild the switch for grins.. but that's not the problem  :o...
The 'giveaway' is in the first post where the OP says' and turn key off meter reads full again'. This says 2 things , first he has connected the meter before the ign. switch ( 'prolly Red wire@ switch and ground ), and switch is good as voltage drops to 0 when the switch is turned 'on' showing that the load AFTER the switch is causing the voltage to drop to zilch. Either there is a 'hard' short to ground after the ign. switch ( was A.O.K. before ) unlikely, or crap main fuseholder unable to pass the 4 or 5 amps required for the ign. coils ( common ).... ;)

OK so your saying the main fuse may be bad but not blown? WOuldn't a main fuse being bad not give the bike any power?
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Offline Spanner 1

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Re: Lose power when turn key
« Reply #11 on: January 29, 2012, 06:04:34 PM »
The fuse being O.K. but the fuseholder 'clips' loose/corroded... it's soooo common and won't pass the required current ( amps ) when you switch the ign. 'on'  ;)
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If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline Tews19

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Re: Lose power when turn key
« Reply #12 on: January 29, 2012, 06:15:36 PM »
AHh thats what I was thinking you meant. See i just got my 750. It has a maintenance free battery. I was able to turn the bike over using the start button. But the gas, carbs are all nasty so it wouldn't start. I charged the battery twice now. The last time I charged it for most of the day. It wouldn't get fully charged. I was thinking maybe its my charger. I put the battery on. Turned the key and the idiot lights lit up in the gauges. Hit the start button and lights went out and and a click. Nothing. I let go of the button and lights turn back on. After reading this thread, I putzed with the ignition and by wiggling it, the lights will turn off in the gauges.  I may make a video and post a thread if I can't figure it out. Weird part is this happened once I removed the pods.
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Offline Spanner 1

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Re: Lose power when turn key
« Reply #13 on: January 29, 2012, 06:36:00 PM »
How sure are you that the battery is good ?.... a lot of electrical probs. are solved after loads of frustration, trying this and that until the owner gets a new battery. Bing! You say the battery has been charged all day and won't crank the starter and the clock lights go dim when you hit the start button ? I'd rule out the fuses by working on them as described and go ahead and clean all connections... only takes an hour of your life !! and leads to reliability/enjoyment/confidence in your bike... esp. if it's 'new' to you and has sat for years... :o ;)... that's what I would do.
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If your sure it's a carb problem; it's ignition,
If your sure it's an ignition problem; it's carbs....

Offline Tews19

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Re: Lose power when turn key
« Reply #14 on: January 29, 2012, 06:37:56 PM »
I am not sure if the battery is good. I really do not want to dive into this bike until I get my build done. maybe I can swap out my battery from another bike.
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1969 Honda CB750... Basket case
1970 Honda CB750 survivor.
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