Topic: 1974 CB750K - Voltage Drop when Throttle Switch on Run

[Yoshimitsuu]

Hey all! I havent been around in a while but I am trying to get my bike ready to ride and have noticed that whenever I put the bike in RUN I immediately have a significant voltage drop. When in the RUN position the battery will continue to drain and if left the battery will continue to weaken. I know that it is an old bike but this definitely does not seem normal. I have just replaced the battery so I know that it is good and the only significant electrical mod on the bike is a Dyna Ignition system that was installed by the bikes previous owner. Most other lights are LED on the bike. I fear I will have to go through and redo a lot of the electronics. Any thoughts on what can be causing the voltage drop? Thank you all so much for all of your help over the years!

[bryanj]

I am told Dynas draw substantialy more power than points and that is the only thing you turn on with the kill switch

[bustech51]

Try disconnecting the coils one at a time and see if you can isolate what one or both are drawing.  Don't leave the power on very long as it could adversely after the coils.

Sent from my LG-H812 using Tapatalk

[Don R]

  It's probably a combination of old small wire and corroded connectors and contacts in switches and fuses. Etc. It may not be an issue.
   It's like leaving the headlight on, you would expect it to drain the battery.

[scottly]

Hey all! I havent been around in a while but I am trying to get my bike ready to ride and have noticed that whenever I put the bike in RUN I immediately have a significant voltage drop. When in the RUN position the battery will continue to drain and if left the battery will continue to weaken. I know that it is an old bike but this definitely does not seem normal.

That is perfectly normal! What is the voltage with the bike running?

[TwoTired]

Where are you measuring the voltage drop?

The run switch turns on the coils.  And with the Dyna, both coils will draw power all the time until the crankshaft turns.  What ohm primaries do you have on the coils?  The lower the ohms, the more power they draw.

Depending on where you take measurements, it could be normal, or connection resistance issues.

Cheers,

[Yoshimitsuu]

I am told Dynas draw substantialy more power than points and that is the only thing you turn on with the kill switch

I have heard this too. If I can’t get this to work as good as possible I might look into other options like the Hondaman ignition.

[Yoshimitsuu]

  It's probably a combination of old small wire and corroded connectors and contacts in switches and fuses. Etc. It may not be an issue.
   It's like leaving the headlight on, you would expect it to drain the battery.

I thought this as well and have already purchased a set of connectors to start replacing some of the more corroded and old ones on my bike. Even if it isn’t the issue it’s only going to be beneficial in the long run.

[Yoshimitsuu]

Hey all! I havent been around in a while but I am trying to get my bike ready to ride and have noticed that whenever I put the bike in RUN I immediately have a significant voltage drop. When in the RUN position the battery will continue to drain and if left the battery will continue to weaken. I know that it is an old bike but this definitely does not seem normal.

That is perfectly normal! What is the voltage with the bike running?

That’s a relief to hear! I am at work now but I believe it was at 12.5 when I was working on it over the weekend. I will check when I go home and report back.

Where are you measuring the voltage drop?

The run switch turns on the coils.  And with the Dyna, both coils will draw power all the time until the crankshaft turns.  What ohm primaries do you have on the coils?  The lower the ohms, the more power they draw.

Depending on where you take measurements, it could be normal, or connection resistance issues.

Cheers,

I am taking the reading off of my Daytona Velona gauge that I put on the bike but I also check with a multimeter at the battery as well. I currently have 5 ohms on these coils since I heard that some people had trouble running the green 3 ohm ones.

[Deltarider]

I currently have 5 ohms on these coils since I heard that some people had trouble running the green 3 ohm ones.

Even on my CB500 I never had a problem with the green Dyna 3Ω coils as far as charging. The difference in power consumption is futile and anyone is invited to prove otherwise (simple test). I've bought mine in the late 80s, be it for the wrong reason. One gave up after 50.000km and had to be replaced which was costly.
Now I think of replacing them by my old OEM coils and here's why.
Although the silicone plugcaps of the Dynatek wires are very good, I don't like they require a ferrule on top of the sparkplug. More than once the connection had become loose. The very supple wires are excellent provided they are shielded against riding wind which makes them move quite a bit. On our bikes, they are not. Also the connection of the wires at the coils is a thing to watch. The first time they connect real well, but much less after being disconnected once or twice. You need to crimp the connectors yourself and you'd better do it well. Moreover the caps require R plugs, as the caps don't have resistors in them. Last reason: they are too bulky and may interfere with the routing of the CB500 clutch cable, possibly hindering a smooth operation.
But extra taxing the charging sytem? Not worth mentioning. Extra pitting at the contactbreakers? Not that I've noticed, but I avoid unnecessary idling. And the last 20 years even zero pitting (transistorized ignition with a much reduced current at the points).

[Jerry Rxman Griffin aka MuthaF'er]

Don't forget to check the drop across the ignition switch

[Scootch]

Are you measuring the voltage of the battery when you turn the ignition switch on? Do you have the headlight switched on? If the headlight is on it will draw a lot of current and therefore cause battery voltage to drop.