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Discussion Starter #1
So after a couple hundred miles of great idling on the newly overhauled carbs they've regressed. I normally have the bike set to idle at 1100. Today after about the first 5 miles it started doing something odd. It would initially idle at 1500RPM when I got to a stop, then it would drop down to about 300. Both exhausts feel warm.

The only other variable in this is my battery. After pulling it off the charger it still did not have enough CCA's to start the bike. So I put a jump box on the bike and it fired up immediately. Once warm, the battery was sufficient for a restart. Is it possible that the bad battery resulted in an odd idle?
I did notice that while idling the voltage across the terminals was only about 11.5v. Bringing it up to 2,000 RPM the voltage slowly climbed to 13v before I stopped testing (at a rate of about 0.2 v per second). After removing the battery it immediately showed 12.5v.

1) Is it typical that the voltage increase while holding 2,000 RPM would climb so slowly? (perhaps the alternators field voltage is goofed up by the low system voltage????)
2) Could a weak battery result in low idle (its a bit of a chicken and egg thing because obviously idling at 300 RPM will result in low voltage).

-Robert
 

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..have a vulcan good day!
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I would replace the old battery with a new AGM, then u will better be able to determine IF other possible problems exist.
WilliamTech
 

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When the engine is warm the system doesn't charge at low rpms. Most will produce 14 to 14.5 volt above 2000 rpm. Be careful to remember that the vn750 system was not meant to recharge a battery but was meant to maintain the batteries peak voltage. Bad battery can overwork the system and cook the r/r and/or the stator. But yes , a battery to low of voltage can cause a misfire. The coils are 12v being 12v minimum for proper operation.
 

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Discussion Starter #4
I guess its reasonable to assume that if the battery had full CCA's available that it would have been able to hold 12.5v at idle and therefor the RPM's would not have dropped? What do you typically see at warm idle?

(my battery is not totally dead, its good enough to start the bike warm, but not cold)

-Robert

When the engine is warm the system doesn't charge at low rpms. Most will produce 14 to 14.5 volt above 2000 rpm. Be careful to remember that the vn750 system was not meant to recharge a battery but was meant to maintain the batteries pike voltage. Bad battery can overwork the system and cook the r/r and/or the stator. But yes , a battery to low of voltage can cause a misfire. The coils are 12v being 12v minimum for proper operation.
 

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12.5 to 12.8 is normal battery with the switch off. Running with the normal idle engine electrical draw is 12.0/12.3. Below 12v can/will start engine misfire. This may not have anything to do with your idle problems but if the battery is weak/bad it can cause these problems.
 

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I still don't really understand how, but yes, a weak battery WILL affect the ignition system on a Vulcan 750. You would think the stator would be putting out enough power even at idle to power the ignition system, but apparently not. Having 4 plugs to fire may have something to do with it. I have had this problem after jump starting my bike with a nearly dead battery. It would rev fine, but would not idle. My '85 Goldwing seems to have a similar issue, but it also has fuel injection. It's an old analog system, and draws a lot of current. OTOH, my Honda Rebel and Yamaha XT225 will idle just fine with a dead battery.
 

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Im thinking that if you replace that battery you need to give that charging system another look see. 13VDC @2000 RPM seems a bit week. You should be getting 14-15VDC when the engine is warmed up and the RPMs raised. Even try it at 3000RPMs and see what you get.

If the voltage is bad, one quick thing you could do is inspect the R/R connector and see if its corroded or even burnt. If it looks good maybe check the stator resistance between the windings and to ground.
 

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If the battery is low, even higher RPMs will give you a lower voltage reading than is normal. 14-14.5 at 2000 RPM means the system is working as it should, but voltage alone cannot diagnose beyond that. Unless I mis-remember (it happens when you go through a lot of bikes), our Vulcans require a battery to be installed to run at all.

If your battery is more than a couple years old, it's not a bad idea to replace it anyway. As you can find in numerous posts, a good battery is almost a 'must' with these bikes. So, pretty much x2 to the recommendations for new battery.

Best of luck,
Hippie
 

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Discussion Starter #9
It probably would have reached 14v eventually. Like I mentioned, it was climbing very, very slowly. I suspect its a feedback issue with the field voltage of the alternator. It needs proper voltage to energize the field. What I wish I'd done at the time is to hook my jump box to the bike and see if everything magically fixed itself (since it delivers reliable voltage) or even my battery charger. I know in aircraft systems if the charging voltage is too low the first thing they tell you to check is the age of the battery. 2 years and its in the trash.

-Robert

Im thinking that if you replace that battery you need to give that charging system another look see. 13VDC @2000 RPM seems a bit week. You should be getting 14-15VDC when the engine is warmed up and the RPMs raised. Even try it at 3000RPMs and see what you get.

If the voltage is bad, one quick thing you could do is inspect the R/R connector and see if its corroded or even burnt. If it looks good maybe check the stator resistance between the windings and to ground.
 
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