New to forum/CB650 and my first question!

[JPSmee]

First off - Greetings!

I just purchased my first bike - a '81 CB750c:




The battery is essentially dead, but I was able to get it to run with a charger. Definitely needs some love, but that's half the reason I got it (got to learn somewhere :-P).

I was wondering if anyone knew off-hand what type of battery I need to get? I've looked at the shop manual and a few postings, but I'm honestly just flat out lost.

Anyone out there able to help?

[shinyribs]

Welcome!Glad you decided to hang out with us! That is a pretty nice looking bike you have there.Starting with a complete bike is always niceAs far as batteries go,you will get every opinion under the sun concerning brand name.The AGM batteries you hear about stand for Absorbed Glass Mat,you may already know that.It basically means the fluid inside is absorbed in the fiberglass internals,requires no outside vent and therefore you will have no issues with battery acid eating the paint off of your swing arm and surrounding area.I have only had my bike for about 2 months now,and was not aware of that when i bought mine.Boy,i wish i did! I would go that direction if i were you.

I wish you  luck with your project and have fun!

[JPSmee]

I do like the idea of not needing to worry about the vent.

Now to establish how much of a newbie I am - are there any specs I need to be looking for specifically?

I called the local auto parts store and they did say that they have motorcycle batteries, but they said they can't look up the model/make like they do for cars.

Where do I begin?

[TOOLS1]

Batteries Plus had no problem looking up the battery, for the Toolmatic (1976 CB 750 A.) Welcome to the forum.

[shinyribs]

The guy  at my local bike shops loves these bikes and knows them inside out,so i really didn't have to use my brain when i got my battery from him.He was big on Interstate,so thats what i got.I have had horrible luck with Interstate batteries in my autos,so i went with his advice on this one. I beleive the battery he sold me is rated at 14 amp hours,but i am not near it now to confirm it.So dont hold me to that one!I will get you some better info,but Tools will probably beat me to it,im sureTools1 has all the answers!

[shinyribs]

See???What did i tell ya. Dude you are sneaky!

[TOOLS1]

Not when it comes to DOHC CB 750's I don't
TOOLS

[shinyribs]

What?! Have we found Achille's Heel?!?!  HAHA

[TOOLS1]

Maybe, but try me on a SOHC question
TOOLS

[shinyribs]

HAHA   I have...and will continue      we just totally hi-jacked this thread in 5 minutes flat...i think that a record.

Back to the battery!  I'll see what i can find out,bud!

[JPSmee]

Hehe, no worries about the hijack.

In the meantime, what's the deal with DOHC/SOHC?

[shinyribs]

DOHC=double over head cam
SOHC=single over head cam

You got the hotrodMO' POWER!

Notice the difference in my valve cover versus yours?

[TOOLS1]

What a creature of opportunity, you are. Showing off your shinny engine there.
TOOLS  

[shinyribs]

Educational purposes only

[JPSmee]

Very cool to know (I am a fan of more power ).

I checked with Battery Plus on their website, and they seem to have what I need:

X2-15L AGM

There's a B+ near my office, so I might just pick one up on my lunch break today. I might even get to ride my new toy this weekend after all!

[sgtslag]

That is a 15 Amp-hour battery, and your bike uses a 14 Amp-hour battery.  The only concern is size of the battery:  the higher the Amp-hour rating, the larger the cells/battery.  Measure the size (height, width, and depth) of your old battery, before visiting B+, and take the measurements, and a tape measure, with you.

+1 on the AGM -- it is the only way to go:  virtually no leakage current (does not need a trickle/smart charger over the Winter, unless accessories are draining the battery); no maintenance; higher resting voltage (typically 13 V, instead of 12.6 V of lead-acid); can withstand colder temperatures without failing.

There are a lot of batteries out there.  I did some research, and I would suggest you look at MotoBatt brand.  Type in MotoBatt, on e-Bay, and you should get a dealer out of Florida.  They sell for less than most AGM's, and they have dual posts.  There was a gentleman who did battery torture tests, and the MotoBatt came out on top -- and it is less expensive than nearly every other brand, to boot.  Haven't had the opportunity to order one yet, as I replaced my lead-acid batteries a few years ago, and I religiously use my Battery Tender Jr.'s, so my lead-acid batteries are running strong so far...  Some day, though, they will die -- then it will be time to order a MotoBatt AGM.  Cheers!

[JPSmee]

Thanks for the heads up sgtslag! I picked up a 14 AH AGM, and will be installing tonight. If I can get some time, I'll get better pics

[rich]

Now that you've got a good battery you should test your charging system out and make sure it is working properly. If it's not charging it'll run like crap (poor idle, missing, backfiring, high speed cutout) even with a new battery. This often gets people thinking they've got an ignition or fuel problem when it's actually being caused by low voltage.

You'll need a voltmeter to check the charging output. Connect it to the battery, make note of the battery voltage with the key in the off position. It should be 12.5 or maybe even 13 volts since you've got the AGM. If it's anything less, stop and charge your battery (Caution: Don't charge your motorcycle battery at any more than 2amps or you'll ruin it. If your charger doesn't have a 2amp or less setting get another one).

If your battery is fully charged, start the bike and note the voltage with the headlight on high beam and the engine running about 1700 RPM, you should be back up the 12.5 or 13 volts you started with. 1700 RPM is the initial charging point.

Rev the engine up to ~3000 to ~5000 rpm, you should see at least 14 volts at the battery now (14.4 to 14.7 is optimal). If it's over 15 - Stop! You've probably got a bad regulator and you're overcharging your new battery.

If your results are close to the above (13.5 to 14 volts at 5000 RPM) you probably just need to clean all the connections from the A.C. Generator to the regulator / rectifier, battery cables, etc. Heck, even if it's is charging or not charging at all you should clean the connections, you never know...

If it's not charging at all and all the connections are clean and tight, the primary suspect is the generator rotor. Remove the A . C. generator cover and check the resistance between copper rings on the rotor.  There should be more than 3.6 ohms resistance if it's good.  Most of the time when rotors are bad there will be either infinite resistance (completely burnt out) or less than 2 ohms resistance (internally shorted.)

Here's a link to a thread with a picture of the way to connect the leads to the rotor.

If it's not the rotor you can test the other components following the procedures in this write up on checking the charging system from http://www.oregonmotorcycleparts.com/FAQ.html

[JPSmee]

Rich,
Thanks for the tips, I wish I had seen them before I started . . . you'll see what I mean in a moment.


So I got the new battery installed with minimal issues (big hands in cramped areas is always fun, plus having the positive lead just *barely* long enough to let me wiggle the battery out gave me a slight frustration). In my excitement, I put in the key, and turned it to the on position. The lights turned on without issue, and I was able to confirm all the lights functioned as they should (and the horn, lol). I then attempted to start the engine, which is where I started to run into problems.

The starter sounded like it either did not have the juice, or was experiencing some resistance. I did however get the engine to start (albeit very roughly). I was able to rev it up somewhat, but it sounded like it was missing and was generally very rough. When I brought it close to idle, it cut off unexpectedly.

That was the only time I've been able to get the engine to turn over with the new battery

I tried to start it twice after, and each one resulted in the starter having a harder and harder time turning until it now only makes a click (much as if the battery was dead . . . which is what the bike was doing when I initially got it).

I'll be checking out the rotor/stator over the weekend to see if that is indeed the culprit. Looks like I will almost certainly need to pick up a charger too (one of those things I've been needing to get anyhow).

[shinyribs]

It'll start! Good news so far!