Starting issues

[chris]

Hello,

Just got my bike back the mechanic. Passed safety aside from a starting issue, non safety related.

Issue is that when the bike is warm it does not start. It will start cold as it always started in my garage and even before I took it to the shop. The tech said the starter is pull 220ish AMP while the battery is only holding 120amp. It is able to start with a separate battery.

2 issues presented are that the battery on the bike is old and uncharged (bike has been sitting for a while before i got it, never really road driven). Or the starter needs to be rebuilt as its location on the bike, right behind the cylinder, can heat up the magnets/wiring , and make them weak or damaged over time.

Cant find anything specific for a starter on a CB750 but a few options for CB700, are these interchangeable? I cant find a 750 on partzilla so I cant get a P/N.

As for now I will put the bike on a trickle charge and see what happens in a day or so. It started fine bump starting as I've had to do as I dont want to over crank the starter.

[Bob H]

It is able to start with a separate battery.
...the battery on the bike is old and uncharged (bike has been sitting for a while before i got it, never really road driven).

Yeah that is death for a battery to sit and get sulfated. Can't really reverse that.
Weak battery = all kinds of grief.

...Or the starter needs to be rebuilt as its location on the bike, right behind the cylinder, can heat up the magnets/wiring , and make them weak or damaged over time. 

I'm calling BS on that. You know the battery is weak.
Decades of Motorcycles have starters behind cylinders like that - it is a non issue.

Cant find anything specific for a starter on a CB750 but a few options for CB700, are these interchangeable? I cant find a 750 on partzilla so I cant get a P/N.

Wait to see if you really need a starter. If so, check Ebay.

[mollusc]

CB700 starter is not interchangeable with CB750.
But as Bob says, go for a battery first.  Seems a pretty likely candidate for the source of your troubles.

[Larry Fine]

Even if other problems exist, if jumping helps, the battery is weak or bad.

[chris]

My update:

I put the battery on a slow charge/repair cycle. Got a good 13.2v in the battery for a few hours after charging. Cranking the bike over I found the starter solenoid was getting really hot and the cable to the starter was also warm.

Took it for a spin and about 30 min into it my bike shut off while driving. Was not able to crank or bump start it to get it going. Was able to swap my buddies battery into it(110CCA) and it started up but only lasted about 1km before it shut off on my again with no crank or start, lights still worked. I had to tie it to my car and tow it home.

Upon getting home, tested the solenoid and found it was working but would keep voltage on both sides of the solenoid. After fully charging the battery again and starting the bike again, it got through two cranks before it stopped cranking for good. I decided to put direct battery power to the starter to try and get it to start but it just sounded like sizzling bacon. I removed the starter from the bike, powered it, and still sizzled, no crank. Took apart the starter and found what appeared to be a short from the brushed to the casing and also every brush was seized in place.

CB700 starter is not interchangeable with CB750.
But as Bob says, go for a battery first.  Seems a pretty likely candidate for the source of your troubles.

I was finding this to be the case but could not find any CB750 starters for the bike but only found CB700/CB750 rated chargers on eBay (400CAD). I decided to take the risk and buy a rebuild kit (50CAD) for a CB700 which arrived today and a starter solenoid just incase (40CAD). After rebuilding the starter, the plate for the brushed fit in. Tested the commutator and armature which were okay so I didnt want to jump right into a new starter.

Installed the starter on the bike and started in one crank. Keep running at idle just as it was when I bought it. Just notice the lights are dim under 2-3k RPM but brighten up after. Solenoid doesnt get hot nor does the cable to the starter.

Will need to road test for a bit close to home to recheck for a shut off.

Basically I think the starter was shorting and backfeeding/draining the battery through constant power at the starter solenoid. 

[mollusc]

A good outcome!  Yes, the starter brushes do wear down over time and can cause shorts -- especially if the commutator pads are uneven and chew up the brushes quickly.

The dim lights below 3k rpm is usual.  The charging system is old and weak.  The alternator doesn't produce more current than the bike uses until it's up to that speed.  It's nothing to do with the starter though.

[chris]

Update again:

I was riding my bike, starts fine cold, went about 10 minutes away, parked it, came back an hour later, started it fine, then people were talking to me about my bike while it was idling for about 10 minutes and it died on me. Would crank but would not start again after trying. It sounded like it would catch a few combustion rotations while cranking but never stuck. I decided i had to leave the bike and come back later. When I came back an hour later, bike was a little cooler, started up and drove home ASAP so it would not die on me. When I got home I just turned it off and let it sit overnight. 

Afraid to take my bike out I did some more investigation into the electrical. Next morning my bike started perfectly in the morning, i had it idling in my garage while I did some gardening, came back after about 15 minutes, turned it off but would not crank on. Found that I cranked the bike over the battery voltage went from 12.7 to about 7 volts. Also found that as soon as I put the key in ignition mode the battery voltage would drop to about 11.5v right away. I don't know how normal of a drop that is but it seems excessive. Charged the battery to full again and same issue with the voltage drop in ignition but when cranking it would start and only drop to about 9v. Revving to about 4k RPM the battery voltage is 15V max then stays there or around 13v.

I am thinking that something on the bike is draining battery power while running and has potential to kill that battery while running. I dont know where to start looking now.

The only things I have found is:
- the starter solenoid gets very hot while running. The 30amp fuse does not blow but what appears to be a power wire (red on the harness side) gets really hot at the connector to the solenoid.
- there are aftermarket led turn signals for the front with ballasts wired in the harness which I do not like. I will be looking for some halogen bulbs and stocks to replace it them.

For now I am trying to figure something out with the solenoid as between the battery and that red cable out of the solenoid into the harness, i am getting a .5V drop within those few inches of wire and it does not seem right.

I will try and post pictures when i figure out how but this is where im at.

[mollusc]

You should not leave an air-cooled bike idling for 10 minutes without a giant fan blowing air across the engine cooling fins and the oil cooler.
The charging system on these bikes is old and weak.  You are draining the battery while idling.  You're not charging the battery until you're up around 3k rpm.

Did you put an aftermarket solenoid on?  If so, did you check to make sure you've got the connections right?  Most of them require you to rotate the plug by 90 degrees.

[chris]

I have attached some pictures of my starter solenoid and drop.

https://imgur.com/a/wFeAUT9

It appears as though it has been changed in the past as there are spliced in the wiring. The fuse, where you can see black, get too hot to touch. The heat also seems to transfer at least a few inches up the red wiring from the connector and the connection shows signs of melting. I see some corrosion in the connector on the solenoid side and am getting a voltage drop of about .6 - .7V from the fuse post to the splice in the red wire.

I have a new solenoid and connector to install but just dont know if this voltage drop is normal across the solenoid. Im also thinking that the red wire for the ignition power to/from the solenoid.

[Larry Fine]

I have a new solenoid and connector to install but just dont know if this voltage drop is normal across the solenoid.

Definitely not.

[chris]

I will replace that in the coming days and update here.

Thank you

[mollusc]

It sounds like you might have the full current of the battery passing through the red wire.  Not good.
That's the OEM style of solenoid.  Aftermarket ones have the more common blade type fuse.
Since you say that there are some splices, I would check the wiring to make sure that each wire is connected to the correct position in the plug.  Maybe it's as simple as someone replacing the plug because the old one melted under similar conditions to what you're experiencing now.  That would indicate some fault elsewhere in the system.

[chris]

I have tired to wire in a new solenoid, but cant get it the solenoid to click. Can get the running lights and everything working but am unable to determine what each wire is for.

https://imgur.com/a/gdoHxIs

I think on the harness side I have:
1 - YR - going to the run off switch
2 - GR - going to the clutch lever to detect neutral, could also be going to the gearbox itself.
3 - RW - going to a box on the underneath the left side cover with i believe is the rectifier? am not sure.
4 - R - going to the ignition switch.

I am not sure if these locations are correct but i have the tank off, split the most of the harness apart to trace these wires.

I have tired to wire the new solenoid 180 degrees from the last one but i was still not able to make the solenoid click. I have tied different configurations but have blown the 30amp fuse in the solenoid a few times. I have been able to make the solenoid click but putting a ground to post 4 and providing power to post 2 with my power probe. Post 1/3 left unattached to harness. I can plug the old solenoid in and get it to click but cant wire the new one to click somehow.

The original harness and solenoid was:
R - Post 1
RW - Post 3
GR - Post 2
YR - Post 4
Im thinking post 1 and 3 are connected for starting conditions?

Is there a wiring diagram to find? I see nighthawk ones for K1, cb750a, cb750k but im not sure what those are or how similar to the 750S i have is.

[mollusc]

There is a wiring diagram in the Honda Service Manual for your bike.  That is the first tool you should get when you get a Nighthawk.

[chris]

I want to finally update this issue as potentially solved after a few days to trying to recreate the issue and benchmark this electrical system.

Things that were done in order:
- fully charge battery
- Rebuilt starter
- new starter solenoid installed, after market with new pigtail wired in
- new rectifier diode
- total of about 150CAD in parts and few weeks of waiting for each item to show up individually as issue was found.

Description of bike now:

Battery voltage still drops about 0.5V in ignition mode. Solved a minor issue where the front turn signals (LED) would blink rapidly until 1500RPM then blink normal. They now blink normal at all RPM. Battery voltage now sits at 13.3V immediately after riding and drops to about 11.5 volts when cranking over.

Bike ran long enough and hard enough to discover another slight fuel issue though, will be new post.

Thank you for reading this thread.