1972 Honda CB500four - lost all power

ZepTepi

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Location
High Ridge MO
Hi, I recently purchased a 72 CB500four from an older gentleman who has been working on bikes for years but just got to old to ride. The bike is in pristine condition with 12k miles and I drove it an hour home and then drove around for another hour at night when suddenly it completely died while going down the highway. The lights and engine shut off and I ended up having to get a tow. Battery read 12.5v, I put it to a trickle charger anyway and got it up to 13v but still nothing, no lights and no horn. My question is, where do I begin troubleshooting? I would really like to think that a connection was knocked loose from vibration but could it be something more serious? Thanks in advance
 
ZepTepi said:
Hi, I recently purchased a 72 CB500four from an older gentleman who has been working on bikes for years but just got to old to ride. The bike is in pristine condition with 12k miles and I drove it an hour home and then drove around for another hour at night when suddenly it completely died while going down the highway. The lights and engine shut off and I ended up having to get a tow. Battery read 12.5v, I put it to a trickle charger anyway and got it up to 13v but still nothing, no lights and no horn. My question is, where do I begin troubleshooting? I would really like to think that a connection was knocked loose from vibration but could it be something more serious? Thanks in advance
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Please check your PMs (private messages). Thanks
 
Since you have battery voltage I would go to the ignition switch and check that there's batt. voltage on the Red wire into the switch. If so then turn the key on and check for voltage on the Black and Brown wires.
 
Howdy!
The folks over at http://forums.sohc4.net/ can probably give you more help.
You can also find a link to download the Honda shop manual there.
It sounds like for whatever reason the bike isn't charging.
To start, check the fuse and check for loose or corroded connections.
The charging systems on those bikes is pretty robust so it's almost always something simple.
 
I'm having trouble removing the fuel tank, I have two hoses going to the petcock, do I just unclamp those and remove? Do I need to drain the tank if the petcock is off?
 
Possibly an issue with the ignition switch connector block or the larger main electrical connectors, or the fuses as noted. How old is the battery too? When a bike has sat if this one has one of the first things to do should be check over the connectors and fuse panel fuses for signs of corrosion from sitting. Fuses can look good yet be broken inside the metal cap area and not working.
Once this issue is solved there is a basic tune up checklist that should be followed. As Cycleranger noted the SOHC forum specializes in these models with free downloads for the FSM to help with maintenance work and troubleshooting issues.
Valve clearances should be checked for sure prior to any serious riding and a cam chain adjustment done to be sure that is tensioned correctly.

Sounds like a nice bike and a great find with low miles.
 
+1 on checking fuses. When you lose power in motion, that seems like the best place to start.

A couple years ago my CB450 blew a fuse as I was making a right turn from a 4-way stop (blinker on). It was an old glass fuse that had some rust on it and I think that was the whole issue. I was never able to reproduce the issue and the replacement is still going strong. (And I now carry spares.)

I know you mentioned no lights/horn, but double check that the kill switch wasn't accidentally bumped to the off position.
 
ZepTepi said:
I'm having trouble removing the fuel tank, I have two hoses going to the petcock, do I just unclamp those and remove? Do I need to drain the tank if the petcock is off?
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If all you need to do is remove the tank there's no need to do anything but remove the fuel hoses. Here's a hint though, twist the hoses on the petcock tubes before you try to pull them off, if you can unstick them by twisting then they come off a lot easier.
 
The culprit was a blown fuse but now I've got another problem. After replacing the fuse, it started right up with push button start so I gave it about a minute to sit and gave it some throttle while I waited. I pulled out of my driveway and immediately went up a steep hill and lost engine power halfway up but this time my lights were still on. Was able to kickstart down a hill only for it die again up a hill. Got back, battery read 11v so I impatiently charged it back up to 12.7v over the course of an hour, started up like last time and died again up the hill. Am I not letting it charge long enough or maybe let it sit idle for longer? Do I just need a new battery or could it be something worse? Right before the engine cuts off, my throttle seems to not do much at all
 
stl360+450 said:
+1 on checking fuses. When you lose power in motion, that seems like the best place to start.

A couple years ago my CB450 blew a fuse as I was making a right turn from a 4-way stop (blinker on). It was an old glass fuse that had some rust on it and I think that was the whole issue. I was never able to reproduce the issue and the replacement is still going strong. (And I now carry spares.)

I know you mentioned no lights/horn, but double check that the kill switch wasn't accidentally bumped to the off position.
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That's exactly what happened to me, except it was a left turn haha
 
Also there is an overflow tube that is spitting out gas sporadically after I stop, this is new and its a good amount
 
If you haven't joined up at http://forums.sohc4.net/ yet I'd urge you to think about it, it's the other friendliest place on the Internet. There are a few simple issues that could be in play in one combination or another, two that come to mind are gas cap vent and float height, but the guys over there will work with you patiently. Just introduce yourself in the new members section and start a thread on your issue in the 500/550 section and be sure to be very clear what bike you have and post lots of pictures.
 
ZepTepi said:
I pulled out of my driveway and immediately went up a steep hill and lost engine power halfway up but this time my lights were still on. Was able to kickstart down a hill only for it die again up a hill. Got back, battery read 11v so I impatiently charged it back up to 12.7v over the course of an hour, started up like last time and died again up the hill. Am I not letting it charge long enough or maybe let it sit idle for longer? Do I just need a new battery or could it be something worse? Right before the engine cuts off, my throttle seems to not do much at all
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Do you mean you bump started (roll started) on the downhill? Not a bad idea in that situation.

Cycleranger mentioned earlier that the charging system may not be working properly. It sounds like it's running completely on the battery, which is a good way to get stranded.

It makes sense to do some basic tests to see what you're dealing with. The easiest is to put a voltmeter across the [fully charged] battery terminals with the bike idling and see if the reading changes when you give throttle. The reading should increase as the engine speed increases and, ideally, you should see a number close to 14 V by the time you reach roughly 4000 rpm. No change in voltage or voltage drop means no charging. Finding out if the bike has the original rectifier and regulator is another good starting place.

Leaky carbs are no fun, especially on a four. As Cycleranger mentioned, could be one of the float needles not seating properly or a float sticking. Tapping the carb lightly might help if you're lucky, but removal and cleaning is probably the thing to do. That hour ride you gave it could have been its longest ride in a while and it's possible that the tank wasn't as pristine as the rest of the bike. Try to have a look in there, because cleaning the carbs won't provide a long term solution if the tank is the real culprit.
 
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Cycleranger said:
Sound like the battery might be bad. Do you know how old it is?

Do you know if the bike sat for a long time before you bought it?
It could be a float or needle valve is getting stuck.
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Battery is believed to be from 2021 but I'd have to take it out to find the date, it's not visible. It looked like it had been sitting, it was overall a little dusty and I had to clean the mirrors to actually use them because they were so dusty.
 
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ZepTepi said:
Battery is believed to be from 2021 but I'd have to take it out to find the date, it's not visible. It looked like it had been sitting, it was overall a little dusty and I had to clean the mirrors to actually use them because they were so dusty.
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If the battery is not an AGM and is of that age, and it sat without any activity over some of that time (more than a few months) it is most likely bad. If there is no vent hose or spigot to attach a vent hose (meaning a standard lead-acid battery type like from 20 years ago) then it is either a gel battery or an AGM
 
stl360+450 said:
Do you mean you bump started (roll started) on the downhill? Not a bad idea in that situation.

Cycleranger mentioned earlier that the charging system may not be working properly. It sounds like it's running completely on the battery, which is a good way to get stranded.

It makes sense to do some basic tests to see what you're dealing with. The easiest is to put a voltmeter across the [fully charged] battery terminals with the bike idling and see if the reading changes when you give throttle. The reading should increase as the engine speed increases and, ideally, you should see a number close to 14 V by the time you reach roughly 4000 rpm. No change in voltage or voltage drop means no charging. Finding out if the bike has the original rectifier and regulator is another good starting place.

Leaky carbs are no fun, especially on a four. As Cycleranger mentioned, could be one of the float needles not seating properly or a float sticking. Tapping the carb lightly might help if you're lucky, but removal and cleaning is probably the thing to do. That hour ride you gave it could have been its longest ride in a while and it's possible that the tank wasn't as pristine as the rest of the bike. Try to have a look in there, because cleaning the carbs won't provide a long term solution if the tank is the real culprit.
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I put it into 2nd gear and kickstarted it while coasting downhill while giving it some throttle and releasing the clutch, the electric push button start only made the lights dim and made a buzzing noise near the petcock. It looks like bump starting may have been a better option and I didnt have to use the kick.

I will attempt to test the battery while idle with some throttle. I don't have any experience working on bikes but I did have the knowledge to at least check the tank before I bought it and it's spotless, the metal almost looks chrome.

I'd like to mention that I have to hold the throttle for some time when first starting it up while in neutral or else it loses engine power, it doesnt want to idle untill I've ridden it for a bit or sat there with the throttle.
 
ZepTepi said:
I don't have any experience working on bikes but I did have the knowledge to at least check the tank before I bought it and it's spotless, the metal almost looks chrome.

I'd like to mention that I have to hold the throttle for some time when first starting it up while in neutral or else it loses engine power, it doesnt want to idle untill I've ridden it for a bit or sat there with the throttle.
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I'm glad the tank is clean. That's one less thing to worry about. Did you try to check surfaces that aren't easily visible when you opened the cap? A friend bought a 350 twin a few years ago and it looked clean below the cap, but we later found that the harder areas to see had a lot of varnish.

Do you use the choke when starting the bike? I've never owned a 550F, but the choke on the 750's is typically a lever that you lift up on the left side of the rack that closes a "shutter" on the backside of the carbs. This richens the mixture while the motor is warming up. You lower the lever before riding.
 
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It could simply be set for a low idle when the engine is up to operating temperature.

If so you'd need to start it with choke closed/on if it's cold and hold the throttle open just a touch for a minute or two before it'll warm up enough to idle on its own, after a minute more try opening/turning-off the choke to see if it'll idle on its own.

There are lots of places to buy batteries and opinions vary, I've been buying the cheap AGM type from www.batterystuff.com, US postal shipping is free and they ship quickly.
Just take the battery out of the bike for the winter and store it indoors where it's warm.

Do you still have the stock airbox?
 
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It does sound like your battery could be pooched and a load test will confirm. If you need a replacement then opt for a AGM one (absorbed glass mat) design, as they are a better built design and withstand longer periods of downtime.
These bikes are cold blooded, so you need to let them warm up for a few minutes before riding off. It may still need partial choke on for a minute or two while riding along initially.
The test to determine if your stator or rectifier is working well as noted above is important as a further confirmation of all is well. If that older battery is a standard flooded old style design, then it could cause most of your problems with running. It may also not show accurately the running voltage check as suggested above too, if there are dead cells inside the battery.
 
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