Topic: Won't start

[Barrett]

Hi. I'm pretty new to SOHC and pretty new to motorcycles generally. I bought a '78 CB750 in pieces and just reassembled it. It started up perfectly and I never had a problem with it starting. I just took it for it's first trip (just around the neighborhood for maybe 20mins.), and now it won't start. I haven't tried kicking starting it because I'm pretty sure the battery is dead. Why would that be? Does anyone know what's going on? Regulator? Rectifier? Is this kinda thing common? Anyway, I'm sorry if this question is naive, but I appreciate any help you guys are willing to offer.

Thanks,
Barrett

[cameron]

Before you catch too much #$%*.. I will tell you:

this problem is easy to diagnose.. and there is a pretty defined procedure for diagnosing it.
Find the manual, search the forum, and respond back to us with the info we need to know to actually help you.

[Gordon]

Are you getting power to anything?  How old is the battery?  Check the voltage, don't just assume it's dead.  If no power to anything then check the fuses.  An already depleted battery, or one that was never fully charged to begin with, can be drained completely by riding around at slow speeds for any length of time.  The charging system doesn't start to make up for the power being used until you get into the higher revs. 

[Barrett]

Thanks guys. I went out and got the manual and a decent multimeter. When I bought the bike it had a new rectifier, which I installed and a new rotor (which is plastic??) that was already installed. It didn't have a battery, so the one that looses power so quickly is new. I am getting some power from the battery -- lights still work, etc. but it's not enough to turn the engine over.   But, I haven't revved too very high. Is it ok to rev it for a sustained amount of time without riding it? When I first got the thing started, I just had it revved without moving and the engine started to steam. I'll see if I can check the ohms after work tomorrow according to the specs/procedures in the manual and let you guys know what happens.

Thanks again,
Barrett

[nccb]

your over heating the engine by just revving it up.  They are air-cooled so it would be best to take it out.  You could just take it into advanced or wherever and have them charge it.  Mine, just died too but luckily work had a battery tender.

[cameron]

I'll see if I can check the ohms after work tomorrow according to the specs/procedures in the manual and let you guys know what happens.

Now we are cooking with butter.

[MoMo]

Was the new battery fully charged before installation?...Larry

[70CB750]

I would spent 20 bucks or so on an intelligent charger/floater.  Does not hurt to eliminate the battery question from your troubleshooting

[KRONUS0100]

Barrett.....what rotor are you referring to as being plastic???

another place to look......................the trans sprocket cover on left side of motor where the chain disappears into.
there are alternator connectors under there that get hard and brittle with heat and age.  check them to make sure there are no bare spots in the wiring.  should be a cloth sleeve that covers them, sometimes the sleeve moves.
make sure to clean all connectors and use dielectric grease in them.

is your starter realyu good?  can you crank the motor over by jumping the starter relay with a screwdriver?

[Barrett]

Yeah Larry, I bought the battery from batteries plus and they charged it for me. I'll take it by advance or autozone and get them to recharge it. In the mean time, I've take the rectifier off, like the service manual said, and tested the ohms accordingly and I got nothing. I tested the regulator too, but it had continuity (or, at least, numbers appeared on the digital read-out). The thing is, though, the rectifier is new! Could something have fried it already? Or could I be testing it wrong? I've got the ohms set at 200 and I'm connecting each lead the way I think the service manual is telling me to. I've tried about every combination of lead and connector.
As per the rotor being plastic: that's me being an idiot. It's usually dusk or dark when I get home from work and I have my bike behind the house in shed with no light. I just thought it looked plastic in the light of a small halogen flashlight.

I really appreciate everybody's help here.

 

[WarwickE36]

If the battery is dead, the coils wont fire, and it wont go "bwahhh" ... now as to why it isn't charging... get a DVOM and the manual and play

[MoMo]

Yeah Larry, I bought the battery from batteries plus and they charged it for me. I'll take it by advance or autozone and get them to recharge it. In the mean time, I've take the rectifier off, like the service manual said, and tested the ohms accordingly and I got nothing. I tested the regulator too, but it had continuity (or, at least, numbers appeared on the digital read-out). The thing is, though, the rectifier is new! Could something have fried it already? Or could I be testing it wrong? I've got the ohms set at 200 and I'm connecting each lead the way I think the service manual is telling me to. I've tried about every combination of lead and connector.
As per the rotor being plastic: that's me being an idiot. It's usually dusk or dark when I get home from work and I have my bike behind the house in shed with no light. I just thought it looked plastic in the light of a small halogen flashlight.

I really appreciate everybody's help here.

 

the rectifier is a series of diodes that conduct one direction only, you will get an open one way and resistance the other on each diode.  You definitely need to get the  how to out of the service manual, which usually can be downloaded from this site if you don't have one...Larry

[cameron]

It's usually dusk or dark when I get home from work and I have my bike behind the house in shed with no light. I just thought it looked plastic in the light of a small halogen flashlight.

Holy crow this kid is the real deal.