DOHC Starting Issue

[wickerRUG]

I've searched all over for a similar problem and have not come across anything exactly like this.

I have a 79' CB750 that's been running great for over a year after a rebuild. I last rode it last weekend and came back on to ride it today and it won't start. With the turn of the key, the lights and the gauges turn on but i get absolutely nothing when i press the start button. The battery is in great condition and charged, just checked and its sitting at 13V and change. the RectReg is new with the rebuild, as well as the stator. I wasn't able to take the kill switch fully apart because ether bike is sitting in a lot next to my apt building. but i believe i had continuity in the kill switch when i opened it up to check. I was not able to reach the actual start button. I was wondering if anyone had any ideas. idk if coils or maybe actually having a short in the switch would be the culprit. i would like to try narrowing it down before tearing apart the harness in a wendys parking lot. any help is appreciated. thanks!

 

[Sonreir]

Is the engine turning over when you press the start button, or does nothing happen at all?

 

[esmoojee]

Do you hear a clicking? Your starter solenoid could be shot or you need a new starter. Also, check the connections and make sure they're clean. Check you're main fuse on your solenoid as well. Could be blown. One other thing if these check out. There is a small diode in the loom that gave me hell until I replaced it. Try those. Check resistance on your coils. They could be shot as well if they're stock. A lot of guys with dohc's use cbr600 f2/f3 coils.

 

[Tune-A-Fish]

Do your metering at the solenoid... batt 12+V cont from to starter jump the solenoid and see if it spins

 

[Grazz256]

What everyone else said.... but check your battery leads, if they are corroded or loose you can have flaky things like this happen.

So...
1. Check your battery leads.
2. Check your fuses.
3. Try to start in both neutral and with the clutch in (in case the diode is shot).
4. Report back... noting if the engine turns over, if you can hear the solenoid click, and if the gauges/lights turn off when you hit the starter.
5. Profit?

 

[Mags]

Do you hear a clicking? Your starter solenoid could be shot or you need a new starter. Also, check the connections and make sure they're clean. Check you're main fuse on your solenoid as well. Could be blown. One other thing if these check out. There is a small diode in the loom that gave me hell until I replaced it. Try those. Check resistance on your coils. They could be shot as well if they're stock. A lot of guys with dohc's use cbr600 f2/f3 coils.

Could you explain where this diode is? I'm having a no spark issue myself and I'm trying to locate this diode you speak of.

 

[Grazz256]

The diode on the DOHC cb750 is on the starter circuit not the spark, if your bike is in gear, clutch in and the starter spins then the diode is fine. If your bike will start in neutral but not with the clutch in then your diode may be fried, if your neutral light comes on with the clutch in then the diode is shorted.

Your kill switch on the other hand will kill power to the CDI boxes and prevent spark.

Cheers

 

[wickerRUG]

Went through your checklists and this is what i found...

the lights and gauges turn on with the key then dim/turn off when i press the ignition button
battery leads are good, and fuses are in good shape
When the button is pressed, literally nothing happens (except of lights dimming), there is no sound, no click, no movement of any kind
This happens whether I'm in neutral or in gear, i don't suspect anything with the diode.

Could this be the stator? i replaced the rotor a while back and almost every other electrical component besides the stator, starter motor, and the coils

I will be checking the coils for resistance tomorrow after work, and possible the stator if i have the time to do so

 

[Grazz256]

With the lights dimming it makes me think that you either have a bum battery (reading fine when unloaded but dropping quickly once you try to use anything that takes some juice); or possibly a shorted starter solenoid (which seems unlikely to me tbh).

Try to disconnect the starter and hit the switch, see if the lights still dim.
You can also try to bump the starter directly on the battery to see if it will kick over.