1980 CB750F - Starting Issue Advice needed.

[epfromnyc]

Hey there fellow 750ers... Okay I have a 1980Cb 750F supersport. There's a ghost in this machine. I've been having an interesting time trying to start this bad boy. I hit the start run button and nothing happens. Initially I hear the starter motor kick off but then stop. Can't seem to find the short.  Changed the inigition assembly and key with a new one... so thats good.  The headlamp wasnt coming on but now it is. Changed the start run button assembly with a clean used one... so that seems good. The headlamp switch (hi beam/low beam) has been changed for testing purposes ( using one from a 1982 cb750 which doesnot have the external wires to connect the horn but that shouldnt matter).  The fuse box (on the handle bars gets hot to the tough on the headlamp fuse connectors) but does not blow a fuse. The fuses are all 10amps.  I have disconnected the starter motor and tested with 12v battery and it spins when i give it power. So I think its good.  The starter solenoid has been replaced with a good used one... the 30amp fuse is not blown. So I get NOTHING when I try to start the bike. All lighter power on... but thats all folks.

Here is my steps: Clutch is in neutral. Ignition key turns to ON position. Neutral Light indicator comes ON, Oil Pressure light comes ON. Turn Signal Lights, Rear Stop lamp works, Headlight Comes ON.

Start switch is in RUN position. I press it... AND the HEADLAMPS DIMS as if turned off.... then nothing. No Sound or action from the start motor.  Sometimes the headlamp does not come back on after I release the start button. Sometimes the Headlamp comes right back on after I release the Starter button.

Any advise would be greatly appreciated. Not sure where else to look... I did check wire for contuinuity and they seem fine... but the heat at the fuse box... definately is telling another story.

[shinyribs]

Sounds like you really checked everything out good. The head light will dim/go out when you hit the start button. Only thing I can think of (but Imagine you probably already checked) is your battery voltage and your battery cable connections. Possibly a weak ground somewhere from the battery to the frame/engine?

[LukeM]

Hmm...starter spins when it's out of the engine.  That was my first guess.  The gleaming ribbed one may be right about a possible ground problem.  Look for a green wire connecting to the frame under your seat.  That (if I recall) is the main ground for your wiring harness.

Just for grins, have you tried to push start it?
Luke M

[epfromnyc]

Hey thanks for the response. Yep a good charged battery is always the best starting step. So the battery is definately NEW and charged ( volt meter says zip-pa-tee-do-dah).  A weak ground.... that could be it. Negative cable from the battery is over 30 years old... so.... and it sat outside for some years. So could be that. Hmm could have some rust where it meets the body. I will clean and recheck it.

Hoping to at least get it to kick over and then I can adjust all other goodies from that point. Wont be sure what the rectifier and charging system is doing until i get it running. etc etc etc.

Thanks much!!!

[epfromnyc]

Reply to Luke: Yep already tested the Starter motor... spins free and strong. Thank you.

[epfromnyc]

@Luke: Nope I havent tried to push start it. I un-raveled the umbilical cord checking for continuity and any wires that might have been broken. So, I have to make that all look nice again... before I can push, run and jump.  Wheres a good hill when you need one :)

[epfromnyc]

@Luke: Hey you mentioned the GREEN wire. Now that I think about it... that ground location is to the body right where the ignition coils mount on the bike. Hmm, I painted that area nicely and put everything back. I will check the paint and ground situation in that area as well to make sure its a good connect. Much Thanks!

[motogrady]

I'm not sure, but I thought I read somewhere,
when start button is hit headlight switches off,
then juice goes to starter....this taking place behind
headlamp.

Let us know what you find.

[Hellbilly429]

Not to be a dick or anything but is the kill switch on? Honest to god that killswitch has fooled us all before, whether anyone will admit it or not, it gets me all the time when I bump it on accident.

If the killswitch is off then check to make sure that it is actually off, they are known for shorting out and causing issues.

[epfromnyc]

Nope kill is in ON position

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From: "Hellbilly429 [via Honda CB750'S]" <[hidden email]>

Date: Tue, 4 Sep 2012 16:57:03 -0700 (PDT)

To: epfromnyc<[hidden email]>

Subject: Re: 1980 CB750F - Starting Issue Advice needed.

Not to be a dick or anything but is the kill switch on? Honest to god that killswitch has fooled us all before, whether anyone will admit it or not, it gets me all the time when I bump it on accident.

If the killswitch is off then check to make sure that it is actually off, they are known for shorting out and causing issues.

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[motogrady]

Try starting to look for voltage at the starter, when cranking, first.
Put a 12 volt meter ground directly on the neg side of the battery.
Check the meter by touching the pos side of the battery.
Put the probe right on the hot lug of the starter,
and crank.  Voltage should show.  
If not, move back to the next part in line, I dunno, what is it, the silinoid(sp?), is voltage there when cranking?
Could be a bad wire between the 2.
If you have voltage going in the wire, but not showing at the starter,
bad wire.
Move back towards the battery, first place you lose it when cranking, look there.

Heck, take a piece of #10 copper wire and jump right from the pos side of the battery
to the hot lug on the starter, if it jumps,  it's somewhere in the circuit from bat to starter,
if not, it has to be the bad ground...............

[epfromnyc]

Yep you are right, the headlamp should dim as power is directed to the starter ( when you hit the starter button).

[epfromnyc]

Okay... yeah good advice and way to check if the power is actually getting to the starter.  I do have voltage (over 12 volts) which shows going through the starter solenoid.

[epfromnyc]

Didnt get a chance last night to work on everyone's advise... but I will be trying it all tonight and will post the results.

[epfromnyc]

General question about the spark units (pair of aluminum bodied squares) usually bolted to the frame underneath the seat.  Currently I have them connected but dangling off the side of the bike since I was troubleshooting any contuity issues. Do they need to be bolted on the frame while testing? Just wondering if screwing them back onto the frame provides a ground for them? Important? Not Important?

[LukeM]

This is not important for now.  Having the spark units hanging off the frame shouldn't have anything to do with your engine cranking problem.

You can check a schematic of your bike to see if there's a ground wire on the harness, or if the unit is grounded through the frame.

So, we've ascertained that the starter spins freely when out of the bike, but doesn't spin well when installed.  It's possible that there are issues with the gears in the starter clutch on the left side of the engine.  Stupid question: is the motor seized?  Can you turn the engine over by push starting it, or by putting a ratchet wrench on the crank and spinning the motor by hand?

To get to the starter clutch, center stand your bike, get a small catchpan, and remove the left side cover.  You'll see a couple of reduction gears that connect the starter to the crankshaft.  You'll also have a little bit of oil that drips out when you take the cover off.  This is normal.  Make sure the gears spin freely in one direction, and resist going the other direction. You can also leave the cover off and see how the starter motor spins the gears.  This may help diagnose your problem.

Hope this helps.  I'm doing this from memory, as I've been without my CB750 for 2 years now.  *sniff*  Yeah, I miss it.

Luke M

[epfromnyc]

Hey Luke: Yep I have the cover off so that I can see the gears. Motor is not seized... but you are right about the push starting approach. I will try all the other electrical check first and if no results... then I will secure all the dangling wires and peices so that I can push start it.  The picture is of my bike which was a barn yard find. It doesn't look much like this anymore since I replace anything that was rusted e.g. exhaust, shocks, crash bars. Pulled off the windsheid, storage trunk and mounting gear.  Alot cleaner now.

[epfromnyc]

I think that even if the starter couldn't turn the motor... it would still make some sound ( a click or something). For now, there is just silence whilel it is in place on the bike. Spins strong while tested off the bike.  I will be tackling this tonight. Most likely, with all the advise here... I will resolve it.

[motogrady]

Looks like you have some oil showing under the heads also.
Mine was also, retorqued the head  and it stopped.

Yes, it can be a lot of work, but you get that thing sorted out, it's great.
  I mean, rev's like a freaking 2 stroke.

[epfromnyc]

Motorgrady: Well... there were field mice or chipmunks that were once living in the Airbox assembly. I pulled more leaves and nut shells outta the airbox than in my yard. So whats a little re-torquing? Hopefully better days are ahead!