'78- No spark, no gas, no clue.

[Sigmatel]

78 cb750k sohc

Initially my friend and I were trying simply to get the starter turning over, and having a dead battery, we were jumping with his spare car battery. After trying for a while to diagnose the electrical problem while wired to the car battery and without doing practically anything, the starter suddenly had power. Any ideas why that would happen??

So now it's not starting..

Rundown of symptoms.
Starter is turning the engine over.
Not smelling any gas out the exhaust pipes.
Pulled the spark plug and looked for a spark and I'm not seeing one.

I'm not sure quite where to start on this one. It *seems* like the carb should be getting gas (valve is open). It's been sitting about 3 months, maybe it needs a carb cleaning???

Also, being this is my first bike, maybe we just don't know how to start it after it's been sitting. Any rundowns online that might be helpful?

Thanks in advance for any help!! I know this is just kind of a spray of general no start questions, just looking for a good starting (no pun) direction, or good parts to start replacing.

[LukeM]

Hmm.  There might be a dead spot on the commutator in the starter.  It's possible it got bumped onto a live spot, and started working again.  Other choices may be a flaky ground connection from your harness to the frame.  Look for a green wire bolted to the frame, possibly under the seat or along the backbone near the gas tank.

The mantra is:  suck, squeeze, bang, blow.  Compression in the engine is usually not a problem for these engines.  That's the first 2 steps.  The bang part comes when the fuel and spark mix.  

You can go to the home page of our forum, and put in a search for "spark test".  I'm sure it's been written up a bunch of times.  Get the ignition system sorted out first.  After that, you can check the fuel/carb systems.

If you haven't downloaded the Factory Service Manual, please go to the home page and do so.  Go to the Maintenance chapter and start checking stuff.  These bikes are pretty easy to work on, but all the systems need to be in good working order for the bike to run.  Take it slow, be thorough, pay attention to detail, and take lots of pictures.  You'll get it going.

Luke M

[TOOLS1]

Start by checking, for voltage at the coils, and then the points (the points should show 12 volts when in the open position).
TOOLS

[Sigmatel]

So now after checking continuity in for starter and testing coils, calibrating points, replaced condensers- still no spark. Plugs seem just fine and #4 fired when testing coil straight from battery but only after 5th try- suspect short somewhere maybe?
Thanks in advance- tired of gigantic paperweight.

[sgtslag]

Did you measure the Resistance (Ohms) of the Coils?  The correct value is listed in the FSM.  It sounds like the coils might be bad, though, if it took five tries to get a spark connecting it directly to the battery.

Did you test the plug wires to make sure they have continuity throughout?  If the end caps are not connecting with the solid copper core, no juice will flow to the plugs.  Check the condition of the end caps, as well:  look for cracks, broken/bent metal clips within, etc.  Replace, if needed, but be sure to avoid carbon plug wires, only use the 'old fashioned', solid copper core wires.

You have several options, if the coils are bad:  replace with stock (new, or used, if available); replace with after-market coils; or replace with GM automotive coils (information can be found either here, or on the CB750 Custom web site, link, join, then search their forum for "GM coil" -- make sure the GM coils have the correct resistance, though, as some GM coils are 0.3 Ohms, and your ignition system may require 3.0 Ohms -- maximum power transfer occurs when the coils' resistance matches the OEM specification).  Best of luck.  Cheers!

[Lucky 1]

Coils almost NEVER go bad.

How old are you? Are you at least high school age? Just asking.

You do not need to remove a spark plug to see if it has spark.
Just clamp a DC inductive timing light onto the plug wire ...any plug wire.

Also when you removed the spark plug to see if it worked
 did you have it grounded to the cylinder head?

Just asking.

Also if the points and condensers are old or worn then you may have NO spark.
You need to replace those . BUT FIRST GO GET a shop manual for that motorcycle.
You will have to have one.