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No spark

jamman
Not getting any spark on cb750f 1977 model. Can anyone tell me where the coil is on this model?  Any suggestions to the issue would be appreciated.
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Re: No spark

LukeM
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They should be under the gas tank.  Trace the spark plug wires from the engine, and the coils will be on the other end. :-)

Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Re: No spark

jamman
Just checked the points and one of them doesn't move at all when cranking the engine...could that be the reason I have no spark?
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Re: No spark

LukeM
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That would explain why one pair of plugs isn't sparking.  If the other point set is opening when it should, you should be able to see spark on the plugs.  Get the Factory Service Manual for your motor, and go through the points adjustment process a few times.  You should be able to get pretty close with static adjustments.  Dialing it in will take a little patience, but that will come later.

What basically happens is when the points open, the voltage change across the points induces a voltage change on the low side of the coil.  When that change goes through, the spark level voltage is induced on the high side of the coil.  It then discharges across 2 of the 4 spark plugs.  While it does fire both plugs, on the one NOT on the compression/power stroke, it's wasting the spark on the exhaust portion of the cycle.  It just saves the complexity of a distributor.

Look for loose wires (around the connection for the points and the condensor), good connections for the feed wires to the coils (one is blue, the other is yellow, if they're wired like my '79 motor), and the fuse for the IGN section.  You can check the point gap and when the points open from the manual.  Just make sure you have the key OFF unless you like getting little jolts of electricity when you put your metal feeler gauge inbetween the points.  It's not fatal, but it IS annoying.

I hope this helps.  Welcome to the wonderful world of manual points.  When it works, it's a beautiful thing.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Re: No spark

jamman
Thanks for the info!  I just tried checking all the plugs... no spark in any but I tried them on another machine and they get spark.  I ajusted the point so it opens now but still no spark.  Think it moght be the coils? Not sure where to go from here.
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Re: No spark

jamman
In reply to this post by LukeM
....also, in the past 2 weeks I have heard this loud clicking that increases as the throttle increases and decreases accordingly.
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Re: No spark

LukeM
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OK, since neither coil is firing, find a good schematic of the wiring for your model motorcycle.  Trace down the wiring from the battery through the coils, then through the points, and down to ground.  You should have +12V somewhere, and if you measure from the points to ground, opening and closing the points will show a change in voltage.

You should get the same results on both sets of points.

If you don't have +12V anywhere in the ignition circuit, I'd look for a blown fuse or a broken wire somewhere.  Re-Run has a 1978 model of your bike, and should be able to help.

Keep at it.  Don't get discouraged.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Re: No spark

jamman
So you think it would more likely be a fuse or broken wire rather than bad coils because the likelihood of both coils going out at the same time is nil and that the clicking is a short? ... Just so this electrical novice is understanding correctly.  ;^)
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Re: No spark

jamman
In reply to this post by LukeM
Oh yeah....I do actually have the original manual and schematic for the bike!
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Re: No spark

LukeM
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In reply to this post by jamman
Generally speaking, when troubleshooting electrical networks, you can either start at the far end and trace it back to the battery, or start at the battery and trace forward.  Since neither coil is firing, either both sets of points are bad or out of adjustment (which could happen).  If you were tracing back, verify the points are good, and you are getting the voltage changes you expect.  If they are there, then move up the cables (blue and yellow) to the coil connections.  Again, unless both are disconnected, it's unlikely both coils are bad.  On the other side of the coil should be the connection to the battery through a fuse.  This is why I suspect there's a blown fuse.  One of the coil connections should go back to the +12V on the battery.  I just can't remember which.

Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Re: No spark

jamman
Well, so far I've checked for power at the coil and points as I run the starter and no power. All the main 3 fuses are ok but still searching for short.
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Re: No spark

LukeM
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I will find a copy of the FSM or a schematic for your machine, and get back to you.  
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Re: No spark

jam718
Upon inspection of the points, the one that was not opening is really black and pitted. The other one is not great. I'm going to cle those up and regap. Question on that is - all the info I read talks about turning the crankshaft until the points are each at their max opening. How is this turning done?  Just by pushing on the kickstarter, I'm guessing?  I really hope this winds up being the issue.  I will need to test the voltage at the points as well.
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Re: No spark

LukeM
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You can put a socket on the crankshaft, and get very precise with a socket wrench or breaker bar.
Yup, pitted points will cause problems.  If there's enough meat on the contacts, you can use some emery cloth or a points file.  Just make sure you clean them REALLY good before putting them back on the bike.

Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Re: No spark

jam718
So if I understand correctly, I will put the socket on the crank on the points side, not the left side of the bike right?
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Re: No spark

jamman18
In reply to this post by LukeM
Getting the new points this week. After installing and adjusting them, is it mandatory to do the timing or should that still be the same as it was before?
Phil. 4:13 " I can do all things through Him who strengthens me"
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Re: No spark

LukeM
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Setting the timing via the static method should provide spark.  Once that happens, then you can get out the dwell meter and timing light to more accurately set the timing.

Keeping fingers crossed for you.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Re: No spark

jamman18
In other words, once your point go out and they are replaced, the timing will never be the same as before until you time it again, correct?
Phil. 4:13 " I can do all things through Him who strengthens me"
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Re: No spark

LukeM
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Not quite correct.  If you haven't loosened the screws holding your points baseplate (the round piece that holds the point sets), then you shouldn't have changed your original timing.

With that being said, there is always a little differentiation between point sets.  It's worthwhile to check the static timing on the point set after installing them.  Once the static timing is set, then you can put the dwell meter on and adjust the dynamic timing.  

This is assuming, of course, that you get good spark after putting on the new points.  If the new points don't fix the problem, then we'll have to look elsewhere in the chain for the problem.

I'm hoping for updates to follow.
Luke M
Used to have a 1979 CB750L, sold it as a parts bike, now riding a slightly modified 1984 VT700C. Network/Field Engineer. Central OH, USA, Earth, Sol System, Milky Way Galaxy, Universe.
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Re: No spark

jamman18
OK, so will a timing lght work or do I need to use a continuity light do do the job when I get to that point? I appreciate you input!
Phil. 4:13 " I can do all things through Him who strengthens me"
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