Topic: Can you help? (Solved)

[Marv]

Good evening all,

I have a really simple question (for someone!), but I am struggling!  I have searched the forum and although there are multiple questions about this topic none seem to answer my question.

1973 CB500 Four
No spark at plugs.

I have full power at the black and white wire going into the coils.  With the yellow and blue wires that runs from the coils to the points, I have one which has full power and when I turn it over the other only has half the power (literally 0.02 V until I press the start switch when it increases to circa 5/6v).  This must be the reason I am not getting a spark but I don't know what is causing this.  The plugs have fuel and the points are creating a good spark.  The coils are well earthed.

I have re-wired the complete bike and the only problem I can think off is I have used 5amp wire for the rectafier / regulator, would that cause the problem?  The thing that makes me think not is because I have had it running since I finished the wiring.  I stopped it with the cut off switch and it hasn't worked since.

Any thoughts about what I should test would be appreciated.

Thanks.

[mrfish2]

Have you tested the resistance of the plug caps?

[Marv]

Just tested them, the plug caps give resistance figures of....

12.1
1 (Broken?)
10.18
10.12

Two of the HT leads had very little core at the end so I also shortened them to get more of a contact area.  it is still not sparking though.

[mrfish2]

They may not the singular issue for your no spark condition, but you definitely need new plugs caps for at least #1 and #2. The optimal resistance for the caps on your bike is 10k ohms, so getting a reading over 12k and infinite means they are way out of spec and need to be replaced.

I don't think new 10k ohm caps can be found anymore though, but new 5k ohms caps are just as good and are what I put on my 550. I can't find the thread right now explaining the difference between the 10k and 5k caps, maybe someone else will chime in and explain better than I can.

[Marv]

Thanks Mr Fish....I will get some new plug caps to see if it helps.

If anyone else has any other things I can check please let me know 

[Marv]

Ok, so I got new plug caps today for the two which were not in the best shape, replaces them, but still no spark.

I also changed the wires where I had used 5 amp to 17 amp to see if that made a difference, it didn't.

The only think I can think it is now is that there are 3 yellow wires coming from the rectafier and going to the alternator.  At the connector have these got to be a certain way round?

Anything else I could check?

[flybox1]

the FAQ section of the forum usually has helpful info...

I used this .pdf extensively when testing my charging system, before enlisting the help of the forum.
gather the test info, then report back...
http://www.hondachopper.com/garage/troubleshooting_charging_system/troubleshooting_charging_system.pdf

excerpt from hondachopper.com....
Testing coils
The coils on these bikes are very easy to test. There are two sides to the coil. The primary and the secondary. The primary
side is the small connectors that attach to the wiring harness. The secondary side is the spark plug wire connectors. We
are going to test resistance, so the coil must be isolated and de-energized. Basically, disconnect the wires from it. You do
not want it attached to the wiring harness for testing.
PRIMARY RESISTANCE
Set the multimeter to measure ohms. Use one test lead on each of the small terminals that attach to the wiring harness. If
using stock coils, you should get a reading of approx 5 ohms. If using DOHC or most aftermarket "hot" coils the reading
should be about 3 ohms. If your readings are very far off from the given values, the coil is probably bad. If you do not get
a reading, or if it flickers in and out, then there is a broken wire and the coil is bad.
SECONDARY RESISTANCE
Set the multimeter to measure ohms. Once again we are going to isolate and de-energize the coil. Use one test lead in
each of the spark plug wire connectors. You may have to wiggle them around to get them to a spot where you are getting
a reading. Depending on the coil you should get a reading of 5Kohms to 20Kohms. I don't have a manual with me so I will
edit later for the actual cb750 specs. If you get no reading, the coil is bad. This test can also be performed with the plug
wires and caps in place. Due to the extra resistance though you should get a reading between 10Kohm to 50Kohms
depending on the coil, wires and caps. If this is done first, and there is no reading, do the test without the wires. If you get
a reading with no wires and no reading without, then the wires are most likely your problem.
INSULATION BREAKDOWN
These coils have a solid metal core running through them. It is used solely for mounting. It is not a ground. Sometimes the
insulation inside the coils starts to break down and the coil will short to the metal core. Set the meter to measure ohms, or
use the continuity function if your meter has one. You can use resistance(ohms) because even if it has no resistance it will
read 0 ohms. This shows that there is continuity. Continuity means that electricity can pass. A wire has continuity, but if
you cut it in the middle, it does not.
What we are going to do is use one test lead on the center mounting bar. The other test lead is going to go to each of the
primary terminals, and each o the secondary terminals. In effect we are doing 4 tests. If any one shows continuity to the
center core, then the coil is bad.
One other thing to remember is that the coils do get hot with use. It is possible for a coil to test good, but once it gets hot,
the insulation inside starts to get gooey and breakdown. If it starts to misfire after it is hot, let it warm up until it happens
and check the coil while it is hot. This can be tricky because it still needs to be disconnected first.

[Marv]

Thanks for the response flybox1, I have done all said tests and the values are coming out smack on the values stated in your post which would infer they are fine....

That's one thing to check of the list.  Any ideas on what's next to test?  I have read numerous posts on this site and others but and still stuck.

[Marv]

Hi all,

Just though I would update the thread in case anyone else suffers from this.  Where as I thought the problem was electrical it was in fact a problem with the fuel system.  The plugs were completely covered by dry fouling which prevented a spark.  I am sure most of you already know this but it is caused by the fuel / air mix being too rich.

I am going to check the timing and if that is ok have a look at altering the air / fuel mix.