Topic: Ignition Issues: Out of Ideas

[Findlater]

I have a 1970 CB 750 I run fairly regularly.

It was bogging down recently and discovered cylinders 2 and 3 (the middle ones) stopped firing.
What Ive done so far:

Re-gapped the Points
Changed the condensers
Replaced the Coil for the 2-3 cylinders.

NO SPARK.

Im out of ideas anyone know whats wrong? Maybe I need to replace the point? It didnt look worn out. Any ideas?

[Johnny5]

Yeah, I would replace the points and condenser together. Personal preference.

[technojock]

First off make sure there is power going to the 2-3 coil.  Unplug the connector to it and check it with a volt meter.  

If that's OK, set the meter to ohms and check the resistance from the where the point wire connects to the coil to ground.  With the condenser unplugged, you should see a clean break from 0 to infinite resistance as you manually open and close the points.

Tony

[garagedoreen]

check to see if your connection for the point and condensor is not grounded out. If you have those little terminals in the wrong position it will ground out on the back plate and you will not get spark.

It happened to me.

good luck

[Johnny5]

check to see if your connection for the point and condensor is not grounded out. If you have those little terminals in the wrong position it will ground out on the back plate and you will not get spark.

It happened to me.

good luck

Excellent point and very easy to do.

[Findlater]

check to see if your connection for the point and condensor is not grounded out. If you have those little terminals in the wrong position it will ground out on the back plate and you will not get spark.

It happened to me.

good luck

Excellent point and very easy to do.

Can you link a picture so I can verify the position of my terminals? Thx Im thinking I might have mixed them up when reassembling but not sure.

[Findlater]

My terminals are okay but still no spark- Im going to check the power going to the condenser terminals in a few mins here. Anyone else know what it could be?  I also saw a post that to gap the points dont use the manual and gap the points at their highest and not on the "F" mark. ias this ture for the '70 and the 76?

I have also now checked the power to both 1-4 and 2-3 coils and they both have about 12 volts. In fact the 1-4 coil which is firing is only holding 11.6v while the new 2-3 coil has a solid 12v

[bryanj]

If your manual says gap on the F mark its a crap manual as the F is the firing point

[Findlater]

If your manual says gap on the F mark its a crap manual as the F is the firing point

Well lol..... its the Clymer manual for CB750s from 1969 to 1978 so Im sure theres a few people with this manual Regardless of what it says I have it gapped just like they say and its firing on 1-4 but Im not getting ANY spark off 2-3 doesnt matter where I have it timed to at this point. I just need it to fire.... and its not.

I re read my manual and realized it was to move the point to its highest and gap referencing a picture where it was set on the firing point. Anyways I've gapped properly now and my bike seems to have a little more power but it still is NOT firing on 2-3. So anyone care to take another shot at this?

[Findlater]

 

If that's OK, set the meter to ohms and check the resistance from the where the point wire connects to the coil to ground.  With the condenser unplugged, you should see a clean break from 0 to infinite resistance as you manually open and close the points.

Tony

Hey Tony what do mean here exactly?

[bryanj]

If you have power into the coil, and power at the wire connected to the points(when disconnected), with the wiring in the correct place  the points are either not opening or not making contact when closed

[Findlater]

If you have power into the coil, and power at the wire connected to the points(when disconnected), with the wiring in the correct place  the points are either not opening or not making contact when closed

How do I check power at the points?

[Johnny5]

If you have power into the coil, and power at the wire connected to the points(when disconnected), with the wiring in the correct place  the points are either not opening or not making contact when closed

How do I check power at the points?

With a 12v test light: Put the bike on Run, turn on the ignition. Ground the lead to the engine case and touch the positive lead to the point arm when the point is open. The light should be on when points are open, off when points close.

[Steve F]

Yeah, I would replace the points and condenser together. Personal preference.

Ditto

[mystic_1]

Diagnostic:

At the coil connections, swap the yellow and blue wires (thus swapping the coils) and check for spark on all four.  Note, bike will not run like this.

Problem remains with 2/3... bad coil.

Problem moves to 1/4.... problem in points system.

Put yellow and blue wires back where they belong.

mystic_1

[Findlater]

Thats a good idea mystic thanks! Im going out to do that now- I will let you know how it goes in about 10mins.

I did a volt-test and when 1-4 open it came up 11.4v and when 2-3 open it came up 0 So I think it is a point problem but Im not sure howto solve it.  Im going to try mystic's trouble-shooter here to confirm that.

[Findlater]

After switching the wires and performing the volt test when 1-4 0v and when 2-3 open 12v it seems to be a problem with the yellow wire? where could this be grounding out? or what may be the issue here?

[mystic_1]

check to see if your connection for the point and condensor is not grounded out. If you have those little terminals in the wrong position it will ground out on the back plate and you will not get spark.

It happened to me.

good luck

I'm leaning back towards this, double-check the terminals where they connect at the points.  The order of the terminals and fiber washers is important.  If you take a resistance reading (bike off) between ground and the end of the wire (at the coil connection) you should have continuity when the point is closed but no continuity when the point is open.

mystic_1

[Findlater]

OKAY SO
What Ive done so far:

Re-gapped the Points
Changed the condensers
Replaced the Coil for the 2-3 cylinders.
Replaced the Point for the 2-3 cylinders

Checked the voltage when 1-4 open 12v
Checked the voltage when 2-3 open 0v

When I switch the wires around 1-4 open 0v
2-3 open 12v HOWEVER, at this point when I test 2-3 for spark, still NO spark,

Also what is the order from nut to nut and everything in between for these washers?

[Findlater]

Okay based on the most recent voltage and resistance readings:

Both coils are at 12volts when their respective point is open and at 0 when it is closed.
Both coils show resistance when the points are closed.
The point on 2-3 continues to show resistance when the point is open along with its open circuit voltage.

Does this mean its grounding out at some point? If so the most likely spot would be? perhaps this really is an issue with the fiber terminals? I will post a picture of what I have in about 2-5mins once I upload.

[Findlater]

[Johnny5]

The green wire on the 2-3 side is sorta attached at an angle, could this be grounding against the points cover?  Or is it the same with or without the points cover on?

[punch455]

There is an easy way to check if your yellow wire is grounding out somewhere.  
1.  Disconnect it from the point.
2.  Get your ohm meter.
3.  Take one lead of your ohm meter and connect it to the yellow wire.  Take the other lead and connect it to engine ground or the negative battery terminal.
4.  Look at your meter.  It should be in the range of MEGA Ohms or O.L.  If it's less, you basically have a short to ground.  I would move the wiring harness where the yellow wire is located back and forth a bit to rule out any intermittent grounding/chaffing.

Let us know what the meter reads! Good luck!

[Findlater]

There is an easy way to check if your yellow wire is grounding out somewhere.  
1.  Disconnect it from the point.
2.  Get your ohm meter.
3.  Take one lead of your ohm meter and connect it to the yellow wire.  Take the other lead and connect it to engine ground or the negative battery terminal.
4.  Look at your meter.  It should be in the range of MEGA Ohms or O.L.  If it's less, you basically have a short to ground.  I would move the wiring harness where the yellow wire is located back and forth a bit to rule out any intermittent grounding/chaffing.

Let us know what the meter reads! Good luck!

Im not going to lie Im pretty damned frustrated at this point... mostly because Im not an electrition. I could tell you what numbers I see but I dont even know what to put after it to determine what type of value it is.  All I know is when I did I was told above I had the same number whether or not the point was open and that number was 047 to 052 somethings of ohms. I had my meeter set to 2000K ohms so short?

[punch455]

Ah, no problem.  But make sure the wire is DISCONNECTED from the point. Do not just open the point.  Once the yellow wire is HANGING and free, then do the test.  I know it's frustrating.. I have the same problem with my bike except that it's 1-4 that aren't quite firing.  If it helps, imagine the meter as sending a little bit of current down that yellow wire. You can use the other meter lead to probe around to check for that current.  If it finds it, it will tell you by showing a low resistance (ohms) value. 

Here's a very easy example..

Touch the two meter leads together, you should see a VERY low resistance value (usually less than 1 ohm).  The meter is telling you "Hey man, there is a good connection here!"
Now hold the meter leads apart.. the meter should read a very high (infinite) amount of resistance.  There is no physical connection between the leads.
Now, hold each lmeter lead in your hands and let them touch your skin.  The meter will read a very high, but not infinite, resistance level.  Your body is making a physical connection between the two meter leads, and the meter is giving you the electrical resistance of your body.

So, if you have a low resistance between the yellow wire and the engine ground, you know that they are connected, which means you have a short to ground. 

As far as the ranging goes, leave it on the 2000 (aka 2k) setting for now.  If the needle (i'm assuming is an analog meter) pegs all the way high, go up one setting.  If it does not peg, give us the reading.

Hope that helps a little!
Max