'77 CB750 point probs

[jamman18]

        The new coil I got from a company on ebay came with new wires and boots, etc.  It fired the bike up fine for 5 minutes about 1 month ago but wouldn't start again after that.
        What I don't get is that the way it should work (as far as my novice head understands it) is that as each point set takes a turn at opening, that causes the circuit to send a pulse back to the secondary side of the coil which sends it through to the plugs.   If there is no power when a single point is open as is my case, then there would not be any pulse generated back to the coil.  That's why I thought maybe I did something wrong.  Am I understanding the way it works correctly or am I totally screwed up?    

[TOOLS1]

Yep your screwed up LOL But you almost have it.
The way the point ignition system works; is when the points are closed it completes a circuit that allows the coil to build a charge. When the points open the coil then discharges this stored energy out it's secondary side to fire the plugs. This is when the condenser absorbs any residual energy keeping the points from arching, and burning up. Since one power wire supplies both sets. Both points would have to be open for you to get a reading, and as you can see; there is no point when both points are open at the same time.
I just had a thought. When you are checking for spark. Are you grounding the plug to the head? If the plug is just hanging in the air; it will not fire. Also both wires must be connected to plugs that are both grounded.

Are you anywhere near Kansas City?
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[jamman18]

...or should I not get a reading just as is and that means the system is working correctly?

[jamman18]

Well, I'm glad I understand the system ok.....When I check for spark I take the plug out reattach the plug wire and lay it on the head. Then I try to crank it over. Do I need to hold it there with a bit more pressure or will I get shocked?  Remember, you are talking to a novice, albiet a novice that has learned much in the last 2 mths since the bike went south!  Oh, and I'm not anywhere near Kansas City, though....Cañon City, CO actually.  Thanks for your knowledge!

[TOOLS1]

Since Honda painted all it's engines with silver/ gray paint; you may not be getting a good ground. Now in a perfect world. You should be able to just hold on to the cap to apply some pressure and not get shocked. But then in a perfect world you would not need to work on anything Now the ifs, if your wires and caps are in good shape, just hold it with your hand. If you have any doubts, hold it with a pair of pliers with insulated handles.
I know that Denver is a 10 hour drive, and that's not too bad. Just load the bike up and bring it, and an ample supply of beer, and I will get it going for you
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[jamman18]

If I don't get a spark, what is the easiest way to test the coils to eliminate that as an issue>  I mean, I got it off ebay by a reputable seller and it did work at first......not sure if there is any reason that it would have gone south or not....

[sgtslag]

The coils should read 3-5 ohms resistance on an Ohmmeter.  If they read infinite/open, then they are bad.

Regarding the comment on Iridium spark plugs, they work exceptionally well on my '79 CB750K.  My bike was requiring me to grind the starter for 3-7 seconds, before it would fire.  When the engine was cold, it would require two grinding attempts before it would run, but I would pause for 20-30 seconds to let the starter cool down a bit, before hitting it again.

I suspected the coils were weak, so I priced new ones:  $160+ for the coils I thought would be superior to OEM.  I priced NGK Iridium spark plugs, at $8 each, so I figured $32 was a lot better option than the $160+.  

The Iridium plugs are supposed to require less voltage to jump the gap; they supposedly fire more consistently; they supposedly give a 2-3 MPG increase in fuel efficiency; in automobiles, they typically last for 60,000-100,000 miles -- in a motorcycle, they should last 20,000+ miles.  The admission price was accessible, so I took the plunge.

When I hit the starter button after installing them, I about jumped out of my skin -- the bike started instantly, and I was grinding the starter for a half-second while my brain processed what my ears/hand were telling it, that the bike was running already.  The old spark plugs (12,000 miles), standard NGK's, original from the bike's uncrating and assembly back in 1979, looked great, and they worked after the bike started, they just took a bit of coaxing to get it going.

I now have 9,000 miles on the Iridium's, and it still starts almost instantly.  It idles smoother, but the MPG has not changed that I can tell.  I switched my '93 Kawasaki Voyager to NGK Iridium plugs as well, but on that bike, I really couldn't tell much of a difference.  Still, they will outlast the standard plugs by more than double.  On the Voyager's more robust ignition system, they should maximize the performance while extending the maintenance period.  With fewer misfires, they will reduce emissions, and that is a good thing, too.  Cheers!

[jamman18]

Well, I was shocked when I went to check my coil again yesterday I also checked for spark at the plugs by holding with pliers. Got a good spark so I tried turning it over and it did! Rode it around the block and then restarted it to make sure I wasn't dreaming. After lunch I rode it over to relative's about 1 mile away and stayed a couple of hours. Went back out to start it up and nothing!  Checked power to points and that is still fine but no spark at the plugs!  I did, however, remember hearing this loud "clicking" sound when I rode it last. Maybe grounding out somewhere?

[LukeM]

Or maybe not grounding out somewhere.  Voltage for spark plugs needs to go to ground somehow.  Maybe the ground for the coils is loose or corroded?  On my 1979 model, there's a green wire which is bolted to the frame near the coils.  Maybe there's a similar connection for your bike that's just not making it?

It's a total guess, but if you got some spark, your pieces parts for ignition are working.  So I'm thinking loose connection somewhere, or maybe something's getting shorted to ground where it shouldn't be.

Maybe it's time for a qualified mechanic to take a look at it?  I hate to say that in front of a bunch of aficianados, but maybe it's time.

Luke M

[jamman18]

Yeah, I have a couple of people around town I found that I can contact....I had already found the green wire and wd-40'd it. Thanks for the help!

[TOOLS1]

I think I know what your problem is, and I wish I had mentioned this sooner. The old 750's had problems with the fuse blocks. The connections on the wires in the back are not very good, and they build up heat , and blow fuses. I have seen people put screws and 1/4 inch bolts in place of the fuses
I will bet some one has installed a breaker in place of the 15 amp main fuse on your bike, and that is the clicking you heard. A breaker is better then a fuse, but it does not cure the problem. I cut the wires off the back of the fuse block, and then wire in 3 in-line fuse holders (blade type). Its easy, and only takes about 20 minutes to do. I also use a 15 amp blade type breaker for the main, and 7 1/2 amp blade type fuses for the lights.
Hope this does it.
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[jamman18]

Wow, wish this were the problem, but all the fuses are new and in good shape.  The sound was actually coming from the area up front where the plugs are so it is probably not coming from the fuses; was way forward of that.  

But thanks for the idea!

[jamman18]

Latest update is I have power all the way to the points but no spark. Checked for shorts and have not found any yet.  If used ebay coil is bad how do I check for that besides checking for spark at the plugs?

[TOOLS1]

Do a continuity test. Take an ohm meter and check for continuity between the pos, and neg leads. Then check for continuity between the two plug wires. You should have around 3 ohm's resistance on both.
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[jamman18]

Great!  I will try this test.  Also, I was thinking....if the points slipped and either have no gap or not enough gap now, then they would not allow the secondary side of the coils to operate power to the plugs......correct?  If so, then it seems that it would explain why they worked at first but then don't now.  Comments?

[TOOLS1]

Of course it would. Are your screws stripped out? I mean the bikes. LOL
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[jamman18]

Yeah, sometimes with this problem I feel like my screws ARE stripped out....but the bike on the other hand does not have stripped out screws. I was just thinking that I might either not have the points adjusted right or did not tighten the screws enough.  I will look at this and check contin on the primary and secondary sides of the coil and let you know.

Thanks loads!

[jamman18]

Well, I tested the coils as you said by checking ohms with 1 probe on black primary and the other to the yellow and blue
wires respectively. Then tested the 1-4 and 2-3 coil plug wires with the caps off.  I got readings of zeros on all these so
from what I understand about continuity, these are the readings I should get for good coils.  That being said, here is what I
do know at this point: the plugs are good, points and condensers new and adjusted right. I have 12v through to the points.
The bike HAS started and run 3 times since this problem began. I have continuity through both sides of both coils. When I try
to get a spark on the plugs by holding the plug against th eengine block I get no spark, although I have before the bike
started the 3 other times. The ignition switch has been bypassed by a main toggle switch which renders the ignition switch
and kill switch inactive. I just toggle the power on to start the bike and toggle off to kill the motor. The starter runs
fine. By everything I know, the bike SHOULD work but it doesn't.

Let me ask this: can the timing be so far off that it won't let a charge go through to the plugs to get a spark?  Seems to me
if it was running before (very well too!) that this would not be an issue but I'm grasping for straws at this point.  Seems
that there is more likely an intermittent short that occurs somewhere in the wiring that is keeping the bike from
starting....but WHERE I don't know since the only thing downline from the points where I AM getting power are the coils which
seem to be working fine. Did I test these correctly?  Any other suggestions?

[TOOLS1]

Since the coils checked out, and it would be improbable that both coils would be bad at the same time; I am going to have to go back to a short. Have you tried running a jumper wire from the battery to the coils?
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[jamman18]

No, but that is something I will try. Thanks!