Topic: electrical problem

[rmac]

I have come to my wits end and hopefully you all can help me out. When it comes to electrical problems I am useless so bear with me. First of all I have a 74 cb750k. I was on a couple day ride last summer and on our way home I slowly was losing cylinders. It was running on 2 or 3 for a while but eventually completely quit. No spark on any cylinders. After messing with it for a while I finally decided to put a whole new harness on along with a new rectifier, regulator, one new coil, points, and condensers. I have power to the blue and yellow wires going to the points, but still no spark. I would greatly appreciate it if you could walk me through what to go through and try next. I've tried timing it, but no matter how I set it, I cant seem to get a light to come on. I have no idea what I am doing wrong. Any help would be appreciated. Thanks!

[HondaMan]

First thing: check the Blue and Yellow wires near the oil tank, by the frame on the right side near the rear brake switch. Make sure the connectors where they plug together are clean and tight-fitting. These are the wires that go to the points. Then, make sure the points have power when they are open: slide a business card (or similar insulator) between the point contacts (to simulate that they are open), and then check the Blue and Yellow wires right there at the points to make sure they show about 12 volts (or use a test light, if that's all you have).

That's the first step. When the points are closed, they are 'charging' the coil that is connected to that wire (either the Blue one or the Yellow one). When the points open, the coil discharges to make the spark.

One thing that you didn't mention: the sparkplug caps. If yours are not new, now would be a real good time to get new ones. Most bike shops have them: use the 5k ohm (5000 ohm) variety (they come in several ohms ratings, so ask for this one in particular). Then clip about 1/4" off of each spark wire when installing the new ones: they screw into the end of the wires and old wires may not allow good contact.

Then report back with what you find?

[MoMo]

Also, make sure you have power to the coils at the black and white wires.  The most common problem I find with erratic or no spark is dirty points...Larry

[Rigid]

Dead battery?  I bought one new last week and after adding acid it registered a whopping 3.7 volts.  Points are just switches.  They turn on and off with the opening and closing of the cam lobe.  They "switch ground" or the return path of the current to the engine case. Black wires at the coils provide power, yellow and blue supply the connection to the points.  Points turn the ground on and off.  A test light touching the nut on the points and grounded to the cooling fins will turn on and off as the points open and close. If this isen't happening, points are not making contact or the wiring is not conducting.  The first part of your post describes a dropping battery voltage scenario exactly.

[rmac]

Ok I checked the power to the points with the points open and no power. But I disconnected the blue and yellow wires from the points and then I had power to just the wires. Do I have them hook up wrong? or bad condensers? I'm not sure whats going on. Thanks again!

[Xnavylfr]

Have you changed points or disconnected any wires at the points plate???
If you have= check where the wires connect to the points ear, the little plastic/phenolic washers have to be in stalled so the wire or the points spring DOES NOT touch the mount ear. I'll try to explain it,   The small bolt and nut should have 2 brown phenolic washers on each side of the attach ear, the wire goes between the washers on one side of the ear and the spring goes between the washers on the other side of the ear.. If any part of the blue/yellow wire connectors are touching the backing plate then it has grounded out the points . Check this by attaching your test light wire to a engine fin and touching the point to the backing plate with the key on. You should NOT have a light on the test probe.
Make sure your battery is FULLY charged. It doesn't take much power to make the bike run but if the lights are on they pull a lot of amps then it could be enough to keep it from starting!

Xnavylfr(CHUCK)

[HondaMan]

Ok I checked the power to the points with the points open and no power. But I disconnected the blue and yellow wires from the points and then I had power to just the wires. Do I have them hook up wrong? or bad condensers? I'm not sure whats going on. Thanks again!

Yeah, like "X" is explaining above. It sounds like the connectors on the wires, at the points, are touching the metal of the mount instead of the moving arm itself. Maybe they got installed wrong? Sometimes the crimp lug on the wire is mounted upside-down, which lets it touch the metal where it shouldn't. Just flip 'em over, if so.

Shorted condensors are rare. More often, they have "shrivelled" inside, making them leak electrically (like a 10,000 ohm resistor), but this won't drop the voltage noticeably.

[rmac]

Well that was easy, thanks guys!

[bollingball]

Well that was easy, thanks guys!

Why not share what you found out for the next guy?

[rmac]

Oh sorry. Just tried what they told me. The washers were on the wrong side allowing the wires to ground

[HondaMan]

Oh sorry. Just tried what they told me. The washers were on the wrong side allowing the wires to ground

Ah, 'simplicity'. SOHC4, spelled with more letters.