75 CB750K STILL has me stumped!

[CB750Winger]

  To continue my post from a few weeks ago, I put in new spark plugs, both coils, (caps all test good) new points and condensers. The carbs have all new parts and are synced almost perfect. This has a stock air box and 4 to 2 MAC baffled exhaust. The needle jets are in the middle position and I have played up and down with them. The air/fuel mixture screws are set to the book and seem to be ok. The bike idles perfect but sputters between that and about 3000 rpms. When I timed it with my light yesterday, 1 and 4 seemed to be ok but when I put the clip on 2 and 3, as soon as I get above idle, the light quits but the bike still sounds ok as long as there is no load on it. I went back and forth with the timing light between 1&4/2&3 and had a hell of a time setting the timing, still don't know if it will stay put. I do know the cam position was right on when I had the cover off and the cam chain is tight. The timing advance seems to be working as it should but what would make the timing light go out above idle on 2 & 3??? Do I need to replace the springs on the advance? They seem pretty tight. I'm lost now.

[cbRider]

With the timing light going out I would say that you are having an ignition problem. Have you swapped the coils for a test? If you swap the coils and the timing light problem follows the problem you could have a weak coil or, wire or connector. If you have points and condensers, it could be a low voltage problem like a connection to the points or possibly a condenser.
Good luck

Karl

[CB750Winger]

 I don't recall the timing light going out on the old 2/3 coil but I guess I could put it back in and try it. I'm on my 3rd set of new points and condensers but here's one wierd thing, on both sets of new 2/3 points, I noticed sparking on the back side of the contact and the wires look ok and the connections are well away from the breaker plate. Both times I've put the old original points from 2/3 back in. It's almost like the new points contact is not in the hole tight or something. Maybe I have a wire harness short to the 2/3 points? This is just too wierd. I bought the entire new breaker plate assembly so the wiring is also new up to the frame where it plugs in. Maybe I need to replace the wire all the way to the coil?

[cbRider]

I noticed sparking on the back side of the contact

It looks like that's your problem.
#1. Make sure your points are installed so they open and close without binding and are perfectly aligned.
#2. I would disconnect the harness up by the frame and with an ohm meter check that you have zero
resistance across the point when closed and infinity when the point are open.

[CB750Winger]

  The point contact is good and aligned with no binding. The point springs are tight too. When I put the old points back in, there is no spark on the back side but very little spark between the contacts unlike #1&4 set. I get the same symptoms with old and new coils. I'll try what you suggest with an ohm meter. Should I check the wire up top where it plugs into the coil? Maybe I have a bad or pinched wire somewhere? That's about the only thing I have not done. Thanks guys!

[cbRider]

There shouldn't be a big spark at the points. A big spark could indicate a bad condenser.

[CB750Winger]

  Ok, so I don't have huge spark at the 1/4 points but constant. Maybe that's why 1/4 plugs are sootier than 2/3? Can hardly see any spark at the points on 2/3. I'll put the original condensers back in and see what happens. Dumb question, when I check the ohms, I stick one probe in the wire and where do I put the other probe at the points, on the back side? I'm not good on the electrical as you can tell. Maybe I can do away with a lot of headache if I just put electronic ignition but really don't want to spend the cash right now.

[Polymer-2]

You might try disconnecting the wires at the points and at the negative side of the coils then temporarily run jumper wires directly from the points to the neg side of the coils and see if it makes a difference. If so it is probably a harness problem. Also make sure you have a good strong 12v at the positive side of the both coils with the key on.

[CB750Winger]

  Sounds like an easy test polymer, I might try that. I think I have enough wire and some connectors laying around. Thanks!

[Polymer-2]

Just be sure to connect the correct point to the correct coil.

[Tech78]

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[pena750f]

I checked all your replies and hadn't seen any where you may want to look into your electrical. These bikes can be difficult if the littlest electrical problem pops up. The battery, the charging, all the wires and the ignition switch. It only seems that you have done about alll there is to do with the coils, carbs, and wires, you may want to look into other areas that will affect your bike. I can't give any place to start but it may not be where you think it should.

[Tech78]

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[Cricket]

Hmmm, you said that it sputters between idle and 3K. What does it do above 3K?

First of all I know nothing about the '75's ignition system, but I used to own a '67 VW bus and it had all manner of little electrical problems like you're describing.

If the spark is escaping to parts unknown above idle (i.e. your timing light is going out) with and without the old parts installed it may be that a wire or component after the spark leaves the points is being vibrated in a way that causes it to short either against another component or ground.

Have you inspected all the stuff after the points? Anywhere a plug wire touches anything is a potential short point. I didn't see if you replaced your plug wires, but maybe a test of them would reveal something (resistance, but also continuity between wires 2 & 3). Maybe 2 & 3 are swapping spark somewhere in their run to the plugs. As their cylinders are next to each other I would suspect they spend more time together than wires 1 & 4. Try jiggling wires 2 & 3 with the timing light hooked up to one the other and see if it suddenly starts working.

Again what do I know? Just another way of looking at the problem.

Anyway, good luck and be sure to let us know what the problem was when you fix it (and you will fix it).
Jimbo

[CB750Winger]

  Thanks again for all the input guys! For some reason, my own post locks up my computer at work so I'm home now. Jimbo, I have brand new coils, wires, plugs and caps, entire new breaker plate with points and condensers that came with new wire up to where it plugs in behind the oil tank in the frame. I have not followed the wires the rest of the way to the coils so there might be a problem somewhere. The bike idles like new but sputters between 2500 - 3000 rpms. Anywhere else is great and it goes like a rocket.  Another suggestion above was to run jumpers from the points to the coil and sounds like a great idea. The previous owner did some half-assed things on this bike and I can't stand it, things have to be right. I know I probably paid too much for the bike and I've sunk another $1500 in it this year. It pretty much has new everything, except wire harness. I'll dig into that next. I really appreciate all your help and support on this, it's making me crazy.      Paul

[CB750Winger]

   Status - first of all cudos to all of you for your ideas and polymer for the one about running jumpers from the points to the coils. This morning I ran new wires direct from the new point wire connectors up to the coils and made this permenant while I was doing it. It fired right up with no missing or sputtering what so ever no matter what rpm. I ran it about ten miles at all rpms and has never run this good or smooth since I've had the bike. This thing should be as good as new now because almost every component IS new. I also replaced the starter button today because it was sloppy and found out the dumb a_s before me rigged it with no return spring but still not make contact or cause a short. If you've ever replaced one of these, it would help to be a magician or have four hands. I'll give the bike a good run tomorrow but it seems this problem is gone so thanks again to you all!        Paul

[Cricket]

Paul,

Most excellent news. I was just thinking about your problem this morning and it inspired me to look at my '81's ignition schematic. Coils after the points (or pulse generator in my case), hmm?

Given that, jumping the wiring between the points and the coils made sense (you're a genius Polymer ;).

Mostly I was thinking about how bummed you sounded in your last post about how much money you've put into the bike, etc.

I'll take this opportunity to say that we own bikes that are worth putting money into even if we never see a financial return on our investment. There's a lot of guys out there that ruin these bikes by trying to customize them (without knowing what they're doing) or just plain old running the shit out of them without putting a dime into them.

You're obviously the type that sees the value of the bike and naturally thinks it should be maintained. If it weren't for guys like you there wouldn't be any 78 CB750K's left for future generations (this includes the children of the jackasses I mentioned above).

Besides, there are worse hobbies you can sink your money into and get no return. I golfed for years, spending thousands of dollars to smack a ball around with high-priced sticks. It's a great game but nothing like the satisfaction I get from riding a classic motorcycle that I personally repair and maintain. Plus, I can easily spot my 81 750k or 74 Moto Guzzi Eldorado in a parking lot full of Road Kings and Ultra-Glides. (By the way, my buddy just paid $1300 to the Harley dealer to diagnose and replace the fuel pump on his Road King just so he can strut around looking like a member of the Village People... I love him anyway.)

Keep up the good work,
Jimbo

[CB750Winger]

  I appreciate the comments and yes, I really enjoy these old bikes and won't quit until it's perfect. It ran good and smooth on the way to work this morning so will keep an eye on it. No missing, sputtering or popping at all. It doesn't seem to have the guts it had but maybe it's just that I'm not used to it running smooth. I hope it's running on all four but there is no smoking or backfiring. I'll run it a couple days and pull the plugs to see what they look like. Maybe I need to give it more fuel mixture. I don't think I rechecked timing after the re-wiring but it was right on before.