Author Topic: 78 CB750K Update! Oh yeah, I need help!  (Read 924 times)

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echoesofwonder

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78 CB750K Update! Oh yeah, I need help!
« on: July 28, 2008, 09:22:09 AM »
Can I just start by saying, OH MAN!

I hope you are ready for the ride of your life (that won't happen because the bike still doesn't work).

Let's just say, if you read my new member introduction, you'll understand that I got myself into quite the headache.  I know that this amazing motorcycle is 30 years old this year, and someone had to have owned it before me (seeing as I am only 22).  I wish my husband would have bought a bike from someone who weren't trying to turn the bike into a chopper, especially since they didn't know what they were doing.

Restoring the bike is about enough to make me want to cuss.

I have come to learn that the hardest thing to diagnose is stupidity.  Let me give you an example.  We pulled the carbs off the bike to clean them, because some fool ran the bike without an air filter because they broke the air box and couldn't get back into it.  That was the first thing I got to buy on eBay by the way.  Anyway, we cleaned out the carbs, put everything back on the bike (mind you this was all supposed to be a simple “get it up and running fix”).  We hooked the gas tank back up and the carbs started pissing gas all over the ground.  So we went through everything in the carbs, finally we gave up and took it to a local guy.  I went to pick up the carbs, paid $100 bucks to find out that the same fool before us had the gas tank hooked into the vent (that black tube that sticks out of the carbs) instead of where the fuel actually goes into.  How was I supposed to know???

So, this brings me to my latest and greatest saga in the “Easy Fix”.

We finally got the bike put back together.  And we worked with her to get her to go.  She would turn over, but just nothing after that.  We were patient with her, but gave up after about an hour of intermittent tries trying to get her to start.  The next day my husband got pissed, hit the electric start button and guess what? She came to life!!! 

Here is the sad part… she only fires on 1 and 4.  I know this because I did (forgive me because my husband told me what it was called, but now that he isn’t here I forgot) an in-line test to the spark plugs and I got spark on 1 and 4 but not 2 and 3.  I gapped the plugs (correctly) and 2 and 3 were wet, but they wouldn’t fire.  (I hope all of this makes sense).

My husband checked the resistance to the wires to the left and right coils and they matched, so I didn’t think that would be the problem.  We ordered new points, but we figured out that the points were changed shortly before we bought it (because we had the oldest points, the points that are now in the motorcycle, and an unopened package of points). So it shouldn’t be the points.  It also isn’t the plugs because they were switched and got spark out of 1 and 4.

What the heck could I be looking at here?  Any Ideas?

I just want to say never tell your husband “fix it or sell it” because you will end up with this mess.  ;)

Thanks guys,

-echoes

Offline mlinder

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Re: 78 CB750K Update! Oh yeah, I need help!
« Reply #1 on: July 28, 2008, 09:28:13 AM »
2 and 3 not firing is generally 2 and 3 coil (they share the coil), 2 and 3 points (make sure they are adjusted properly) or faulty wiring for the 2 and 3 coil.

Check points, the work your way back from the 2 and 3 coil, checking for continuity/current.

Good luck!
No.


Offline BobbyR

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Re: 78 CB750K Update! Oh yeah, I need help!
« Reply #2 on: July 28, 2008, 09:45:01 AM »
A quick and dirty method is to put the 1-4 wires on 2 + 3 and see if they fire. Prior to doing that try clipping off the very tip of the plug wire and thread the plug cap back on. corrosion can occur there. If  2-3 fire with the 1-4 wires on them, then you have something wrong with the either the 2-3 coil, wires, points. Also, may I suggest that you get a copy of the shop manual. You can find links to online copies on this site.
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Offline eurban

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Re: 78 CB750K Update! Oh yeah, I need help!
« Reply #3 on: July 28, 2008, 09:57:44 AM »
Yes here is the link:     http://www.mediafire.com/?5ttzdg9yyuz     Take your time and read through it.  30 year old abused motorcycles will take some patience and information / advice to make right again. You will likely kill your wallet if you don't learn to do maintenance / repairs yourself, but between the manual and this site you should be fine.   

Offline mystic_1

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Re: 78 CB750K Update! Oh yeah, I need help!
« Reply #4 on: July 28, 2008, 10:00:13 AM »
The points themselves may be OK but they may not be adjusted right.  Perform the Point Gap Adjustment procedure and the Static Timing procedure, and see if that sorts out the problem.

Here's where you can find a bunch of manuals:

http://forums.sohc4.net/index.php?topic=17788.0

This one should be for your bike:

CB750 K8 F3 Shop Manual:  http://www.mediafire.com/?5ttzdg9yyuz

Good luck!

mystic_1
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