Topic: Back fire, no starting...coils or plug wires wrong?

[Thor's Hammer!]

After searching a little, i know that often times it's common to either have the plug wires wrong or the coils connected incorrectly when the bike won't start but back fire badly.

Bike is a 78 CB750F.

Unfortunately I didn't take  pic before before removing plug wires and electrical connections to the coils (foolish i know now!).

I have not had the head off nor have I messed with the points or ignition timing.  The bike ran about a month ago and only the wire harness and plugs wires have been removed and put back on the bike, nothing else changed.

I have green dyna coils 3.0 ohms.  I have yellow to yellow connected on coil 1 (left side, sitting on the bike) and blue to blue (right side, sitting on the bike).  The ground wires have been tried between the each coils and seem to have no effect.

Coil 1 has plug wires going to 1 & 4 cylinders and coil 2 has wires going to cylinders 2 & 3.

On coil 1, the front most plug (from the coil) goes to #1 and the back most plug (from the coil) goes to #4.

On coil 2, the front most plug (from the coil) goes to #2 and the back most plug (from the coil) goes to #3.

Anybody got a pictures of dyna coils hooked up on a 750 that shows both the plug wires and the electrical connections??

Anyone have ideas what might be the problem if not the connections to the coils or the plug wires??

[Industrial Cafe]

I have pics, BRB
both wires on each coil fire at the same time, so front/back doesn't matter.

my yellow wire is on the right hand coil.

[Industrial Cafe]

left side


right side


right side wires.

[Thor's Hammer!]

Thanks Industrial Cafe.

We must have different year 750's or our coils are wired differently.

I thought the order of the plug wires mattered?  I read several threads where folks seems to report that the way spark plug wires are run into specific cylinders made a difference.  Maybe I read it wrong or misunderstood?

Gonna try to upload pics of my setup here (picasa seems to be down right now).

[Industrial Cafe]

I think blue is for the cyl1-4 and yellow is for cyl2-3

it looks like you got blue and yellow on the same coil.

you didn't take a pic of your other coil.

[Thor's Hammer!]

Oops.  My bad.  I can't get at that side of the bike in the garage right now.

The 3rd pic down shows things a little better.

The blue wires are ground wires for both coils.  They go to the top screws on both coils.

That purple wire goes to bottom screw on coil 2.

The plugs only connect in a certain way such that purple wire can connect to the single blue or yellow going to the harness.  The two ground wires (blue here) can only connect to the black ground wires with the white stripes on them...but the blue grounds coming from the coils could connect to either ground, if this makes sense.

I have read the grounds for the coils don't have an order..meaning, how the ground wires are connected shouldn't matter.  Is this true?  I know the yellow coil wire was always connected to a yellow harness wire.

[TwoTired]

The blue and yellow wires provide ground through the points (or Dyna connection).
The other terminal on each coil should go to the B/W connection.

[Thor's Hammer!]

I am kind of stumped here fellas.  Have to admit I am not sure how to approach this issue for the moment.

The coils seems to be connected properly.  The bike back fires loudly but only in maybe 1 in 4 (or more!) revolutions.  There must be some spark as a result of the back fire?

The carbs are a set I had on the bike when it last ran so I doubt anything is wrong there.  I am leaning towards plugs, plug wires, or electrical as being cause of the back firing without running.

What do you guys think?  Were should I begin?

[Industrial Cafe]

I don't know, why don't you start by wiring your coils up right.
(sorry I'm a smart ass)
I could be wrong but I don't see any ''black with a white stripe'' wires connected to them.

[paulages]

the black and white wires from his harness are going to the top posts on the coils, he just decided to confuse matters by using colors that don't match the connections at the harness.

mismith- try swapping the yellow and blue wires (harness colors) and see what happens. you may have the firing order reversed.

[Thor's Hammer!]

I don't know, why don't you start by wiring your coils up right.
(sorry I'm a smart ass)
I could be wrong but I don't see any ''black with a white stripe'' wires connected to them.

Thanks Industrial...The black and white wires are there and connected...I promise you ... they are plugged into the top wires for each coil.

[Thor's Hammer!]

the black and white wires from his harness are going to the top posts on the coils, he just decided to confuse matters by using colors that don't match the connections at the harness.

mismith- try swapping the yellow and blue wires (harness colors) and see what happens. you may have the firing order reversed.

Thanks Paul,

Technically, "I" didn't use those different colored wires, the bike came this way 

I like the idea of flipping the yellow and blue wires to the bottom posts.  Maybe I was wrong about thinking it was originally yellow to yellow.

Gonna go out and give this a shot.

[Industrial Cafe]

I don't know, why don't you start by wiring your coils up right.
(sorry I'm a smart ass)
I could be wrong but I don't see any ''black with a white stripe'' wires connected to them.

Thanks Industrial...The black and white wires are there and connected...I promise you ... they are plugged into the top wires for each coil.

oh ok.

don't forget to post what you did when you get it running. I'm interested.

[Thor's Hammer!]

the black and white wires from his harness are going to the top posts on the coils, he just decided to confuse matters by using colors that don't match the connections at the harness.

mismith- try swapping the yellow and blue wires (harness colors) and see what happens. you may have the firing order reversed.

BINGO!!

The damn yellow and blue coil wires were reversed...hence, it was 180 degrees out...just like on a car motor with a distributor being 180 out.

I swear the yellow wires were originally together, but being 40 now, memory is something I use to have. 


Now that it runs :D  ... It's only running on cylinder 3 & 4.  Plug wires perhaps?  Going back out to see if I can get that sorted.

[Industrial Cafe]

cool, you're gettin there!

[mystic_1]

I'd strongly recommend getting that wiring sorted out, it'll make the bike much easier to work on, both for yourself and also for any potential future owner.

Get a junk harness on eBay and harvest the wire and connectors you need.

This would also make a great opportunity to inspect and clean all the connections, which you should probably do in any case.

cheers
mystic_1

[Industrial Cafe]

I have a badly chopped up 76k harness. if you need pieces.

[Thor's Hammer!]

Thanks fellas!!  I really appreciate the help and suggestions.

Cylinders 1 and 2 are firing from the plugs...I pulled out each plug and held it against the valve cover...both plugs were firing nicely, but after running for 5 mins or more, cylinders 1 & 2 are cool enough at the exhaust pipes to grab with my hands...

I am thinking i have a fuel problem perhaps but I will work on that and ask questions in another thread after searching and playing.


As for the wiring, I agree that it is a mess and it is going to be my next thing to tackle.  I am going to get new plastic connectors and a rectifier rebuild kit from http://www.oregonmotorcycleparts.com/connectors.html

Industrial, please check PM about the harness.

[MJL]

Before you dig into your carbs, put new plugs in and see if that helps. They could be fouled out.