cb750k 1977...no spark

[skrivan]

so, i rebuilt my carbs and changed the plugs.  now i get no spark.  i opened the points cover and they seem fine, but no spark at points either.  so i changed them and also changed the condensors.  i put the old plugs back in, too.  still nothing.  i checked for power and i think it's correct---i do get 12v at both sets of points.  should i see spark at the ignition points and shouldnt it be obvious.  i checked the kill switch and that shows continuity all the way to the coils....since it ran ok (at least it started) before i changed the plugs i must have knocked something out somewhere but i swear i cant find it after 3 hours plus of searching.  totally frustrated now.  i just cant believe it's the coils because they were fine 3 days ago.....anybody on here that might know what i'm missing?  thanks....joe

[shinyribs]

No.You won't see any actual spark at the point themselves. And when checking for 12 volts at the points you will only see that is they are open.

Do you have voltage at the coils? The blue and yellow wires?

There is a harness at the rear brake pedal that contains the wires coming from the points to the coils.Check there.It would be very easy to bump that when removing and replacing your carburetors.

[skrivan]

thanks shiny.  that is exactly how it is on this bike.  no spark at points or plugs, but i have power at points and coils when points are open.  i think i'll get coils in any case and try that, but i really find it hard to believe that they both went bad coincidentally with my plug changeover....it has to be something else.  also,  the plug wires were labelled 1,2,3 and 4 and i put them back in the cylinders based on those numbers....however, it looks to me like the wiring diagrams show the plug wires from first coil to cylinders 1 and 2 and the second coil goes to cyl's 3 and 4. i see another wiring diagram that goes form coil 1 to cylinder 1 and 4 what is the correct order of the spark plug wires for a '77?  regardless of that, i still get no spark at any plug when i pull it out, attached to spark plug cap, hold it on engine block, and hit the run button.  starter works fine, but no spark....as much as i hate to do it, i might need to take this to a real mechanic or i'm oging to continue to throw money into it and still not get this problem fixed.  thanks for any insights....joe

[TOOLS1]

1, and 4 are on the same coil, and 2, and 3 are on the other. Have, you checked the Blk Wht wire that goes to the coils, for voltage?
TOOLS

[skrivan]

thanks man....i will check to be sure the wires ar ein the right order.  i do have power at the black/white and also continuity from kill switch B/W wire to the coil.  i hate to admit defeat, but i also know my limitations.

[shinyribs]

Man,if you have voltage at the coils then it should run.

The coil on the left (clutch lever side) is for cylinders 1 & 4 which is the two outer cylinders

The coil on the right is for 2 & 3

If you reverse the wires on the coils is doesnt matter.Like if you revers 2-3 or 3-2.They fire together since they are hanging out of the same coil.

But firing order doesnt mean poo if there is no spark.And the firing order did not change from year to year.They are all the same.

Have you had the coils off? I know there is a  heavy green ground wire that bolts on with my coil mounting bolt.Not sure what it goes to,but maybe a ground is loose/off somewhere?

Do you have a meter? Check from the negative post on your battery to your engine to see if the engine is properly grounded. I know its a stretch,but I hate to see you spend money on a mechanic. Good luck!

[seestheday]

Kind of related: I don't believe the firing order matters at all on these bikes.  This is because there is a spark on the exhaust stroke as well as the compression stroke.

Its usually a wasted spark, but will cause a lot of popping if you are running too rich (just happened to me before I fixed my air/fuel mix).

Still doesn't help with the original problem though.

[Lucky 1]

The spark plugs do have to connected properly. You can switch 2 and 3 or 1 and 4 but you need to make sure one coil is firing 1 and 4 and the other is firing 2 and 3.

If you do not do it correctly the dwell will be wrong and not have enough time to build up a field and release the spark to the plugs, and the spark would be weak.

[Lucky 1]

The points gap must be correct and the dwell must be correct.
If the dwell cannot get within specs it means the points are worn where it rubs on the points cam.


Also ....You CANNOT adjust old point with a thickness gauge because the points gap will be way off.
If you don't believe me then check the dwell. You will see what I mean.

[seestheday]

when I first got my bike it was set up 1,2,3,4 when it should have been set up 1,4,2,3.
It seemed to run fine. Maybe I was just lucky. I've now switched it to 1,4,2,3.

Sent from my BlackBerry device on the Rogers Wireless Network

---

From: "Lucky [via Honda CB750'S]" <[hidden email]>

Date: Mon, 30 Apr 2012 08:20:05 -0700 (PDT)

To: seestheday<[hidden email]>

Subject: Re: cb750k 1977...no spark

The spark plugs do have to connected properly. You can switch 2 and 3 or 1 and 4 but you need to make sure one coil is firing 1 and 4 and the other is firing 2 and 3.

If you do not do it correctly the dwell will be wrong and not have enough time to build up a field and release the spark to the plugs, and the spark would be weak.

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NAML

[Lucky 1]

The firing order of the CB750 is 1,2,4,3.  Look it up.

You may be getting the numbers of the cylinders mixed
up with their position in the engine.

[TOOLS1]

I once had a problem like this. It turned out that one of the condenser wires had shorted out.
TOOLS

[skrivan]

sitting on the bike, facing forward and looking down, the cylinders are 1,2,3,4 in that order....right?

so, there are 2 coils.

 is this correct:   the left one is to fire cylinders 1 and 4 and the right side coil fires 2 and 3.  it doesnt matter which coli wire goes to which plug, as long as the left coil serves 1 and 4...and right coil fires 2 and 3....am i hooking these rigs up right?  thanks

[TOOLS1]

That is correct, but the wires from the points must also be connected to the right set of coils. However this would not have anything to do with it not firing. If, you have a timing light connect it to a plug wire, and crank the engine. If the light flashes, then the problem is in the caps, or plugs. If the light does not flash, then it is from the wires back. However I will bet that there is a wire shorted out somewhere.
TOOLS

[skrivan]

thanks for all the help,guys..... i got this bike started an hour ago.  i chased all the wires again and found that a repair had been poorly done inside the throttle and the kill switch wire was severed, but hanging and it could intermittently be connected....so, 2 minutes with a solder iron and it sparks.  i have other issues, but i'll ask in a new thread on those.  at least i'm starting again so i'm stoked on that.  my other problem is that when i accelerate quickly, the engine dies....but like i said, new thread for that one.

[LukeM]

Vroom vroom.  One problem down, on to the next one.  Nicely done.

Just take your time, and diagnose the problem(s).  

Luke M