Topic: cylinders 1 & 4 not firing on '78 cb750 (update: lead to #3 carb fell off)

[deafcaferacer]

Was out riding today and # 1 & 4 kicked out, then back in.. then near the end kicked out again..  brought it home and did the sizzle test..  #2 & 3 very hot, 1 & 4 warm to the touch but not boiling the water off.

Other than checking plugs, what else should I look at and in what order?

Thanks

[Vinhead1957]

Points set that the blue wire goes to!

[bollingball]

If that is OK then check the little jumper that goes from one coil to the other.
 You are looking for any thing that is common to both cylinders.

Ken

[deafcaferacer]

Thanks for the quick responses. Pulled both plugs, they were coal black. Have replaced them with NGK iridium DR8EIX's and fired her up, took her for a spin, so far so good.

I'm a little suspicious of both 1 & 4 quitting at the same time so I'll continue to monitor how things go.

[deafcaferacer]

Well, #*#&$@.. that isn't the problem. 1 & 4 quit after I got about 2 blocks down the road. I am checking connections now.

EDIT: Shut it down to check connections for plugs, then started it up again all cylinders fired, took it out for a ride, then the 2 cylinders kicked out. Also, when coming to a stop where the motor just idle, it dies down.. has done this since I installed the new plugs.

I am taking a look inside headlights to check all electrical connections. It feels like a connection issue to me because of the way the cylinders haphazardly decide to kick in or kick out.

[deafcaferacer]

This issue is getting more weird..    it appears that all four cylinders will fire up once I start the bike.. I checked all connections- looks fine.

I will let bike cool down and start up again, and if all 4 cylinders fire, then I would think it is the fuel system, and not electrical. I just took it out for a spin, no problem with idle.

[deafcaferacer]

Ok..  I started it up again.. and again fired on all 4 cylinders.. then 15 minutes later, 1 & 4 kicks out..

Forget what I said about poor idle.. I figured out that it's related to only 2 cylinders firing.

I'm stumped.. no loose connections as far as I can see. Fuel system? Don't know how it could be this since all 4 fire up every time I start.

[Smoke Detector]

Switch 1-4 and 2-3 coil wires and see if problem moves to 2-3 with the coil. From reading on this site( search "bad coil" )- I believe that coils can fail as they heat up, which sounds like what you are describing.

[Vinhead1957]

Condenser before coil

[Xnavylfr]

GUYS, You always tell people to switch coil wires BUT you forget to ALSO tell them they have to swap the sparkplug wires..
SWAP BLUE wire and plug in to YELLOW wire from points
SWAP yellow wire and plug in to BLUE wire from points
Change sparkplug wires from 1/4 cylinders to 2/3 cylinders
Change sparkplug wires from 2/3 cylinders to 1/4 cylinders.

Take a ride LONG enough for the symptom to reappear, if the symptom has moved to the 2/3 cylinders then the COIL attached to those wires COULD be going bad. Before you call it a bad coil , if you can remove your sparkplug caps? Remove them and cut the wire back 1/4 inch then reinstall the caps on the wires and try for the problem again, if it shows up, then the coil attached to the dead cylinders is going out when it gets HOT.

Xnavylfr(CHUCK)

[Vinhead1957]

If you swap the blue and yellow and the problem moves then it is not coils plugs or caps but you really can't do that cause the timing would be affected. You  can swap condensers and check the points gap and surface. That is the easiest. Since it is 1 and 4 it kinda pointing at the blue wire set! Also check the bullet connectors on the harness for corrosion.

[Stev-o]

Sounds like the coils are cutting out when getting hot. Happened to Tom's K7 two weeks ago.

[deafcaferacer]

Thanks everyone for your input, I am leaning towards the coils as being the  culprit because as mentioned it cuts out when the bike gets hot which is pretty much what is happening to me consistently, so that rules out poor connection, et al. Also the wiring harness is new so no chance of corrosion there.

Steve, would you mind giving me a link to Tom's thread, if you could? I'd like to see what he did with his issue.

In the meantime I'll remove the gas tank and take a look at the coils, take a pic and post here.

[Stev-o]

Steve, would you mind giving me a link to Tom's thread, if you could? I'd like to see what he did with his issue.

It happened on the Rally, we repaired it at the campground!

See reply #23...

http://forums.sohc4.net/index.php?topic=119595.0

[BobbyR]

Trace the wires coming out of the points casting and going up to the coils. I had the insulation wear out and short on and off. I also had a dyna unit that crapped out when the motor got hot, which has been a real issue with those units. You will always lose 1-4, 2-3 due to the waste spark system if the paired fails to fire.

[Xnavylfr]

HEY Vinhead,,
Cylinders 1/4 are controlled by 1 coil and cylinders 2/3 are controlled by another coil. IF you just swap the BLUE and YELLOW wire you are correct, but IF you also move the spark plug wires to the other cylinders as I described before, your bike is STILL in time, all you have done is caused your ignition to fire from a different set of points and coil. BUT if you put a timing light on the INDEX marks and have the light wire pick up on #1 wire you will see #2/3 timing marks when the light fires, NOT 1/4 and just opposite if you have the light hooked to #2 wire, it will show # 1/4 timing marks.

Xnavylfr(CHUCK)

[Vinhead1957]

Xnavylfr,

I had to think about that one!  Yup makes sense

[I Zombie]

What coils are you running?
Are they stock or after market?
You said you put in new DR8ES plugs, are you still running the resistor caps?
I'm no expert on this but, I'm sure others with more knowledge will chime in.
Could this all boil down to too much resistance @ the coils causing them to cut out?

[deafcaferacer]

Well, I had some time to look at the bike again yesterday and before I could get to the point where I could be 100% sure that the problem was isolated to the ignition coils, I hit a block on the list of diagnosis.

I pulled the new plugs to look at them to see how they were firing. The old #1 & 4 were sooty and black, and when I pulled the new ones, #4 was fine, but #1 had immediately coated over, so I decided to pull float bowls from #1 & 4 to have a look see, and when I pulled #1, it was overflowing bad.

I'm now looking through the archives in SOHC for repair options, but feel free to comment.

[deafcaferacer]

Settled upon the very scientific method of giving carb #1 a lovely tap to loosen things up to see if a sticking float was the problem, per some of the suggestions in the archives, and that seemed to work.

Took it out on the road for a spin and it seemed to work for the most part but then started misfiring every now and then. Came back and checked all the connections to the spark plugs and to my surprise, the terminal on the lead to #3 carb fell off in my hand. Seeing that the wires are still protruding from the cable, should I just try to screw it back on or trim the cable sleeve a little further back, to expose more wires?

[BobbyR]

Just trim the cable just a bit to give the screw some fresher copper to bite into. Good find!

[Vinhead1957]

Roll the rubber seal back up on the plug cap before you screw it in and once it is in tight roll it back in place .

[flybox1]

sorry, haven't read your whole thread, but if your caps are off, measure their resistance, and get new ones if you need.
makes a huge difference good caps and good coil wire connections. 

[deafcaferacer]

Thanks all.. I didn't measure their resistances.. have to learn how to do that. I do have a volt meter.

exposed about 1/8" more copper wire, rolled the sleeve up like Vinhead described, screwed the cap in good and rolled sleeve back over it, and took it out for a run. Everything working so far.

Then the transmission springs an oil leak.. rather than open up a new thread, I think I'll just head back to the project forum and post on my thread there.