Sparks not sparking Correctly :-(

[Gena]

Hi Guys,

I am new to the site so please bare with me. First off all I want to thank you for the amazing site, it has helped a lot with my very own project. My father passed down a 1981 CB750c that belonged to his father, so it means a lot to get it fixed properly.

When I got the bike, it has been on the road for close to 2 years, so I started with the basics. Carb and engine service. Now all is good and it goes well except for the past weekend when something went wrong.

Fist cylinder 1 & 4 did not fire and I thougt the coil was shot as they are both lnked to the same coil. Swapped the coils around and cylinder 1 going now but 3 & 4 was not firing. After a few hours trying to figure it out I got to the following conclusion and here is where I need your help please.

As soon as I remove the cap of the sparkplug, the plug gets life and the cylinder runs. When I place the cap back, it is dead. This applies to Cylinder 3 & 4. As soon as all four caps are on there respective plugs only 2 cylinders are running. I have wrapped my brain around what could possibly be wrong but to no avail.

Please can you guys help me with this one.

[LukeM]

Welcome aboard.  Nice that you're keeping the bike in the family.  Amazing that these Hondas are ridden by grandchildren of the original owners. I like that. Maybe I can keep mine running that long.

The coils fire both plugs at the same time: one ignites the mixture at the start of the power stroke, the other ignites the other cylinder during the exhaust stroke.  This way, you don't need a distributor (saves money, parts, and time).

If memory serves, one coil fires cylinders 1 and 4, and the other fires 2 and 3.

I'm guessing if/when you get it running in this condition, it's down on power and won't accelerate well at all. So, here's how to diagnose it:

Unscrew a spark plug from the engine, and lay it on the engine case.  Crank the engine and see if it sparks.  If it does, the coil is good and the wire is good. Do this for all 4 plugs.

If a single plug does not fire, here's what might cause that:
bad connection from the coil wire to the plug boot (might have to take 1/4" off of the end of the wire and reattach the boot), plug is internally shorted (try again with a known good plug), coil is intermittently shorted for that output (try again with a known good coil), ground connection on coil is intermittent (check and secure ground wire/bolt), or the problem lies elsewhere.

If both plugs on a single coil do not fire, here's what might cause that:
Bad coil or coil wires (swap with known good coil)
Bad connection at the coil (+V from points, bad ground for coil)
Issue with spark controller (check the sensor at the left end of the crankshaft, check the wiring from here to the coil)

It's a pretty simple system, but finding the problem is sometimes tricky.  You can search our site for ignition issues.  This problem has been there a lot.

Good luck, and keep us in the loop.
Luke M

[Gena]

Thank you very much for the response. It is very special considering it is being passed down. I am slowly but surely in the process of rebuilding the bike and giving it a bit of a modern twist.

What I was able to conclude is that it must be the wire and the boot. The coil is fine because when i move the plug boot the issue follows. What also made me worry is that the spark is firing at back of the cap where the wire is screwed in as soon as you hold it close to the case.

Plus the plug boot that is giving me issues is not stock.

I am going to post pictures of all my mods I am going to make on the bike as the time goes by.

[seestheday]

Buying new NGK wires/boots made a noticeable difference in consistency for me.

It is a pretty cheap thing to try.  While you're at it you might also think about getting iridium spark plugs and/or Accel/Dyna coils depending on your budget.  The spark system on these bikes is a known weak point.

When I put iridium plugs in I got a noticeable seat of the pants and audible performance increase.

[Lucky 1]

Coils almost never fail.

[Lucky 1]

One thing you can do is unscrew the plug connector of the spark plug wire. Then clip off about 1/4 inch of spark plug wire. Screw the spark plug cap back onto the spark plug wire.

It always helps to get a better connection.

[Gena]

I am planning on getting new wires and plugs. As mentioned by "seestheday" the spark system is a weak point. I am also going to try and find iridium plugs in my country or I am going to have to import from yours.

Eitherway the electrics on the bike is going to be replaced. Step by step. If am going to import the plugs I might as well just import the performace coils that was mentioned.

[FastCletus]

Another thing to keep an eye on may be is the spark units back by the battery.  I just had one go out the other week, 2 cylinders fire and 2 dont. replaced 1 spark unit and vroom

[Gena]

I just bought new caps, wires and plugs. Just needs to replace the old ones and fire the old lady up to see what se does.

Thank you all very much for the assistance. I am going to add all my pics with the resoring/mods I am doing.