left cylinder not firing consistently

[Steveo123]

Hello everyone;

I have a 66 CB77 that is running pretty well, except for one significant issue; I seem to have an intermittently firing left cylinder. I have new Magna-style coils, new spark plug cables and NGK caps, as well as correct spark plugs, and new condensor, and new pistons, rings and honed cyilnders. I also put on new points. The carbs have been very well cleaned out with a sonic parts cleaner. I also have a fresh battery and hook up the battery tender whenever it's not running. I have the original exhaust system on installed. The carbs are stock, with stock sized needles/jets, and have been synced. I first noticed a problem when I couldn't make a strobe timing light work on the left plug wire.

While it's running, I can remove the left spark plug cap without any change in idle, etc. When I move the cap back over the spark plug to reinstall it, I get occasional firing of the plug. However, I can't hold the cap in any one place to keep the cylinder firing.

As I say, the bike runs pretty well; it fires right up from cold, and will idle with the choke on; I can run it at idle without the choke after literally a few seconds. I am assuming the root of the problem is non-spark, rather than a compression issue; as I mentioned, based on the temp of the exhaust pipe after a ride, it's warm, but not hot, like the right cylinder pipe.

Any thoughts? Could the rectifier (which is original) be contributing to the problem?

Thanks in advance for any/all troubleshooting advice/thoughts you have!

Stephen

[Steveo123]

Brief update; when I pull the left plug and cap with engine running, and ground them on the engine, no spark. However, if I hover the plug and cap a quarter inch or so over the engine, a spark arcs over to the engine AND the plug Sparks. Any ideas what's going on, and possible fixes?

Thanks!

[DJM]

You seem to have replaced everything but haven't mentioned the plugs themselves, even if they are 'new' one could be fouled inside. Have you tried another pair or even just swapping the plugs from one side to the other?

What grade of plug are you using and are they gapped correctly.

Years ago I used to race one of these things and ran a 'total loss' battery system, IE NO charging system.

When the battery was about gone it always dropped first onto one cylinder at low revs but would run OK at higher engine speeds. Can't remember now which side used to misfire.

Unlikely to be a problem with the rectifier if you are starting with a fully charged battery, what volts are you getting with ignition switched OFF and again with the engine running.

[G-Man]

Stephen

Have you tried swaping components left to right? Not a particularly easy task but worth a try.

Start with the plugs and work backwards to the caps, coils etc. Whichever part makes the fault move left to right is the culprit.

G

Brief update; when I pull the left plug and cap with engine running, and ground them on the engine, no spark. However, if I hover the plug and cap a quarter inch or so over the engine, a spark arcs over to the engine AND the plug Sparks. Any ideas what's going on, and possible fixes?

Thanks!

[DJM]

The plug is fouled, that's why your not getting a spark when you ground the plug against the engine, the current is passing straight to the body of the plug (ground) without trying to jump the gap.

When you move the plug away from the cylinder head the current is still passing to the body of the fouled plug but then jumps the gap across to he engine.

If you're getting a 1/4" spark I'd say the rest of your ignition system is fine, try a new pair of plugs then work out why the plug is fouled in the first place! Try float heights or a sticking float on that side for starters.

[Steveo123]

Awesome, guys! Thanks for the input. All great guidance.

Again, thanks, and enjoy the rest of your weekend.

Stephen