CB 350, Intermittent spark, need thoughts.

Alright guys, I'm in need of some guidance.
(Apologies, this got long)

I'm SO STINKING CLOSE TO putting this bike on the road in time for the great spring riding weather here in East Tennessee, but I've been going round after round making the last step to "completion"...

For a little background, I stripped the bike down & did paint and deletes and all this fun stuffact, and then wired the thing from scratch for just the necessities.

Well, here I am, scratching my head. I'm missing on the right cylinder about 15-20% of the time. I initially blamed cheapo bits in the carbs and redid them, but that did nothing.

Well I finally took the timing light (strobe light) to the thing after I had confirmed that I'm getting spark, and sure enough, you can watch the strobe light miss along with the sound of the miss! Bingo!

I know my timing is spot on, and the thing runs down the road fine aside from this miss, but the miss gets worse as the bike gets warm, and the cylinder will nearly quit at idle if I don't stay on the throttle. It won't quit completely as long as I keep the revs up, at least it hasn't yet.

I went through the entire ignition system with my trusty Fluke the other night, and came out as clueless as I started. I hit every single connector with dielectric grease, checked my grounds, cleaned the points & checked the connections to the springs, checked them for possible shorts VERY carefully (and again in the dark watching for stray sparks), swapped out a new spark plug, swapped condensers, then swapped plug wires from one side to the other and the trigger wires so it would start, and the miss moved to the left side.

So I narrowed my issue down to the point or the coil. In the dark you can see the spark in the points, and in the right one it's weaker & inconsistent, and fails visibly when the thing misses. Let me just say again that I swapped the condensers and the issue did not move to the other side.

I re-checked for shorts, wrapped all the connectors in electrical tape just in case, trimmed and re-seated the coil end of the plug wire, re-checked the connections at the coil, held the points wires away from the head, no luck.

Metered out the coils with everything disconnected, they were both in spec according to DCC, but they were different by about .4k Ohms. I'm out of town and away from my notes and can't tell you which coil had the higher reading or exactly what it was, but I felt like it was alrigt.

The battery was at 12.5v when I started for the day, and 12.6 when I gave up.

Since I went through all this, I've realized I didn't check my voltage at the battery with the thing running, but I know it was charging well when I first got the bike back together. It does have a new rectifier, and a roughly 1 year old battery.

At this point I'm at a loss. Do I just replace the suspect coil? I have a hard time believing it could be the point now that I've checked it's connections & cleaned it's surfaces...

Any opinions are really appreciated!

Using the strobe, did you check the timing at both idle and full advance? Good for both, I assume?

These Honda twins are a little funny in some ways, and a lot of them have to do with the alternator. Because the charging systems are weak, it can lead to a host of other problems that leave us scratching our heads.

Generally I don't advocate just buying new parts and I know your battery is reading good voltage, but try a new one. On a Honda twin, intermittent miss on the right cylinder is a classic symptom of imminent battery failure.

Alright, one thing to add to all that. At the end of the day of testing, I realized it will lose spark for a noteworthy period if you crack the throttle quickly. That left me stumped. I can't figure out how throttle position could affect spark like that. The throttle cable is not putting any pressure on any wiring...

Here is a video I forgot I had taken of the behavior.
https://youtu.be/41scdUMui_Y

NiftyHalden said:

Alright, one thing to add to all that. At the end of the day of testing, I realized it will lose spark for a noteworthy period if you crack the throttle quickly. That left me stumped. I can't figure out how throttle position could affect spark like that. The throttle cable is not putting any pressure on any wiring...

Click to expand...

At low RPMs, the alternator isn't producing enough juice to run everything. When you crack the throttle, you're increasing engine RPMs, which decreases the period between sparks. This leaves the ignition coil less time to charge between firings. Because you're working with a 180° twin and the left side fires first, there is a longer duration in which that coil can charge because the right point is open and that coil isn't drawing power.

Sonreir, I did not check the timing on Sunday when I went through this, but I know I checked it when I re-gapped the points last & I remember the advance being OK.

Do you have any recommendations on batteries for these bikes? The new one I put in last year is a cheapo thing from NAPA.

NiftyHalden said:

Sonreir, I did not check the timing on Sunday when I went through this, but I know I checked it when I re-gapped the points last & I remember the advance being OK.

Do you have any recommendations on batteries for these bikes? The new one I put in last year is a cheapo thing from NAPA.

Click to expand...

Most anything that fits and is between the 9Ah and 12Ah range is good enough for me. Lean toward higher Ah if you do a lot of city riding or spend longer-than-normal amounts of time idling.

Something quick you can try: Use jumper cables to hook up your car battery (keep the car turned off) to the bike and see how things run while it's connected. If your problem goes away, it's definitely a battery issue.

That was a great explanation of what could be happening. Wow. This is why I finally spoke up.
Thank you!
I will try attaching a good car battery Sunday when I get back to town and can look at the thing, I will also check it's charging output, and report back.

NiftyHalden said:

That was a great explanation of what could be happening. Wow. This is why I finally spoke up.
Thank you!
I will try attaching a good car battery Sunday when I get back to town and can look at the thing, I will also check it's charging output, and report back.

Click to expand...

You can save the thanks until it works.

If everything is good on the charging front, you should see about 12.5 at idle all the way up to 2500 RPM or so. From that point, you should see voltage climb steadily until about 4000 RPM, which it should cap off in the mid-to-late 14s.