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Slight mis-fire at higher rpms

guitargeak99

Well-known Member
Joined
Oct 8, 2020
Total Posts
81
Total likes
5
Location
Oakdale
I'm dealing with a slight miss at around 4k to 5k rpm, and I'm seeking some help here. It's not a bad mis-fire, but it's driving me nuts.

My 72 CB350 has always ran utterly amazing.
Recently after a ride with my vintage bike club, she starting missing really bad when I was only about a mile away from my home. Popped off the points cover and the right side points were really sparking - I knew it was the condenser.
I replaced it and the bike still had a "small" miss at 4k to 5k rpm.
It starts right up, idles, and will accelerate up to 6500 rpm no problem (under these conditions I can't hear the miss). But, when I just level it off and cruise at 4k to 5k rpm there is a small miss, that was never there before. Tried cleaning up the points with no luck. Replaced the spark plugs, replaced the points with genuine Honda points, gapped them at .014, and set the timing per the manual........ No change.
Battery resting voltage is 13.0v.
Any ideas? Should I experiment with point gaps? I still have room to decrease the gaps, but I don't want to go too narrow for fear of heating up the coils.


  • 3 years ago I replaced the coils
  • New R/R combo unit from Sparck-Moto.
  • New Motobatt battery.
  • Valves are adjusted.

Thanks in advance
 
When you say checked the timing per the manual - did you put a strobe light on the rotor and check the advance?


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No I didn't check the timing dynamically. I used a 12v light.
I don't have much experience on motorcycles. My experience is with musclecars. And I have never seen bad timing cause a mis-fire. Bad timing usually causes detonation, surging, or sluggish performance.
Since my bike was running great before the condenser failure, my thinking was that my bike should return to previous performance when the condenser was replaced.
Am I wrong?
I did check the advance and everything is clean and the mechanism operates freely.
Thanks so much for your reply.
 
My experience is with musclecars. And I have never seen bad timing cause a mis-fire. Bad timing usually causes detonation, surging, or sluggish performance.
Since my bike was running great before the condenser failure, my thinking was that my bike should return to previous performance when the condenser was replaced.
Am I wrong?

I don't think you're wrong, but you also have to remember that unlike a V-8, your 350 uses 2 separate and independent ignition systems - 2 sets of points, 2 coils, 2 condensers but operated (points that is) with a single breaker cam lobe and advance mechanism. There have been instances where excess wear on the advancer caused intermittent issues with one or both cylinders, so these bikes present somewhat of a separate challenge beyond the typical you're used to. I will agree that it seems oddly coincidental though.
 
If you have a timing light it might be worth checking the advance on each independently. I believe 4k is where the advance should *just* hit it’s sweet spot.

If one cylinder is a bit off it might cause an issue.

For what it’s worth, I’ve always needed to fine tune my bike after setting timing statically by using a timing light. The 350s seem to prefer being dynamically timed.

Check 66Sprint’s thread for how to check and adjust timing on a 350 - useful info.


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I don't think you're wrong, but you also have to remember that unlike a V-8, your 350 uses 2 separate and independent ignition systems - 2 sets of points, 2 coils, 2 condensers but operated (points that is) with a single breaker cam lobe and advance mechanism. There have been instances where excess wear on the advancer caused intermittent issues with one or both cylinders, so these bikes present somewhat of a separate challenge beyond the typical you're used to. I will agree that it seems oddly coincidental though.

Thanks AD, makes sense.
 
Check the voltage at the coils, KO EO should be very close to battery voltage, then check KO ER at idle and 3500 rpm. Voltage should be again very close to what is seen at the battery .... which should be around 14v. I ended up running a hot from the battery on one bike and took it for a test ride to finally figure out a partially broken wire in the harness that fed the coils. High resistance in the kill switch, key switch and wiring connections in the headlight are usual suspects.

Non resistor plug with resistor cap or resistor plug with non resistor cap .... if resistor cap check resistance, if it hasn't been done trim the hi tension lead back and thread the cap back on. I like to expose the wires and bend them back then screw on the cap.
 
When you say checked the timing per the manual - did you put a strobe light on the rotor and check the advance?

Update: It was the timing! Fine tuned her, and 4200 rpm seemed to be the sweet spot.
One thing that I'm learning about this bike is if everything is in spec, she runs like a top. But if the ignition or fuel system isn't dialed in, then you'll know it.
Thanks all - I'm all smiles now.
 
One more thing to look at.
You said you had to replace your condenser due to serious arcing.

Arcing is Like WELDING.

It will transfer metal from one side of the point contacts to the other, creating little mountains and valleys on the surface.
This will cause the EFFECTIVE Point GAP to change.
You may need to take some fine Wet/Dry Sandpaper to the contact surfaces to smooth them out then clean up with contact cleaner.

I just pull a thin strip of 600-800 thru once or twice each side to get the contacts cleaned up.
Allow the opposite side contact to keep pressure on the paper when you pull it thru so the contacts remain flat and parallel.
 
One more thing to look at.
You said you had to replace your condenser due to serious arcing.

Arcing is Like WELDING.

It will transfer metal from one side of the point contacts to the other, creating little mountains and valleys on the surface.
This will cause the EFFECTIVE Point GAP to change.
You may need to take some fine Wet/Dry Sandpaper to the contact surfaces to smooth them out then clean up with contact cleaner.

I just pull a thin strip of 600-800 thru once or twice each side to get the contacts cleaned up.
Allow the opposite side contact to keep pressure on the paper when you pull it thru so the contacts remain flat and parallel.

You missed the part in my post that I replaced the points. However, I did sand the damaged side down with some fine sandpaper and finished with some fine crocus cloth - saved the used points for another time.

Thanks
 
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