Topic: 78 cb750 cuts out at high rpm

[2wheels]

With the spark getting crazy when the problem is happening i would suspect the coil or all the wiring on the bike that is directly related to the 2/3 ignition.

But, as you know, it is a weird problem.  So who knows what it will be.

[TwoTired]

If the points are excessively arcing, it's either bad/wrong condensers, or there is way more back EMF from the coils than is normally found in a stock set up.
The back EMF is related to how fast the spark event occurs, as that is when the energy field is collapsing to produce the spark in the secondary.  The collapse also creates the back EMF voltage in the primary, where the points and condenser are attached.

If you have 3 ohm coils, you can insert a 1 ohm resistor in series with the coil power, to reduce the current draw and a little bit of the stored coil power.

If you have the proper secondary spark lead resistance, this also slows the coil's field collapse.  That would be spark plug caps (at least 5KΩ) and/or resistor spark plugs (also 5KΩ).  There is also the resistance within the wires option.

I would start with known good condensers, though.  (Which can't be guaranteed by the term "new".)  They are supposed to quell some of the point sparking in stock ignitions.  Changing coils can also effect ideal values for the condensers.

Cheers,

[750Moto]

That's what I am thinking too, just can see anything that would cause them to ground out. Does it matter what side of the point bolt/screw the capacitor and power wire are attached to, I'm pretty sure it doesn't matter but you never know

[750Moto]

All of the things you guys have said is things I believed to have cause the problem previously but so far those have been debunked. Primary and secondary resistance of coils along with the 5ohm cap resistance have been checked and we're perfect. I have swapped coils between 1and4 and 2and3 and nothing changed, I have also swapped condensers between 1and4 and 2and3 and nothing changed, 2and3 continued to be the misfiring cylinders. On top of swapping parts between cylinders I have put a set of used coils off a running bike and nothing changed. For the condensers the old ones never went bad I just changed them as preventive maintenance just like I did with the points I have swapped back to the old ones and used every combination of new and old points and condensers with no good results. This is why I lean toward ground fault but I just have no idea

[BobbyR]

This a very long shot. There was a case on here years ago someone had the same problem. After testing everything he finally changed the spark plugs. One plug was defective. As I said a long shot.