Sputtering Under Hard Acceleration

Thanks Patton. I'll have to check voltage at the battery while running when I get home tonight. The last test was (not running) 12v at the battery and 11.5 at the coils.

Plug wires look like twisted metal wire.

Topper wrote: Thanks Patton. I'll have to check voltage at the battery while running when I get home tonight. The last test was (not running) 12v at the battery and 11.5 at the coils.

Plug wires look like twisted metal wire.

Plug wires look like twisted metal wire. Good, because that's metal core, which fitted with non-R plugs, leaves only the resistor caps as a single resistance component between each coil's secondary terminals.
I believe (but am not positive) that the bike at hand came new from the factory with metal core plug wires and non-R spark plugs and resistor plug caps.

Would also bear in mind that even a "bad" battery may measure 12v between the terminal posts, and still be too weak to do very much such as turn the electric starter motor, etc.

Of possible concern is that the battery may fail to accept a charge or fail to hold hold a charge, even where the charging system is producing a 14~15v charging rate.

That's why the load-test is important on a supposedly fully charged battery.

Usually, a free load-test is readily available at any auto parts store such as AutoZone or Walmart or other places where new motorcycle batteries are sold.

Good Fortune!

So I'm getting about 12.5v at the battery with the key on, not running. Running at 4k+ RPM I get about 13.6.

I had the battery tested at the auto store not long ago and it checked out fine. I don't recall if they load tested it. It was a quick test, they didn't keep the battery overnight or anything. Does that mean it probably wasn't load tested?

about 12.5v at the battery with the key on, not running --- acceptable.

Running at 4k+ RPM I get about 13.6 --- not quite up to specs, but should be enough to keep the battery charged.

had the battery tested at the auto store not long ago and it checked out fine --- was most likely a load test, which only takes a few seconds.

With ignition switch ON, engine not running, battery @12.5v, coils receiving 11.5v, is 1v loss between battery and coils.

Presumable, 13.6v between battery posts @4000 rpm would have the same 1v loss, whereby the coils are receiving 12.6v, which should be sufficient for a good spark under hard acceleration load.

What is plug reading?
Does the issue persist immediately after installing brand new spark plugs?

Could disconnect the headlight and do a test run to see whether the issue persists, as this test may provide slightly stronger plug sparks.

Are compression test figures acceptable?
Are valve clearances within specs?

If compression is weak (with correct valve clearances), could perform a poor mans leak down test on each cylinder to determine exactly where compression is being lost.

Too tight valve clearances may allow good compression test figures, but result in loss of compression at normal operating temperatures.

One less likely condition that might cause an excessively rich fuel mixture during mid-upper throttle openings is a worn needle jet (where the jet needle interfaces with the needle jet orifice).

If a vacuum operated petcock is fitted, a leaking petcock diaphragm might possibly allow fuel to pass through the vacuum hose into the vacuum nipple, whereby the mixture is enrichened.
Could detach vac hose from carb nipple, cap/plug the nipple, and test run with petcock in prime position.

Good Fortune!

Having the same issue with WOT on a '80 KZ750 Twin. However, I have a 1 into 2 carb intake manifold with a 400ex carb. Plan on changing needle clip position first, and then maybe look at changing main jet.

Fiddling with main jet and needle clip position hasn't seemed to have any effect.

I did realize that the spark plugs I was running were a little cold and changed those. Stock plugs for 76-77 KZ750 was B8ES, but for 78-79 it was B6ES.

I was running B7ES plugs and changed to B6ES and noticed a significant change.

I can now go WOT up to 7k rpm without issue. It starts to pop and sputter a little at 7k rpm now, but this is a dramatic improvement. Previously it would misfire and cut out between 6-7k rpm.

Could the change from B7ES to B6ES plugs really make that kind of difference? Or is it more likely just fresh plugs making the difference?