13volts at the coils start bike then I have 11volt

The meter is an expensive digital FLUKE
Ive been using engine ,frame for ground with the same readings

richard wrote: . . . idles good but under load it feels like the spark is week ,feels like a twin . . . 1976 Z1 . . . coils . . . turn power on and bike has 12.8 at the battery(disconnect head light)12.8 at the Y lead that powers the coils
and 12.8 at the coils them selves
now start the bike (again with head light disconnected)and it idles to 13.5 at the battery ,13.5 at the Y connector that feeds the coil
but at the coils I get a 11.2 reading...this only happens while the bike is running????
. . . .

Could temporarily connect both ignition coil primary terminals directly to battery positive via hot wire -- which will provide full battery power to the ignition coils regardless of what's indicated on the voltmeter at hand or any other meter -- and test ride the bike while so hot-wired to determine whether there's any change or improvement in engine performance under load.

Remember to disconnect the hot wire immediately following the test ride.

Also, if not already done, would assure there's no possibility of shortage at coil primary terminals to a mounting spacer or otherwise, as with Dyna coils is prone to happen.

If determined to continue using ignition coils of higher resistance than those designed for igniter system ignitions, would also fit solid core plug wires and non-resistant plug caps and non-R plugs, all being with the objective of reducing resistance in the secondary loop.

Good Fortune!

While riding and experiencing the mis-firing under load, could perform a throttle chop and immediate plug reading to help diagnose the cause.

Are plugs black and sooty? Fuel saturated?

Has clear tube testing been done to assure correct fuel levels?
If too high in any carb, can result in combustion issues under power, regardless of otherwise perfect carbs and perfect everything else.

Good Fortune!

richard wrote: The meter is an expensive digital FLUKE

As I said: digital meters have a 10 MEG Ohm input impedance and the leads from the meter can act like antennas and pick up the hot radiated EMI off the coils or coil lines and read goofy. I use a needle meter for troubleshooting around the ignition. If you can get an analog meter, use it.

As I said before, the solution is easy: you take a a voltmeter and walk down the power line from the battery to the fuse to the ig switch and finally to the wire connecting to the pos side of the coil. Record the readings and see where the voltage drop is. That isolates it to a particular wire, connector, switch, or whatever. If there actually is a voltage drop (and not a false meter reading) there MUST be some kind of resistance causing the voltage drop. That resistance could be a corroded connector, harness plug, screw terminal, wire or any of the above. If you are losing voltage, it has to be across something.

Just a thought
If the coils are heating internally then the resistance will change.
voltage = ampage x resistance
Coils must be secured to a solid heatsink to aid in cooling, usually the frame.

Voltage can be fine with no current flowing but not with current flowing.
Do a drop test between the battery and the coil lead .
Meter on volts dc
Positive probe on positive battery terminal
Negative probe on connector at coil.
Start bike and ensure no/low voltage shows on voltmeter.
Do the same for the return circuit .
Positive probe on Ground/earth point of frame
negative probe on battery negative
again run engine and check for no/low volts

current always follows the path of least resistance.
The wiring should all be low resistance but the internal circuit in the meter has high resistance.
The current should always want the flow though the wiring

Another thing to add

Where does your bike feed power to the coils from?

If the cdi supplies power to the coils is it possible that the power in watts is maxed out?

power = amperage x voltage

to many amps drawn will reduce voltage.
possible not but again an explanation

the - to the coils is switched on and off to energize them effectively grounding that + wire at the frequency the bike is running at. what your seeing is a average voltage. if you looked at it with a o-scope, you'd see a sawtooth pattern.
measure to the coils with the meter on AC volts and you'll find the missing volts. does that meter have "peak hold" ?
or "high resolution" ? try those too