1991 zxr750 (J1) breaks down under load.

My zxr750 starts and idles well can be riden softly ok but as soon as you open throttle it will break down. It doesn't feel like a misfire on one cylinder only, it seems fairly even over all four cylinders. Have sent ignition module away for testing and repair tried a different one, tested coils, rebuilt carbs twice (second time by qualifed m/c mechanic) replaced inlet manifolds, tested fuel pump and relay, checked fuel lines taps strainers etc and repeatedly smashed my head against the wall. Any ideas would be much appreciated. If you have had this problem and I have mentioned it above let me know and i'll try it again.

This bike was running fine and then parked in the shed for a few years. After sitting for some time it was pull out of the shed and had this problem.

Try cleaning the coil mounting bracket mounting points for rust or paint or running another earth to the coil wires....

I have a 93 L model. I did a tune up on it. New plugs, sync'd carbs etc. Put it back together and the same thing happened. After a hell of a lot of head scratching and swearing and testing just about everything you have done, I found that I hadn't connected one of the pipes back onto the airbox. Fixed it straight away. I couldn't believe it.

Not sure if the air box on the J Model is the same, but worth a try anyway.

If it idles indefinitely ie for more than 10 minutes, replace your plug leads and make CERTAIN the copper has good contact with the coil sockets.
Also try riding with the fuel cap open/ajar. You might have a blocked air relief port in the cap.

As previously stated, it sounds like an ignition problem. You can check for bad high voltage, (ignition) leaks by spraying a fine mist of water spray on the wires. Do it in a semi dark environment and the leaks will make themselves known by the blue sparks. This only works on high voltage ignition leaks, (spark plug wires, coils, etc.). the 12 volt side does not have enough EMF to allow current flow through the spray mist. Another way I use is to just feel around with my hands and when I shock the shit out of myself I've found the problem. Then it's Jim time.

Thanks for the replys will be trying a few of these tips and will post a result if i get one.

Finally a result. after going through everything I started over. The igniter module, which was sent away twice then replaced (with another second hand one) was pulled apart by an electronic whiz I know. I don't know what he did but it is now running fine.