Engine has no power under load

Starts & idles fine but has no power & badly misfires when put under load, can I adjust the timing on a distributorless motor? I've replaced the plugs, leads, the ignition coil pack & the air filter. I've done a compression test, checked the resistance (all 15ohm)& cleaned the injectors. I've looked for leaks in the vacuum hoses & they all seem fine. I'm about to check the pulse power to the injectors so i'm not 100% sure there. I have a horrible feeling it's got something to do with the timing or maybe one of the valves, what do you think. If you could drive the car yourself, I have no doubt that you'd be able to diagnose the problem easily by the sound & the feel. It sounds like it has a big hole in the muffler & feels like it runs on 3 cylinders, once the car is up to speed it sounds & feels not so bad, it just takes a while to get there. Thanks for providing this service, i'm very greatful. Kind regards, Tim.