205 Mi16 Misfire

[chopper205 0 1 Cars]

I have had this problem for a few months now and am really stuck. It's a mi16 on dellorto 45s and is misfiring really bad. Changed leads, distributor, cap, plugs, amp,and refurb carbs but still no change. Got a new coil today and fitted it got 100 metres down the road and it totally died would not start. After 10 mins it started and manage to crawl home, think the ignition amp has gone but only replace it a while ago and it still made no difference. Could it be a earth fault making it misfire and then blowing the amp. Some times it runs ok and other times it won't run at all. I have tried everything and really thinking of getting rid of soon