My 84 Sportster at about 100 ks/3000 revs gets a clapping type sound that sends the motor out of control like it is all out of balance.

At this point I have no throttle, I drop back down the gears till I get to first gear the bike back fires then motor rights it self up again and off I go.

Have changed points, condenser, flyweights, new later model cv carby, but I had this problem with original carby. This only happens if I am doing a constant speed of  100 ks which is annoying because this is my main highway speed.

Regards Mal

It's probably a sticking rear exhaust valve.
Give it a good dose of upper cylinder lube and some good fuel.

It may need the heads pulled and a decoke plus a valve seat job.

And run the rear exhaust valve clearence a bit loose(wild). They get hot.

Hey Mal, I had a similar problem which was electrical, now I'm not saying it is but if you have done any wiring yourself double check it. Mine was a short in two places that caused intermittent engine failure where the old girl woud just stop and take off again.
Hope this helps.

Have you considered speeding...???

Other than that I did similar to Bucket and sold the ironhead to a fella I didnt like then bought sumthin else...