Last problem first. No exhaust pipe/muffler(box). A bit NOISEY aren't we? Fix the exhaust pipe first, there is no back pressure to the engine which requires it. Besides the problem of carbon monoxide poisoning in the cabin.

The lack of back pressure will cause the motor to run lean and exhaust the cylinder too fast, drawing unburned fuel out to the exhaust causing back firing and loss of power.

Your fuel filter may be full of water and dirt, has it been chainged since 1986? It is also possible that the fuel pick up hose in the tank is on it's last legs.

The automatic choke may also not be fully opened when the motor is warm, besides it sounds like it is completely closed when cold. Or your timing could be off a little since changing the distributer.
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95 854T, 88 780, 88 245, 01 A4 Audi