I have a 1986 Chev Van, Series 20 with a 305 (155K miles).

When you first start and run the van, all is fine. When it warms up, and you place it under load (say, going up hill or accellerating hard) it will knock.
If the van is pulling a heavy load when warm, like my Boy Scout trailer, it will knock violently when you accellerate. The van looses power, and the heat skyrockets. Only going to idle stops the knock (knock stops completely).
I thought it was just my van until I rode in another Chev that did the same thing. Owner said he owns 2 identical vans, and both do the same thing.

I took it to a shop, and they said the timing was right, and all cylinder compression was reasonable. New plugs, cap, etc didn't help. Mechanic brake torqued it to get the knock, and then aded gas to the carb. He said the noise stopped. He wants to rich the mix. I guess it is possibly the problem, as the chock plate would be closed when the van was not hot. However, the van use to get 20 mpg at 50K, and now gets 10. I would hate to loose even more milage.

I have a feeling that the advance is pulling the timing out of range.

Is there a known issue with these engines? Any other thoughts?

Thanks for any and all help!