First post, weird problem

[eazzrider]

Hello and thanks in advance for all the good info on the site. Here's the deal, Son recently purchased a 72 C10 with the 250 and three speed on the tree. 56,000 miles super clean and rust free. Truck has been setting for a long time. Runs like a champ but won't idle smooth. I have the idle turned up to keep it smoothed out. You can get going down the road, put the tranny in neutral, hit the brakes and the engine will die. It does not have a vacuum assist on the brakes. When the front end dips the motor dies. I can go in reverse and do the same thing and the engine speeds up. With it idling fast in the driveway I can bounce the front end and make the rpm's swing all over the place. What gremlin is in this thing? Thanks,