It's not a Chevy but I hope someone can help. this summer we did a heads, cam, intake, and carb on my friends 69 olds 350. It ran great for several weeks. then one day he got into a race and near the end he had to hit the brakes hard. Motor died and wouldn't start. we towed it home and jump started it now it runs weird. If you rev it up the RPM's take a long time to come back down to idle and it backfires some times. He put the old carb on just to rule out the carb and it was no better. Any ideas?

 

High idle + backfires usually indicates a vacuum leak. Maybe on the hard brake the power booster diaphram tore. Unhook the hose and plug it to see if it helps. If not, keep looking cause the symptoms are classic vacuum leak. Also check timing.


I'm not sure, but I think Olds have only one bolt on the cam and the bolt also retains the drive for the fuel pump. Sad I don't remember cause I used to own a 442. Fords have this setup too, and I've seen those bolts come loose when torqued to the factory spec. So pump drive loosens and fuel pressure drops. I've learned to give bolt extra torque and loctite them. Loose bolt causes engine to run like crap, but doesn't cause high idle.

 

Almost sounds to me like the distributer wasn't bolted down tight enough, and moved slightly, changing your timing.

As angel suggested, check your timing.

 

we couldn't find a vacuum leak anywhere. Couldn't check timing we didn't have a light handy.