cant fugure it out.....can u?

[chevychevy]

so i got me a 73 chevy stepside witha 350 in it and for some sdang reson it acts funny at high speeds (only when im on the freeway)
it always backfires thru the carb but when im on the freeway it starts making a noise that sound like a rod smackn the engine and when i pull off the freeway it keeps makin the sound and acts like it wants to die now if i turn it off for a while it will run fine.....what the hell????......questtions/comments...any one?

 

[Pontiac1976]

Sound like timming and carb ajustment when the last time it had a tune up, is it still points in it. if it been running to long this way you might have piston trouble.

 

[drabo]

The rod smack could be a lifter that is shot or a bent pushrod.

 

[OLDIRON69]

It kinda sounds like the vaccum advance on the distributor is bad. Does it do it at all at low speed acceleration?

 

[T-bone]

Maybe!

I have a 86' Silverado and it would backfire a little when you race the motor. And sometimes it would sound like something inside would click and going down the road it would start to lunge or act like it was not getting gas or something. Two weeks ago I was replaceing the thermostat and found that some of the vacuum lines made of rubber were just wore out. It was cold and the more I worked with them the more I broke. After I got them all replaced I no longer have that lungeing and my mileage has improved. It feels like it is running better and the backfireing has stoped. My meaning is that there was some bad lines and I didn't know it untill I moved them around working on something ealse. May help you maybe not, just an ideal. By the way its a 305.