i have a 1991 f150 with a 302. when it is at idle it is knocking like a desail. and it idles at 500 rpm can i chage it and how?

 

You need to determine where the knock is coming from - it could be as simple as a collapsed lifter, which is easy to fix; or, it could be a rod bearing, flat lobe on your camshaft, etc..... - not so easy to fix.

 

I had a problem that sounds similar to yours, it turned otu the harmonic balance had slipped on the rubber and was bumping the timing cover at idle.

The truck had been in an acident a month before and the body shop didn't look at the balance. I was trying to figure out what was knocking and saw the balancer wiggle. $80 and it was fixed

 

pull a spark plug wire one at a time, if its a rod the knock should go away