In my 73 f100 4x4 truck, the engine is knocking. This is fairly loud at idle and get louder and more rapid with acceleration. Also, the vacuum gauge fluctuates down with the knock from 15.5(standard idle) to ~13.5 From what I can tell, it seems to knock only once per revolution.

This started after I had been driving and for some reason unknown to me, the rocker arm on one of the exhaust cylinders became misaligned. After I opened her up and saw this, I checked the pushrod(still straight) and the spring(not broken) put it back together and started her up. I now have the knocking but the rocker arm does not come off anymore(have not tried driving with it as I'm scared to damage something so it may still at higher revs). I have not let the engine run longer than maybe 2 minutes since this has occured, again, scared to damage something(more).

What are some things I can do to troubleshoot this to isolate the issue? And what tools might I need to invest in?

 

Take a chunk of old garden hose and use it like a mechanic's stethoscope. That way you can pinpoint where the problem is.

 

Sounds like a rod or a 'spun' crank bearing to me any knocking from your engine is usually BAD NEWS !!

 

Take it from me just pull it out and rebuild it. My truck used to have a 428 in it and I couldnt hear for the knock and it ended up throwin two rods right through the pan and crackin the block in two cylinders. I would pull er out and tear it down put some new bearins, rings, gaskets and timin chain with a good cam and put er back in. If it needs to get bored out then I would do that too.

 

You got a stuck lifter. You need to pull at least this lifter and replace it. Usually this is caused by dirty oil, but not always. You may have worn rocker arms from not getting enough oil to lubricate this will knock too. Pull the valve cover and start up and quickly check to see if all lifters are pumping oil up to the top. If not replace lifters and rocker arms and pivots on all.