Hello,

Have a 67 F100 with FE 352. Recently had a refurbished stock carb (Autolite, 2bbl) for this engine put on. Ran fine for a few days, then got a loud knocking sound when driving just a few blocks from home. Was able to get it back to the driveway, and didn’t lose any power in acceleration or ability to shift gears (its a 4-speed manual with granny gear, used after market transmission installed by previous owner).

the knocking is loud and speeds up when you give it gas, but it’s there when I’m idle too. Sure sounds like it’s coming from under the VC maybe on passenger side, but hard to tell.

Here's a video with the knocking sound:
Any ideas?

I only mention the carb install because timing of the problem seems significant. Truck has always run fine. A little sluggish at certain spots and always needs to warm up, but it had a carb I didn’t recognize before, hence the change to a refurbished stock carb.

 

Pull the passenger side VC.

 

Did you get a chance to dig into this yet? It's a very serious knock, but the first thing I do now when I hear something like that is to remove the V-belts from all the accessories and see if it changes.
The tone of the knock as heard through the computer is pretty dire sounding and not like you'd usually hear up in the valve cover area. I would think it's more down in the crankcase, but the last time I said that and thought the engine was on it's way out (a 302 in a '69 Bronco) they took it to a mechanic and he tightened the belts!
You could have knocked me over with a feather because I've been working on engines since the late sixties and had never "fixed" a knock that bad with a quick tensioning of the belts for the water pump, alternator and power steering.
Apparently that mechanic had more years of experience under their belt than I did!

That's just by way of saying, don't give up and think the worst just yet.
It might still be a main bearing knock, or a busted piston, or a tossed pushrod (happens more than you'd think these days), but I'd sure hate to see you dig too deep before ruling out the external sources.

I still say that tightening the belts just put enough pressure on the crank to quiet the bearings. But that's even more far-fetched sounding than having that noise come from a loose belt!

Paul

 

Thanks all. Turns out, there are pieces of metal (a nut or washer maybe) hanging out on a piston and another one around a valve seat! This is based on camera probe pictures, so won't know for sure until we get in there and dig it out - still waiting on replacement gasket and head bolts to do the job.

Had to have happened by mistake when the carb was installed (not by me) - noise started shortly after. Unbelievable. If you want something done right...

 

That's funny. I built a SBC 327 one time and had an Edelbrock carb on it. Out test driving one day shortly after rebuild in order to tune the carb. I stopped to make a spring a rod change and I used one of those nut driver handles with the socket on the end to tighten the rod covers on the carb and when I did, APPARENTLY the socket came off and stayed on the top of the carb. Down the road a ways I did a WOT and sucked that socket right into the cylinder bore. BAM! Socket turned into 15 pieces. It required a new piston and valves!

 

Yikes. Yeah, it doesn't look like there's any damage to piston or valves but won't know until we get in there.

 

Circling back here. Of all the things, turns out the screws holding the throttle plates were defective / over-torqued and one snapped off and obviously got sucked into the engine. Pictures below. Couldn't believe it. When trying to remove another one of those screws. it snapped in two pretty easily, so clearly going to replace all 4 screws. Thankfully, no damage to engine, other than a little ding on top of the piston head.

Having replactement screws sent, but had a questions for the forum. This is for an Autolite 2100 2bbl refurbished carb (manufactured in 67 based on the tag serial number). Do those screws have a particular torque value? Should Loctite be used and if so what type? Anything else I should be thinking about so this doesn't happen again? Thanks so much!

 

Once you get the new screw tight, squeeze the threads with vise grips so that end of the threads is oval. Now the screw can't come back out.