Finally think I figured out '68 F100 backfire
 

First, thanks for all the help, advice and encouragement. Second, I'm thankful to have an understanding wife. So I've been trying to diagnose and repair a backfire thru my carb on a swapped 460 motor in a '68 F100. No matter what I did, rebuild the carb, replace the entire fuel system, work with the ignition in every possible combo and degree angle, I STILL have the popping noise in my carb. VERY frustrating. Yesterday I was about to start smashing **** when I remembered a trick my dad taught me years ago. With the motor running I began taking one plug wire at a time out of the ignition sequence. Every other wire I took off would make the motor bog a bit, but otherwise run ok. When I took the #6 wire off there was no change in the motor. I also revved it up to about 2000 RPM. No popping, ran strong, had that nice deep growl at high RPM. NO POPPING. Then to check the plug wire I switched #6 and #7 wires. #7 cylinder ran fine with #6 wire, #6 cylinder with the #7 wire still had the popping noise, so the wire at least is good. I did not check to see if the #6 plug had spark as the motor was too hot. Additionally, the day before I ran a compression test and ALL 8 cylinders had good compression and were within 5-10 pounds of each other. Anyway, I let the motor cool down yesterday afternoon, and removed the valve covers. Here's what I found: #5 intake rocker arm is very sloppy. Almost enough slop to get my little finger between the end of the rocker arm and the end of the valve stem. #6 exhaust rocker arm had even more slop. Without tool measuring the rest of the valve train, just going by feel at this point, I found that most of the rest of the train is snug and when I measure it I'm sure it'll be within tolerances. Felt a couple that were not quite snug and plan to measure all. Pushrods were not micced but rolled out and none seem to be bent or out of round. My motor does not have adjustable rocker arms.

Seems to me that on the #5 and #6 cylinders the valves are stuck closed because the pushrods are not long enough to do their job. I also reckon that would cause the popping noise I can't fix by adjusting ignition or fuel systems. Ran good if #6 cylinder was taken out of combustion sequence, but a comp test shows #6 has good compression.

I ordered a few longer pushrods to replace in the sloppy spots and standard length pushrods to just replace them all in any instance. What are your thoughts on replacing lifters at the same time? Most of the rest of the valve train parts are functioning as they should, but since I'm gonna be in the general area, I'm giving it some thought. Another aspect of this motor is that my early goal is getting it and the tranny mechanically sound and reliable. I'm retired and have plenty of time. Once I get it mechanically good to go, then the next phase is performance, paint, interior, chrome, etc. If I have to pull the motor apart for performance purposes, I'm good with that, but for now, I want rock solid reliability.

Thanks,
Joe

Backfiring or popping out the carb is generally caused by timing or valves not opening and closing properly.If the pushrods and the rocker arms are OK I think you should pull a lifter and check for wear sounds like you may have wiped out a lobe on the cam.

Yes ditto on collapsed lifters and or cam lobes wiped out. No zink in the oil I have heard can do this but never have seen it yet. Maybe the longer rods will work for a while like a band-aid. But I think you are in for the long haul over haul.

Something to think about is that there are a lot of good running 460's out there for cheap. At least around me and mine anyway.

no zinc in the oil is a major problem I work at a local repair shop and have a run on flat cams mainly on Chevrolet and one ford a 75 302 only way to know is pull a lifter

There is many threads on here about oil and zinc and that stuff but after years of reading them I still don't have a clue what oil to buy for like my 85 Bronco with the 351W. I just add zinc. Or use Delo 100 or 400 that I think has zinc. What do you suggest jacks61ford?

the delo and rotella are no longer advertised as having zinc I buy the additive at napa every other oil change is what I run in my 223 and 400 in my fathers ranchero ...if you like Lucas it still has zinc in it also in its additive. on another note my sister just moved to bethel alaska with her new hubby you anywhere near that I don't know much about Alaska haha

I would be betting on the bottom of the lifter worn out &/or flat cam lobe. As with Jack what I would see back when I turned a wrench professionally was Chevrolets but you definitely have something wrong. If a collapsed lifter you would hear it ticking. If you do end up putting a cam in it buy a timing set for a pre '72 460. It will move the cam to 0 instead of 4 retarded and give you more power.

you can just pull a valve cover and run the motor for a short time and make sure all valves move evenly and Steve riff says hello

I am on Prince of Wales Island just north of Ketchikan. That is just a few hours boat ride from Prince Rupert in Canada. Or a few hundred miles north of Seattle. Bethel from me would be like Frisco is from Seattle. It's a very long way and a whole different world up there. Alaska is so big it should be it's own country.
This is where I generally make some silly comment about how many Texas's we can put in Alaska and still have room for most of Montana. But that would be rude.:D

wow I never realized how big Alaska is nor did I ever think I would have family up there ..I just hope everyone up there is as friendly as people down here lol