hope this isn't to off topic but my hauler a 78 C20 isn't running right. this developed back in october coming to washington from socal. i rebuilt Qjet no change, did complete tune up with 8mm wires and HEI cap has brass terminals (made in canada) no change. here is video i did today with my phone. as you can see it idles great. i'm leaning towards cam issue. thanks!
A tune up and carb rebuild wont fix that racket Sounds tinny on my end A vacuum gauge can show you a flat cam A compression test will to and a dynamic compression test definitely will.
The audio on my I phone isn't that good but if its popping back through the intake it is usually an exhaust lobe worn flat. The exhaust gasses have no where to go and push back through the intake valve.
Double valve springs??? maybe is 1 broken, (had that on my 350, idled great, but not enough pressure to close the valve above idle) or maybe pulled the bottom out of a rocker..
sounded like my K 30 CS when it did it ( about 75K miles ) started bucking a little on the highway in the Pa mountains under load . at idle you could see the carb back spit or fog if you unplugged the cylinder if the timing chain cover never has been removed its more than likely a flat cam as that was the years of the recall for it ( up to 1980 ) verify with vac gauge like said, also check, and pull the rocker covers and use the timing light method for checking lift it will show up like a sore thumb . the recall repayment ended about 1990 as mine went in 87 , great time to replace to a better cam as the factory cam ( car cam ) was garbage for towing or milage with any gear higher than 3:55 ( I had 4:56 on a 16.5 )
thats what it was doing loaded through the mountains bucking a lot! hope this thread helps someone else!
if you find the offending cylinder and need the truck you can pull the plug wire or zerogap the plug ( which is better for the hei ) and drive it for a little bit ( about a week ) but try not to put a load on it as the uneven heating of the head can screw it up .
Little hard to tell in the video, but if it seems to be coming back through the carb, then I am going with the consensus, cam going flat. Intake lobe, exhaust won't do that. But pull valve covers first, could be a rocker off, or a push rod punched through if it has a lot of miles.