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 br*** 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 g***es 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.
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