I'm back to working on this truck again.
The truck still has this same problem of making popping sounds in the exhaust when idling at any speed. When a pop is heard, a small fluctuation in manifold vacuum frequently coincides. It fluctuates down by as much as 2inHg from 20inHg. It will also suck a piece of paper into the exhaust pipe when the pop occurs. Additionally, the engine will frequently shudder slightly when there is a pop.
When driving under load, the problem does not seem to occur and the engine seems to work fine with plenty of power.
Checked for burned valves by doing a compression test. All cylinders showed 120-123psi. Can valves be burnt if the compression checks fine?
Readjusted the valves.
Checked for vacuum leaks by removing the carburetor, blocking off the place were it was, removing the pushrods, and pressurizing the manifold with compressed air. Only air leakage found was some seepage past the valves and guides. I also checked for vacuum leaks with the engine running, spraying acetylene around the joints. No change in idle noticed.
If I retarded the ignition to the point that the idle slowed considerably, the popping seemed to mostly stop but the truck then couldn't accelerate.
This is really quite frustrating. If I somehow put the camshaft in with the dots on the timing gears not matched up, could it cause this problem? Is it worth replacing the entire ignition and fuel systems? Worth removing the head to check the valves?
