Resolved: Carbureted ZZ4 350 High Idle and Hard Starts

[The Mindless Philosopher]

Symptoms:
- Exhaust pops on quick deceleration
- High idle (1,200 RPM range instead of 800), and it wouldn't settle down
- Harder than usual starts (high quality, low mileage "Carburetor Mike" tuned Edelbrock 1901, so no reason this should occur)
- Mechanical knock (louder than a lifter tick) from the front right side of the engine block (Chevy ZZ4 350)

I first inspected the rubber vacuum plugs on unused carburetor ports and then checked the two vacuum lines: one to the choke and one to the distributor.

The carburetor stud bolts were tight, and the base plate gasket on the intake manifold was fine.

I was beginning to think it could be a bad intake manifold gasket on one of the cylinder heads or perhaps an exhaust manifold/header gasket leak until I checked the last of the low-hanging fruit and found the culprit...

Root cause: loose spark plug on cylinder 4.

[Puff]

Whenever my ZZ4's exhaust pops on quick deceleration, I have found that replacing the plugs
with new ones takes care of that issue. And Adrian always suggested we gap new plugs at .035
not .045 like some do. My bike is at 103,000 miles at the moment. Nov 2023

[The Mindless Philosopher]

Bill, my plugs are gapped at .045. I'll gap the next ones at .035. Thank you for the info!