Resolved: Carbureted ZZ4 350 High Idle and Hard Starts
Posted: Sun Oct 29, 2023 5:00 pm
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.
- 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.
My bike is at 103,000 miles at the moment. Nov 2023