* Governor issues: The governor controls engine speed. A problem with the governor linkage, spring tension, or the governor itself could cause erratic idle speed and kickback. It might be trying to compensate for something else going wrong.
* Ignition timing: Incorrect ignition timing can lead to poor idling. While less likely after a carb replacement, a slightly off timing can manifest as kickback at low speeds but not affect higher RPMs.
* Air intake restriction (even with a new carb): Something might be partially restricting airflow even with a new carburetor. Check:
* Air filter: Ensure it's clean and not restricted. A slightly clogged filter can cause issues at low RPM.
* Air cleaner housing: Make sure there are no cracks or blockages in the housing itself.
* Intake manifold: Inspect the intake manifold for cracks or debris.
* Weak spark at low RPM: While the spark may be strong enough for high RPM, it could be weak at idle. This could be due to a failing ignition coil, faulty condenser (if equipped), or a problem with the ignition module.
* Carburetor adjustment (despite being new): Even a new carburetor might need minor adjustments. The idle mixture screw might need fine-tuning. Incorrect adjustment could cause a lean condition at idle, leading to kickback. *This is a common culprit, despite the new carb.*
* Fuel delivery issue at low RPM: Though less likely with a new carb, there could still be an issue with fuel flow at very low speeds. This could be due to a partially clogged fuel filter or a problem with the fuel pump (if equipped).
* Flywheel key (re-check): You mentioned the flywheel key is okay, but double-check it *thoroughly*. A slightly sheared or damaged key might be causing intermittent problems that manifest mostly at low RPM.
Troubleshooting Steps:
1. Re-check the carburetor adjustment: This is your first priority. Consult your engine's manual for proper idle mixture screw settings.
2. Inspect the air intake system: Thoroughly examine the air filter, housing, and intake manifold for any restrictions.
3. Verify the governor linkage and spring tension: Make sure everything is correctly adjusted and moving freely.
4. Inspect the ignition system: While more involved, check the spark plug for good spark at idle, and consider checking the coil and condenser (if applicable) for proper operation.
5. Thoroughly re-check the flywheel key: Look for any sign of damage, even microscopic. Consider replacing it as a preventative measure.
6. Check fuel delivery: Inspect the fuel filter and fuel line for any blockages.
If you've checked all these and the problem persists, seeking help from a small engine repair shop might be necessary. They have specialized tools and experience to diagnose more complex issues.
