Symptoms
- Engine RPM rises and falls on its own at idle
- Idle feels rough, shaky, or inconsistent
- May stall at idle even though it revs fine when accelerated
- Idle speed screw may have been bumped or never adjusted
Likely causes
- CommonIdle mixture screw out of adjustmentSmall engines have a mixture screw that, if set incorrectly, causes hunting or rough idle.
- CommonPartially clogged idle jetA small restriction in the idle circuit causes an inconsistent fuel supply at low RPM.
- PossibleVacuum or gasket leakUnmetered air entering through a bad gasket upsets the idle mixture.
- PossibleDirty air filter affecting idle more than full throttleIdle circuits are especially sensitive to airflow restrictions.
What to do
- Check and clean or replace the air filter first, since it's the easiest variable to rule out.
- Locate the idle mixture and idle speed screws per the owner's manual, if your carburetor has adjustable screws.
- Make small adjustments to the idle mixture screw, a quarter turn at a time, listening for the smoothest, most consistent idle.
- Set idle speed to the RPM specified in the manual using a tachometer if available.
- Inspect visible gaskets between the carburetor and engine for cracking or looseness.
- If surging continues after adjustment, the idle jet may be partially clogged and need carburetor cleaning.
Tools and parts
- Small flathead screwdriver
- Tachometer (optional)
Carburetor gasket kitSearch: “small engine carburetor gasket kit”
When to call a pro
Many modern small engine carburetors have limited or no adjustable screws; a small engine shop can identify your model's correct adjustment points or clean the idle circuit.
Safety
Generally low risk
Generally low risk for a careful person. Stop if anything looks different from what this guide describes.