Engine feels weak and down on power

The bike accelerates slowly and feels weaker than it used to, but it runs and does not misfire badly. Low power has many causes, and some are not in the engine at all: a dragging brake or a soft tyre feels exactly like a weak engine. The order below follows the K-Light 125 power fault chart: first the wheels, then the engine's breathing, spark and fuel, then compression.

Is it safe to ride?

Yes, with care. Allow more distance for overtaking and joining traffic.

Quick checks first

• Do both wheels spin freely by hand with the bike on its stand?
• Are the tyre pressures correct?
• Is the choke fully off and the air filter clean?
• Do the revs rise without the bike speeding up (slip)?
• Fuel-injected: is the warning light on?

Diagnose it step by step

1When you open the throttle, do the revs rise but the speed hardly changes?

• Yes: the power is being lost in the clutch or drive. Go to Engine revs rise but the bike does not speed up.
• No, the engine itself feels weak: go to step 2.

2On the stand, do both wheels spin freely and keep turning when you give them a push?

• No: something is dragging. Go to Bike feels held back.
• Yes: go to step 3.

3Are tyre pressures correct?

• No: Check tyre pressure. Look for a slow puncture.
• Yes: go to step 4.

4Is the air filter clean and the choke fully off?

• No: Clean or replace the air filter.
• Yes: go to step 5.

5Is the spark plug in good condition and the correct type?

• No: Spark plug service. A black plug: see Engine runs rich. A white plug: see Engine runs hot and lean.
• Yes: go to step 6.

6Fuel-injected: is a code stored?

• Yes: Read engine fault codes. Many bikes reduce power when a sensor fails.
• No, or carburettor: go to step 7.

7Does it lose power mainly at full throttle, or is fuel flow weak?

• Yes: see Bogs down or chokes at full throttle. Check the tap and lines, the filter or the fuel pump.
• No: go to step 8.

8. Is the exhaust blocked? Two-strokes build carbon in the exhaust and port: Exhaust blocked with carbon. On four-strokes the K-Light 125 manual lists a catalytic converter blocked by burnt oil.

• Clear: go to step 9.

9Is compression correct?

• Do a compression test with a gauge (K-Light 125: 1.0 to 1.2 MPa). Low: check valve clearance first, since wrong clearance is the most common compression fault on small engines, then see Low compression.

Confirm the fix with a road test.

Causes, most likely first

Reference values (K-Light 125)