Surges or jerks at steady cruising speed

You hold the throttle still at a steady speed, but the bike surges forward and back, or jerks. Two very different things feel like this. Either the engine power rises and falls (a lean mixture at light throttle is the classic cause), or the drive has slack that snatches: a loose or tight-spotted chain, worn cush drive rubbers, or a slack throttle cable that moves as the bike bounces.

Is it safe to ride?

Yes, with care. Check the chain first: a very slack chain can jump the sprockets.

Quick checks first

• Is the chain slack correct and even all the way round?
• Does the jerk come with a clunk from the rear when you open and close the throttle?
• Is there a lot of free play in the throttle?
• Does it happen only at one light throttle opening?
• Fuel-injected: is the warning light on?

Diagnose it step by step

1On a road test, watch the rev counter or listen. Do the engine revs rise and fall during the surge?

• No, the revs stay steady but the bike snatches: the drive line. Go to step 2.
• Yes, the engine itself surges: go to step 4.

2Is the chain slack correct and even? Check at several points around the chain.

• Too slack or tight in places: Check and adjust chain slack (K-Light 125: 20 to 40 mm, upright). Tight spots: see Chain is tight in some places.
• Correct: go to step 3.

3Hold the rear brake and rock the rear wheel by hand. Does the sprocket move a lot before the wheel turns?

• Yes: worn cush drive rubbers. Check the cush drive rubbers.
• No: scooters, see Scooter judders when pulling away. Manual bikes, check the throttle cable (step 4).

4Is the throttle free play correct, so the grip does not move the throttle when you hit bumps?

• No: Adjust throttle free play (K-Light 125: 3 to 5 mm).
• Yes: go to step 5.

5Does it surge only at small, steady throttle openings, with popping or a white plug?

• Yes: lean at light load. Find an intake air leak. Carburettor: check the float and clean the carburettor. See Engine runs hot and lean.
• No: go to step 6.

6Fuel-injected: is a code stored?

• Yes: Read engine fault codes. Oxygen sensor faults upset the fuel correction at cruise: Check the oxygen sensor.
• No: check ignition: service the spark plug. If it misses rather than surges, see Misfires when accelerating hard.

7. Revs rise and fall but the speed does not follow: this is slip, not surge. See Clutch slips.

Causes, most likely first

Reference values (K-Light 125)