How To Tell ATV Jumped Time? (In 4 Steps)


Engine timing is mission critical, getting this stuff wrong can spell big trouble. If you suspect timing is out, best not to run the motor.

Checking ATV engine camshaft and crankshaft timing marks, is the correct way to verify timing. It’s simple 4 step process:

  1. Remove spark plug
  2. Remove camshaft cover
  3. Set engine to TDC on compression stroke
  4. Check camshaft sprocket marks align correctly

By the end of this post you’ll understand clearly how to check your ATV engine timing marks, understand why timing jumps and how to fix it and also other causes of bad timing.

Timing

Timing is everything, every component of your engine has an important job to do and some components have to do it at a very precise moment in the engine cycle.

Doing their job too early or too late can make the difference between an engine on full song and no singing at all.

The bottom end of your engine (crankshaft) coordinates the top end (valve train), it employs sprockets, chain, tensioners and guide rails to keep everything in time.

The valves, positioned in the cylinder head open and close sequentially allowing gas mix in and spent gases out.

The spark plug obviously needs to fire at the correct time too or the whole sequence is pointless. Ordinarily when the engine is idling the plug fires when the engine is at TDC and it signifies the beginning of the power stroke.

As the engine speeds up though, the plug is fired earlier BTDC (Before Top Dead Center), as the spark plug takes time to produce a spark. The firing, advancing and retarding of the spark plug is controlled by the trigger (Aka pulse or ckp) and the CDI box (Capacitor Discharge Ignition).

Your ATV engine is a four stroke, simply because it has four clearly defined stages in it’s operation.

1 Induction stroke

Piston moves down the cylinder and draws fuel mix in through open intake valve.

2 Compression stroke

Piston moves up the cylinder compressing the air fuel mix (valves closed).

3 Power stroke

Plug fires and piston moves down the cylinder powering the crankshaft (valves closed).

4 Exhaust stroke

Piston moves up the cylinder forcing spent gases out the open exhaust valve.

Symptoms of Bad Timing

There may be lots of reasons why your ATV engine doesn’t sound or operate like normal. Timing is not one of the first items checked unless it suffers from some of the following symptoms:

  • No power
  • Popping in the carburettor
  • Backfiring
  • No start
  • Engine binding on crank
  • Poor idle
  • Long crank
  • Won’t rev
  • Hot starting issues

Checking Timing

Checking your timing is a straight forward procedure, but you’ll likely need to remove a few items to gain access. This guide guesses that your engine is an Over Head Cam found in most modern ATV’s.

Step 1

  • Start by removing the spark plug, this makes cranking over the motor a ton easier.
  • Remove the camshaft cover, this will allow clear access to the timing chain and sprockets.
  • Remove the side cover flywheel bolt access cap and flywheel timing mark inspection cover.

Step 2

  • Rotate the flywheel anti-clockwise slowly using a suitable socket and ratchet. The timing mark “T” or equivalent will appear in the inspection window. This indicates Top Dead Center (TDC)

Step 3

  • With the crankshaft at TDC, check the marks on the cam timing sprockets. It’s normal for cam timing sprockets to have two marks per sprocket. One mark aligns with the horizon plain of the cylinder head and the other points straight upwards (90°).

Step 4

  • If you don’t see the cam sprocket timing marks align, rotate the flywheel anti-clockwise one full turn.
  • Position the “T” or equivalent TDC mark in the inspection window once again.
  • The sprocket marks should now be aligned on the horizon plain of the cylinder head, and the additional cam marks pointing upwards 90°. The engine is now set to TDC on the compression stroke.
  • If all cam timing marks don’t align, your timing is out and will need to be reset.

Why Timing Is Bad

The timing chain and valve train are all mechanical and require adjustment and maintenance for trouble free service.

The more usual causes of jumped timing include:

  • Worn or faulty hydraulic chain tensioner
  • Worn timing chain
  • Worn sprockets
  • Worn chain guide rails
  • Stretched chain
  • Low oil level
  • Poor quality oil
  • Sheared flywheel key
  • Over revving engine

But consider, there may be other reasons why the timing is bad:

  • Pickup sensor faulty, or out of position
  • Faulty Stator
  • Flywheel magnet position fault
  • Sheared flywheel key or missing
  • CDI box fault
  • Faulty coil
  • Bad plug
  • Valve lash needs adjusting

Resetting Timing

Resetting the timing can be a pain in the ass, but you do need to get it correct. If the timing isn’t correct the piston could possibly impact the valves causing some very expensive repairs.

Your very first job is to find a manual for your engine, the process, torque specs and tightening sequences vary from engine to engine.

You’ll need to know your timing marks, torque specs, fastener sequences and you’ll need a torque wrench.

This guide will give you a general flavour of the job ahead.

  • Remove cam covers and set crank and cam sprockets to TDC on compression stroke
  • Remove the tensioner
  • Remove the chain from the top of the sprockets and hold
  • Realign the sprockets
  • Refit the chain
  • Reload the tensioner – screw-back to reload
  • Fit tensioner plus new gasket and torque to spec
  • Fire the tensioner by tapping
  • Check timing marks
  • Rotate the flywheel anti-clockwise two full turns and check timing again.

A full inspection of chain, sprockets, guides and tensioner is advised. A failing tensioner is a very common cause of jumped timing and obviously if root causes aren’t fixed, timing will jump again and maybe with costly consequences.

John Cunningham

John Cunningham is an certified mechanic and writer on ATVFixed.com. I’ve been a mechanic for over twenty years, I use my knowledge and experience to write articles that help fellow gear-heads with all aspects of ATV ownership, from maintenance, repair to troubleshooting.

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