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 a simple 4 step process:
- Remove spark plug
- Remove camshaft cover
- Set engine to TDC on compression stroke
- Check camshaft sprocket marks align correctly
By the end of this post, you’ll understand clearly how to check your ATV engine timing marks, why timing jumps and how to fix it, and also other causes of bad 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, chains, tensioners, and guides rails to keep everything in time. The valves positioned in the cylinder head open and close sequentially, allowing fuel in and spent gases out.
The spark plug needs to fire at the correct time. 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. Know as BTDC (Before Top Dead Center), this is because the spark plug takes time to produce a spark. Starting the process early, therefore, means the plug continues to fire at the sweet spot in the engine cycle.
The firing, advancing, and retarding of the spark plug is controlled by the trigger (aka pulse, pickup, or crankshaft sensor (CKP)) and the CDI box (Capacitor Discharge Ignition).
An ignition timing issue may be caused by a faulty CDI box or an incorrectly positioned pickup sensor.
Your ATV engine is a four-stroke simply because it has four clearly defined stages in its operation.
The piston moves down the cylinder and draws fuel mix in through the open intake valve.
The piston moves up the cylinder, compressing the air-fuel mix (valves closed).
Plug fires and the piston moves down the cylinder powering the crankshaft (valves closed).
The piston moves back up the cylinder forcing spent gases out the open exhaust valve.
And the cycle starts over.
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 carburetor
- No start
- Engine binding on crank
- Poor idle
- Long crank
- Won’t rev
- Hot starting issues
Don’t rule out some of the simple causes first and they include the following:
- Fouled spark plug
- Wrong plug type
- Bad gas (common cause of issues)
- Valve lash needs adjusting
- Faulty CDI
- Faulty coil
- Faulty or misaligned pickup
- Faulty stator
- Flywheel magnet position fault
- Sheared flywheel key or missing
Checking your timing is a straightforward 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 ATVs. I’ve included a single and a twin-cam, your timing markings will likely be different but not to worry, from the timing diagrams below, you’ll get the picture.
The process goes like this:
- Start by removing the spark plug, this makes cranking over the motor a ton easier.
- Locate and remove the timing inspection hole.
- Rotate the flywheel counter-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)
- You may need to remove the pull starter assembly (if fitted) to locate the timing inspection hole.
- Go ahead and remove the camshaft inspection cover or remove the cam cover.
- This is a single cam engine with the crankshaft set to TDC.
- Go ahead now and check your marks on the cam timing sprocket. It should align with the mark on the cylinder head.
For twin-cam engines, the process is similar, with just an extra sprocket mark to line up. Typically, both sprocket marks align with the horizon plain of the cylinder head (marked as red lines below) and some sprockets may have an additional mark (not in the pic below), and typically they point straight upwards (90°).
- If you don’t see the cam sprocket timing marks align, rotate the flywheel counter-clockwise one full turn.
- Position the “T” or equivalent TDC crankshaft mark in the inspection window once again.
- The sprocket mark(s) should now be aligned on the head (horizon plain of the cylinder head, (Twin-cam)) and the additional cam marks pointing upwards 90° (if applicable). 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.
But Why is the Timing Out?
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
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 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.
Critical information and tools
You’ll need to know your timing marks, torque specs, fastener sequences, and you’ll need a torque wrench. You’ll find a list of all the tools I use here on the ATV tools page.
This guide will give you a general flavor 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 sprocket(s) and hold
- Realign the sprocket(s)
- Refit the chain
- Reload the tensioner – screw-back to reload
- Fit tensioner plus new gasket and torque to spec
- Fire the tensioner by tapping usually
- Check timing marks
- Rotate the flywheel counter-clockwise two full turns and check timing again.
A full inspection of the 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.
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