Understanding Your Four-Stroke Engine: Event Timing


Timing, they say, is everything, and that’s particularly true with engines.  Understanding how and when the individual events happen, and why they happen when they do will help you understand why they have to be set up as they are, and that always makes it easier to figure out what’s going on when problems arise.

Let’s start by going over the basic way the engine works, and what events need to occur and when.  First, we’ll look at the simple basics, then at the reason things happen exactly when they do.  To simplify the discussion, and to make it more closely relevant to dirt bikes, we are only going to discuss single cylinder engines during this explanation. 

The term, “four-stroke cycle” means that the engine needs to move the piston up or down the bore 4 times to complete all the functions that go into producing power from gasoline.  Because the piston is connected to the crank via the connecting rod, each “stroke” takes a half revolution of the crank, or two full revolutions for the four necessary strokes. 

The Four-Stroke Cycle

A lot of folks have seen this simplified version, but let's review. The cycle starts with the Intake Stroke, near top dead center (TDC), where the piston is at its highest possible position, with the intake valve opening and the piston moving down the bore toward bottom dead center (BDC).  This creates a void above the piston that is filled by air from outside rushing in through the intake port to fill it, and that air carries with it the fuel added by either the carburetor or a fuel injection system.

The rotating crank then begins to move the piston up the bore and the intake valve closes, trapping the fuel and air in the cylinder.  As the piston continues upward, the air/fuel mix is compressed, which heats it and increases the amount of force with which it will expand when ignited.  This is the compression stroke. 

The power stroke begins near the top of that second stroke, when ignition takes place, starting the fire.  The crank rotates past TDC as the burning fuel begins to expand, and the combustion force pushes the piston down the bore, creating the rotating force on the crank that drives the whole works.  

As the piston nears BDC again, the exhaust valve opens, and the piston is run up the bore to pump the spent gasses out through the exhaust port to complete the cycle with the exhaust stroke. 

Half Speed

Notice something here: The engine’s crankshaft rotated twice to produce four trips up and down the cylinder for the piston, but each valve only opened and closed once during that time.  To make this work, the camshafts have to turn at one half the engine speed, so the chain and sprockets, or gears, or toothed belt and sprockets used to drive them are set up at a 2:1 ratio. 

Ignition also has to happen right on time, so each ignition system, whether simple ‘50’s style points, or the most sophisticated electronic, has to have something to signal when that is. Traditionally, this signaling trigger has been attached to the camshaft so that the spark occurred only once every other revolution, but engineers seeking to simplify the design of single cylinder dirt bikes found no reason that there could not be a spark on every revolution, so the trigger sensor was mounted at the crankshaft instead.  That means there is a spark on every revolution, instead of only once per each two-revolution cycle of the engine. The second spark happens at the end of the exhaust stroke, so there’s nothing present in the cylinder that would burn.  It also makes setting up the timing during assembly somewhat simpler by eliminating what used to be a common mechanic’s mistake of picking the wrong top dead center position.

Getting Ahead of Things (The Engine is Dynamic)

Simplified explanations of the cycle like the one we started with here always show the valves opening and closing right at TDC and BDC, but if you watch the piston position as you turn an engine over by hand to watch the valve gear operate, you will notice that the valves don’t open and close at the exact top and bottom of their respective strokes. That’s because the engine is a dynamic system, which means it’s something that moves, and it does so at a pretty high speed. Most MX 450’s make peak power at around 9000 RPM,  which means they make two full revolutions and complete an operating cycle in about 13 milliseconds at that speed.  The crank spins continuously, but the intake, exhaust, and combustion all stop and start again while that’s going on.  That means that all of these events actually have only a certain amount of time in which to occur, so they have to be started in advance so that they happen on time.  Again, we’ll look at the intake stroke first. 

With the crankshaft spinning along at a few thousand revolutions per minute, if we were to wait until top dead center to open the intake valve, the piston will travel well down in the bore by the time the valve is open wide enough to let much air into the cylinder, so the intake valve begins to open around 20 degrees or more before top dead center (BTDC).  This does a couple of things.  For one, the exhaust stroke is just ending, and the inertia of the spent gasses leaving the cylinder creates a bit of a vacuum that helps get the intake air moving in.  There’s a little bit of built up pressure right behind the intake valve as a result of the intake valve having been slammed shut on a moving column of air at the end of the previous intake stroke, and that helps, too.  But mainly, we want the intake valve to have time to be open nice and wide as the piston moves through the fastest part of its down stroke so we can get the cylinder as full as possible.  On top of that, we’re going to keep the intake valve open until well after bottom dead center (ABDC) to take advantage of the inertia of the incoming air.

Which brings us to the compression stroke.  The piston is now rising and pushing against the load of incoming air, stalling the flow into the cylinder, so the intake valve closes as this balance is struck, about 130 degrees BTDC.  With both valves now closed, the piston compresses the air and fuel mix to less than 1/10th its original volume to heat it up and to increase the force with which it expands as it burns.  This compression will continue until TDC, but the ignition has to happen well before that in order to extract the maximum power from the burning of fuel.

The Power stroke, then, is initiated before the piston actually starts down.  This “spark advance” allows the burning gasoline time to start at one small point near the spark plug and spread across the combustion chamber to the point where it becomes confined by the piston and must push it down out of the way.  That’s where the power comes from.  If the spark occurs too late (is “retarded”), the piston will outrun the fuel burn and not much pressure will be applied.  On the other hand, if it happens too early (“advanced”) then too much pressure will be created while the piston can’t get out of the way fast enough, which leads to damage from detonation and the like.  The faster the engine turns, the more advance the ignition needs to keep up, so modern systems advance the timing as the RPM increases.

At about 120-130 degrees ATDC, the energy from the fuel burn is so low that it really isn’t putting a lot of force on the piston any more, and the leverage that the piston has on the crank is getting pretty low, so the exhaust valve starts open before reaching BDC.  The pressure that remains from the burn starts the gasses flowing outward, boosted by the piston as it rises and pumps the bore clear.  The exhaust valve remains open past TDC to utilize gas inertia and help restart the intake airflow for the next cycle. 

Am I 180 Out?

People ask this a lot when they have trouble getting an engine running after they’ve set the cam timing up, or when they bring the piston up to Top Dead Center and find both valves open.  This is the common mistake we mentioned earlier, and it's one of the things that's more easily understood when you have a good grasp of the complete cycle.  It’s more of a car thing, but if you have an old classic four-stroke from the ‘70’s or before that uses cam driven breaker points, it’s sometimes possible.  These days, the answer is usually, “no.”  The old way of connecting the ignition to the engine mechanically, that of using a distributor or some other device driven at half speed by the cam, allows a mistake in assembly to be made.  A mechanic could position the engine at TDC, and if not careful to check, he could position the ignition trigger to fire during the exhaust stroke instead of the compression stroke.  This was referred to as being “180 degrees out” because the distributor or point plate was 180 degrees away from the correct position on the camshaft because of this. Actually, going by the crank, the ignition timing was 360 degrees out.

But with the ignition trigger located on the crankshaft instead, as is the case with virtually all modern single cylinder dirt bike 4 strokes, that’s not possible.  Without the cams connected to the crank, one TDC is exactly like another; the rod’s at the top, and the spark signal is given as the crank gets there, every time.  So, the only thing that determines which stroke is which is the camshaft(s), and how they are positioned by the assembler.  That’s why the service manuals for such engines make no mention of checking for which of the two different TDC’s is used.  In operation, there is a second, "wasted" spark that happens near the end of the exhaust stroke. 

What About Automatic Decompression?

This is another area where really understanding the four-stroke cycle helps clear things up.  It's extremely common to hear people tell someone with a modern four-stroke single to "find TDC" before starting, but that's wrong.  When you turn the engine over slowly, you find it rotates fairly easily until you come to a "hard spot".  Without auto decompression, the hard spot is the point at which the intake valve closes to begin the compression stroke.  That looks like the picture below, "Non AD".             

From this point, you would need to force the engine to compress about 80% of it's full stroke length worth of air, and that can be nearly impossible with the high compression ratios used these days.  What automatic decompression does is use a speed sensitive mechanical system to lift the exhaust valve off its seat at very low speeds (slower than the engine will idle at) until the engine gets a lot closer to TDC, but not past it, so that when kicked over from this position (or spun through it by a starter motor) there will still be enough compression to start, and both valves will be closed as the spark happens and the end passes top dead center.  That looks like the "Auto Decomp" picture above.  You can see that there will be a lot less effort needed to compress the air/fuel charge from here than from the normal, non auto decompression setup.  This, by the way, is where you want to be if you have an older manual decompression engine.  If you go past TDC instead of stopping just prior to it, you would have to kick the engine through nearly two full revolutions to get back to the compression stroke again, and it would still be at full strength.

Once you have the whole picture set in your mind, you'll make fewer assembly mistakes, and you'll be able to catch on to problems more quickly.  A crusty old  mechanic told me a long time ago, "The best way to figure out why something works wrong is to know how it's works when it works right".










Edited by grayracer513

6 people like this

User Feedback

Create an account or sign in to comment

You need to be a member in order to leave a comment

Reply with:

  • Similar Content

    • By Riley Zeiler
      sup guys,

      i got a bit of a story. a few days ago i bought a 2004 crf250r for 800 bucks off a guy who didnt know much except a few things...
      1. has an oil leak (shift linkage seal was half out  )
      2. had a 'full rebuild' by the previous owner
      3. had a hard time starting

      so i snatched it up, hoping it would be a quick fix.

      its not exactly going that way.

      i brought it home, cleaned out the carb, and fired up after several kicks. the idle was pretty high, but wouldn't come down without completely dying on me. so as the bike is running and im trying to get the idle down, i noticed two things;
      1. the thing backfires practically every 2-3 seconds
      2. the exhaust pipe was so red-hot, it looked like it was about to turn into a damn puddle of molten metal.

      realizing there was something seriously wrong with the bike, i shut it off. this brings us to my first question.

      why in the frick is my pipe getting red hot and shooting flames after 60 seconds or running ? and whats with the back firing ?

      now for part 2 of story time. 

      with a bit of research my dad's opinion, i assuming that there was some sort of problem with the exhaust valves. long story short, the exhaust valves were perfectly fine. my next thought was to check the timing. take a look at the pictures in the link below and let them speak for themselves. https://www.pinkbike.com/u/Ziggypop14/album/nothing/
      as you can now see, the notch on the magneto cover does not completely match up with the notches on the flywheel. i noticed there are two notches on the flywheel, where should the notch on the cover versus the two on the flywheel? im assuming in between the two? 

      i researched and found a few things that could be at fault.

      1. the cam cog slipped on the shaft. which really cant be it since at TDC the two notches on the cam cog are still completely level, and if the cog slipped, the notches would be off. Right?

      2. the flywheel slipped on the crankshaft. this one makes the most sense because how else can the flywheel notches be off while the other notches are dead on. but how the hell is that even possible? the flywheel nut is so tight that i cant even get it off to see if the keyway is still intact. (still trying to get it off, jamming sockets in gears) unless the idiot who rebuilt the bike didnt put the keyway in (which is entirely possible), i cannot see this as the solution. this brings us to my 2nd and final question.

      Why aren't the flywheel notches lined up even with the piston at TDC and everything else lining up ?
      any help is is appreciated, i need my garage back!!

    • By idratherberiding
      Can anyone confirm that this is true regarding cam timing:

    • By tater1994
      I know this has probably been over a hundred times. But im at wits end. Does anyone know the STOCK timing for a 2000 wr400f? Right now i got it at intake came is at 9 12 and 3. And i got the exhaust cam retarded (clockwise) one link for the yz time? Is this right for a 2000? Ive heard that the 2000 wr400 the stock time was one of the cams retatded one link? Idk..it wont crank for nothin..it ran the other night for 20 min perfect. And hasnt since. Bowl keeps overflowing. Ive adjusted the float and tried to blow through the inlet with the float shut and it was sealed...its getting spark. Seems to have great compression. ..idk WTF else it could be! !???  Im at my wits end with this bike...

    • By ADAVEN6
      I just replaced the rings in my 07 yz450f and when i put it back together it did not kick start so i pull started it and it ran (kinda) but at high rpms it would bog. i know its not the carb because i had it in another bike and it ran fine. but then i could not start it a second time. it would start for 1 or 2 seconds but not run. so i took it apart and found the timing was off (more then i thought) but one of the cam timing marks was always to high or to low. so i got it as close as i could but now it has a ton of compression, is that good? please help 
    • By Jzonts
      I've tried adjusting my accelerator pump so the squirt just barely hits the slide. The best I've been able to accomplish is the squirt starting as the slide is half way up, this is with the adjustment screw back out all the way. The original setting was 4.25 turn out and at that setting the squirt started once the slide was almost all the way up. I recorded the different setting with my phone and went frame by frame to check. I can't seem to figure out how to post a video with slow-mo to show what I am seeing.
      From what I've read I should be able to back the adjustment screw out until the squirt actually hits the slide but I can't do that with my bike. I do get a fairly bad lean bog, although I haven't tested it since getting the squirt to be somewhat faster.
      My question is what would cause the squirt to be so delayed compared to what I've read in other bikes? Anywhere I can start trouble shooting? Thanks!
      (I have the stock 55 leak jet)