Cam and top end question

I am just about to pull my motor apart and put in some new hot cams and a 97mm piston in my 01 WR 426. I have a few reservations left that I want to clear up.

When I tear the motor apart at TDC and then replace the cams, piston and new barrel, how do I know if the bike is at true TDC without the cams in the bike? Do I just have to make 100% certain that the entire time I have the motor apart the crank does not move at all? :)

Do our bikes fire both on power and exhaust stroke? :)

you can turn the crank as you wish. Set it at TDC using the spy hole in the stator cover. The only time the crank must be at TDC is when you install the cams.

I presume by "spy hole" you mean the hole on the left side (driver side) of the bike that you use to align the vertical mark with the notch to find TDC. You can use that hole to find true TDC when the cams are in the bike. But without the cams in the bike how do you find true TDC if the motor has been moved/rotated, it could be 360 degrees off? Right? :)

Yes, it could be 360 degrees out. It will always be the 'true' TDC, just a question if the on the compression or exhaust stroke.

Since the question has never been raised before , I would assume the bike fires on the exhaust stroke too.

Yes, the engine fires at TDC, no mater which stroke. The camshafts are what designates weather it is compression stroke, or exhaust stroke. If the crank is at TDC, that is all you need to worry about. Once you install the cams according to the timing marks on the cam sprockets, then that will make it the compression stroke. Rotate the crank 360 degrees, and it will be on the exhaust stroke. Rotate it another 360, and you will be back at compression. Hopefully that helped.

Jonova, don't sweat being on TDC before you tear it down, it only matters when you assemble. I rebuilt my 01 WR two years ago with a 97mm piston and stock cams. Hotcams is right, TDC doesn't matter untill you install the head. It's not a hard motor to reassemble, just pay attention to what your doing. :)

AH HA, So it doesn't matter which TDC I use....That is what I was looking for. Thanks fellow T.T.ers :)

jonova, I think you need to ponder 4-stroke theory just a little deeper. They do only fire every two revolutions of the crankshaft. The way I read Hamish's and HotCams responses to your question, they may have mislead you a bit. The camshafts are geared at a perfect 2:1 ratio so that the cams turn once for every two revs of the crank. Crank revs are what we refer to when measuring engine speed. So at 4000 R.P.M. the cams are actually turning 2000 R.P.M. And at this speed the spark plug will fire 2000 times each minute. With the cams out fell free to turn the engine as much as you like and with the cams absent the piston has no idea which "stroke" it is on. For ease of assembly, run the piston to the top and it's easy to know where to orient the cams. You could even install them exactly backward and they'd be right, but would now be 180 degrees off and between the exhaust and intake strokes. A 2-stroke fires once per revolution. One revolution is two strokes, the piston sweeps from top to bottom, then from bottom to top. Hope this helps.

Bottom line______________Do our yammy's fire every time the piston comes to TDC????? :):)

Bottow line______________Do our yammy's fire every time the piston comes to the TDC????? :):)

Without actually checking it, I would say YES.

jonova, sorry it took so long to read your question. They do not fire at every TDC. But every other. Imagine the spark plug being tied to the camshafts. One full rotation of the cams, and one explosion being ignited. I'm sure you've heard a 4-stroke referred to as a 4-cycle. Here's a refresher: Starting with the piston at TDC with both valves closed, Stroke #1 (INTAKE) of the "cycle", piston starts down and intake valve opens. The exhaust valve is closed so the piston moving down creates a vacuum on top of the piston as more and more volume is created with the only way for air to fill this vacuum being the intake port. The piston nears BDC and the intake valve closes. Stroke #2 (COMPRESSION): The piston starts up again, but now both (actually 5 in our case, three intakes and two exhausts, for simplicity we'll talk of one intake and one exhaust valve) valves are closed. With all the valves in the head now closed, the piston squeezes the fuel/air charge into a very tight space. Now, at some very precisely timed instant before TDC (depending on engine speed, throttle position, and several other things our computers can somehow keep perfect track of) several thousand volts are sent to the spark plug. This starts an explosion that needs to be thought of as taking some time to complete. As with so many things in life, timing truly is almost everything. Too much spark advance and you're actually putting pressure on the piston before it reaches the top of its stroke, slowing the crankshaft down. Plus pre-ignition problems, incomplete combustion, etc. The free mod of cutting/disconnecting the gray wire changes the ignition timing advance map used by the computer, there are two in our WR's, the WR or YZ map. Fire the plug too late and you're not delivering the strongest blow possible to the top of the piston for the fuel/air charge in use. Stroke #3 (POWER): Now, to get all the boost to the crankshaft you possibly can from the pressure available on top of the piston, you don't want to vent, or alleviate the pressure too soon. So as soon as what the cam grinder considered to be enough time to harvest all he could from the explosion has elapsed, he opens the exhaust cam to get the hot, oxygen poor gases out of the cylinder so we can do it all again. Stroke #4 (EXHAUST): With the exhaust valve now wide open, the piston starts up and pushes the bad gases out. The exhaust valve remains open until the piston is all the way to the top so as not to build any pressure in the combustion chamber before the piston reaches TDC. This would blow backwards into the intake as the intake valves opened and we want air and fuel to rush in just as soon as they start to open. I'm sorry if I have told you lots of things you already knew, but for the benefit of others that might be reading wondering about 4-strokes, I've tried to explain it as well as I can. Good luck with your work. It brings back memories of when I first started taking stuff apart.

From a quick skim it looks like Eightyduece has given us an excellent lesson on four stroke engine theory.

The fact is though that WR's trigger from the crank not the cams so YES it does fire on every stroke. The fire on the compression stroke is the only one that does anything, you would never even know that it was firing on the exhaust stroke as well but it does. As long as you can see the flywheel marks in the sight window when you line up the cams your good to go.

EightyDeuce - I apreciate your effort in reviewing a 4-stroke engine. Back to basics is good.

Is the ignition set up on our bikes so that the plug fires at both TDCs????????

I know that a 4-stk only needs to fire once a cycle, 720 degrees of the crank. But on many modern motors these days the plug fires every 360 degrees because the ignition pulls a signal directly off of the crank.

If the plug on our bikes only fires every 720 degrees, when re-assembling my motor how can I determine the correct TDC (TDC when plug fires) to index my cams with if the head has been removed and the angular location of the crank is unknown?

I can find a TDC using the spyholes but how do I know if that TDC is the true TDC just before the power stroke?

If the plug fires every 360 degrees then I can just index my cams to either TDC.

Luke_n - you answered the million dollar question!!!!! Thank you so much!!!!!

luke_n, interesting, I was unaware of this. Thanks for enlightening me. Does a sensor in the generator send a signal to the CDI telling it where everything is?

There is only one TDC according to the crank. At 4000 RPM, the plug will fire 4000 times in one minute. Trust me, we have installed a cam or two. Like some have said, install the cams at TDC, and don't worry. Once the cams are in, rotate it by hand to make sure there is no piston to valve contact. Put it all back together, and go ride.

One way to think about this is what tells the ignition info. There is only the info from the stater/flywheel about where the crank is ( at what angle). There is nothing in the head that tells the ignition what phase the engine is in, compression or exhaust. Therefore the engine MUST fire every rotation, once after compression and another after exhaust. On FI bikes there is sometimes a Cam sensor that will let the computer know that the engine is in compression or not. Just a little FYI

ryder is ryte on!

Create an account or sign in to comment

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

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now