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Timing with Bike at TDC but not compression stroke?

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My bike has not ran since i adjusted the valves on it. I was told to make sure the timing is correct. The manual says you have to set timing with the piston at TDC on the compression stroke. Is it possible i set the timing with the piston at TDC but not on the compression stroke? Is the purpose of the cam lobes facing towards the rear to make sure the piston is on the compression stroke?Thanks.

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In regards to the piston, TDC is when the piston is at the top of the stroke with all of the timing marks on the right side of the case lined up.

When you install the cam, you determine if it's on the compression stroke by how you orient the cam lobes. If you put the piston at TDC and then installed the cam with the lobes facing towards the rear of the bike, you did everything correctly.

If you installed the cam improperly, your timing would be way off and you'd probably have the piston contacting the valves when you kick the bike over.

Why do you think it's not firing? Valve clearances are still off? I assume you've got the plug wire pushed down on the plug correctly? Didn't damange the plug wire at all? Are you getting spark? You didn't happen to do anything to the coil wire or ground wire when you removed the valve cover did you?

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In regards to the piston, TDC is when the piston is at the top of the stroke with all of the timing marks on the right side of the case lined up.

When you install the cam, you determine if it's on the compression stroke by how you orient the cam lobes. If you put the piston at TDC and then installed the cam with the lobes facing towards the rear of the bike, you did everything correctly.

If you installed the cam improperly, your timing would be way off and you'd probably have the piston contacting the valves when you kick the bike over.

Why do you think it's not firing? Valve clearances are still off? I assume you've got the plug wire pushed down on the plug correctly? Didn't damange the plug wire at all? Are you getting spark? You didn't happen to do anything to the coil wire or ground wire when you removed the valve cover did you?

well here is the thing, i last worked on the bike a few monthes ago. I checked to see if i had spark then and it was def good. Well today i pulled the plug and checked to see if i had spark and i had nothing at all. So i put a screw driver into the plug wire and placed another screw driver on top of the first one and there was clearly spark between the two when i kicked the bike. For some reason i could not get spark through the plug though. I have to wait untill tommorow to get a new plug.

I am going to start a new thread about my timing with pics. Maybe you can help me out there. Thanks.

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OK, here is my take and I think it has been confirmed by two qualified mechanics. When you check the valve clearance, they tell you to put it a TDC on the compression stroke (lobes pointing rearward) because that is how the cam must be oriented to make the measurement. That is the only time you have to be concerned with the lobes pointing rearward. Well, when you put it back on it fits better that way.

Now, if you remove the cam from the bike all that matters is that the timing marks on both the top and bottom end line up. You don't need to worry about the piston being on the correct stroke. As far as the piston and crank is concerned there is no right stroke for the compression stroke. That is determined by the cam shaft. Think about this. The cam rotates at half the speed of the crank. So the piston comes to TDC twice for each rotation of the cam. That is why you can line up the TDC mark on the crank and the cam is in the wrong place to measure the valve clearance. But the piston is at TDC and if you notice, the timing marks on the cam gear are lined up. Push the crank through a full rotation again (half a rotation for the cam) and the all line up again but now the lobes are pointing backward. There are two points in the rotation of the cam where it is safe for the piston to be at TDC and if the timing marks on both ends line up, you are safe. If this were not true, a 4 stoke engine would be impossible. That is why the cam turns at half crank speed.

Poppycock you say. If you don't get it on the right stroke, then the spark will not fire on the proper stroke. It will be 180 cam-degrees out of phase. True if you are talking about an automotive engine that has a distributor to control the firing of the spark. (I might be wrong but I am assuming my car only fires the spark every other time the piston comes to TDC) But we do not have a distributor on our dirtbikes. That is all controlled by the flywheel/stator combination which as far as I know turns at crank speed and not at half of that like the cam does. What this means is that the spark plug fires every time the piston come to TDC. Sound nuts? Well, is the ignition system of a 4 stroke different than that of a 2 stroke? Granted, I am making some assumptions here but I think they function the same and a 2 stroke fires every time the piston is at TDC.

So after I worked this out in my head (and yeah that took many hours and beers) I mentioned it to two different mehcs and did not get any disagreement. Also, and this is what lead me to this conclusion, in the service manual you will find nothing in the sections where you totally remove the cam (top end replacement) that states you have to ensure it is on one TDC and not the other. It only says line up the marks. If the marks line up it is correct.

Now then if there is someone who knows more than I do about this (and that ain't gonna be hard to do) and you disagree with me I by all means want you hear what you have to say. I think my assessment is valid but I also know it is based on some inferences and not necessarily pure knowledge. If my assumptions are wrong, so is my conclusion. But my bike is running so that is a strong supporting point. :)

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OK, here is my take and I think it has been confirmed by two qualified mechanics. When you check the valve clearance, they tell you to put it a TDC on the compression stroke (lobes pointing rearward) because that is how the cam must be oriented to make the measurement. That is the only time you have to be concerned with the lobes pointing rearward. Well, when you put it back on it fits better that way.

Now, if you remove the cam from the bike all that matters is that the timing marks on both the top and bottom end line up. You don't need to worry about the piston being on the correct stroke. As far as the piston and crank is concerned there is no right stroke for the compression stroke. That is determined by the cam shaft. Think about this. The cam rotates at half the speed of the crank. So the piston comes to TDC twice for each rotation of the cam. That is why you can line up the TDC mark on the crank and the cam is in the wrong place to measure the valve clearance. But the piston is at TDC and if you notice, the timing marks on the cam gear are lined up. Push the crank through a full rotation again (half a rotation for the cam) and the all line up again but now the lobes are pointing backward. There are two points in the rotation of the cam where it is safe for the piston to be at TDC and if the timing marks on both ends line up, you are safe. If this were not true, a 4 stoke engine would be impossible. That is why the cam turns at half crank speed.

Poppycock you say. If you don't get it on the right stroke, then the spark will not fire on the proper stroke. It will be 180 cam-degrees out of phase. True if you are talking about an automotive engine that has a distributor to control the firing of the spark. (I might be wrong but I am assuming my car only fires the spark every other time the piston comes to TDC) But we do not have a distributor on our dirtbikes. That is all controlled by the flywheel/stator combination which as far as I know turns at crank speed and not at half of that like the cam does. What this means is that the spark plug fires every time the piston come to TDC. Sound nuts? Well, is the ignition system of a 4 stroke different than that of a 2 stroke? Granted, I am making some assumptions here but I think they function the same and a 2 stroke fires every time the piston is at TDC.

So after I worked this out in my head (and yeah that took many hours and beers) I mentioned it to two different mehcs and did not get any disagreement. Also, and this is what lead me to this conclusion, in the service manual you will find nothing in the sections where you totally remove the cam (top end replacement) that states you have to ensure it is on one TDC and not the other. It only says line up the marks. If the marks line up it is correct.

Now then if there is someone who knows more than I do about this (and that ain't gonna be hard to do) and you disagree with me I by all means want you hear what you have to say. I think my assessment is valid but I also know it is based on some inferences and not necessarily pure knowledge. If my assumptions are wrong, so is my conclusion. But my bike is running so that is a strong supporting point. :)

i can see what you are trying to say but it is a little over my head. I can follow but i am not 100 percent on the assumptions you make. You could be right though. Thanks for the help.

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you are correct, the plug fires on both the intake and exhaust strokes...it is impossible to get it 180^ out of time like you can on a car.

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you are correct, the plug fires on both the intake and exhaust strokes...it is impossible to get it 180^ out of time like you can on a car.

Ha, well to be honest, it is almost a let down to be right. I was really expecting to learn something new from the responses here. Well, I guess knowing what I am doing is not such a bad thing is it. Thanks for the input.

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