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/cgi-bin/YaBB.pl General Category >> Rubber Side Down! >> setting top dead center /cgi-bin/YaBB.pl?num=1200196132 Message started by rocky on 01/12/08 at 19:48:52 |
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Title: setting top dead center Post by rocky on 01/12/08 at 19:48:52 I set the tdc according to the clymer and the tech page. the decomp solenoid was at the correct distance. That is one part of my question as the solenoid was only measureable in one position of tdc and at 180 off from that position it would not extend. I am assuming that was correct. The next problem is no matter which way tdc the rocker arms have no play. I am assuming that the valves are to tight. I need help. Any suggestions? I previously posted this in the wrong section. |
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Title: Re: setting top dead center Post by justin_o_guy2 on 01/12/08 at 22:37:03 In order to be certain you are at TDC on compression, stik a finger in the plug hole & feel the compression, then get a light & get eyeballs level with the end O the crankshaft & line up the marks. Now, adjust the valves, cuz if you got zero clearance it's gonna get hurt. |
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Title: Re: setting top dead center Post by Savage_Greg on 01/13/08 at 06:42:08 To do this you have to wrap your mind around the 4 stroke engine cycle...it is Intake, Compression, Power, Exhaust, but since we are looking for the compression stroke, I'll explain it from TDC at that position at 3:00. There are 2 top dead center positions but the one that you want is on the compression stroke. It might be hard to visualize crank rotation, a piston going up and down, a horizontal paint mark, camshaft rotation, and a clock running backwards...but what the heck, I've got coffee. Remove the spark plug. Remove the timing plug on the left alternator cover. Get a 17 mm socket and a breaker bar, if you have one. It gives better control. Now turn the engine counter-clockwise only to position the paint mark on the rotor bolt with the groove on the engine case at the 3:00 position. To find the compression stroke, watch the valves operate as you turn the engine. When you approach TDC in the compression stroke (going from 9:00 to 3:00) both sets of valves will be closed...in fact, the valves remain closed through a full 360 degrees of crank rotation, but what you can't see is that the camshaft is turning during this time too....and we want the lowest position on both cam lobes. If you pass the 3:00 position do not reverse the rotation, but go around 2 more times while watching the valves again. But watch the valves... ...From TDC of the compression stroke at 3:00, both sets will be closed as the piston goes down to... ...BDC in the power stroke at 9:00, then the exhaust valves open as the piston goes up to... ...TDC in the exhaust stroke at 3:00, then the intake valves open as the piston goes down to... ...BDC in the intake stroke at 9:00, and now both sets are closed as the piston goes up to... ...TDC in the compression stroke at 3:00. Stop. Both sets are now closed, the piston is at TDC, and whether you feel rocker movement or not, you are ready for the real fun :P to be cont: |
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Title: Re: setting top dead center Post by Savage_Greg on 01/13/08 at 09:00:42 I ran outta space in the other post... You have to remember that the valves do most of their movement in the way of open/close during one of the engine cycle strokes. Also, both the intake and exhaust valves operate during one complete 360 degree revolution of the engine or 2 of the strokes, back to back. During the other revolution they are essentially closed but the camshaft is in different positions.... ...so, you don't want to adjust the valves in the TDC position that is in the narrow time between the Exhaust and Intake strokes... If you look at the actual start of the cycle, it begins with the intake stroke. Keep in mind, that I mention the clock position because that is your reference point with the wrench... ...the Intake Stroke starts at TDC at 3:00, the piston moves downward, the intake valves open, a huge gulp of fuel and air rushes in, the valves close before BDC at 9:00 and... ...the Compression Stroke starts at BDC at 9:00, the piston is hurled up, squeezing the fuel and air to 1/8th it's original volume, the spark plug fires just before TDC at 3:00, there is a tremendous earth shaking explosion and... ...the Power Stroke starts at at TDC at 3:00, the piston screams downward under tremendous heat and pressure to BDC at 9:00, a simply incredible amount of torque is unleashed and... ...and Exhaust Stroke starts at BDC at 9:00, the piston slingshots upwards, the exhaust valves open, all the burned gas, oil, dead bugs, and smoke is forced out, a loud THUMP resonates everywhere, the valves close before TDC at 3:00 and... ...if everything is still in one piece, the cycle repeats again :P |
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Title: Re: setting top dead center Post by thumperclone on 01/13/08 at 09:50:50 gregs' methiod needs to go into yhe yech section..even i undersood it!! |
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Title: Re: setting top dead center Post by justin_o_guy2 on 01/13/08 at 11:45:14 If you pass the 3:00 position do not reverse the rotation, but go around 2 more times while watching the valves again. But watch the valves... I've heard this, but really don't unnerstand why not. Seems that as long as the cam is not trying to move a rocker the lash can be set. Would backing the thing up just a few degrees be that big of a deal? Is there enough slack in the cam chain to keep the cam fro0m moving even as the crank turns? I don't see it. So, educate me if ya will. |
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Title: Re: setting top dead center Post by Savage_Greg on 01/13/08 at 12:29:53 607F797E63645565556D7F73380A0 wrote:
I mentioned not reversing the crankshaft above, but anyway... First of all, you don't reverse it because you might be putting (theoretically) slack in the cam chain which means the cam might not be in the correct position relative to the crankshaft. Remember that you are turning that fun little do-da on the other side too. As well, you are turning the cam lobes in the opposite direction against the rocker arms too. Second, you are seeking the smallest diameter portion on the cam lobe, and that position is directly opposite the full lobe height of the cam. There is a gradual increase in cam profile from one point to the other, and since we don't know exactly how much, go for the lowest point which is at the 3:00 position. Third, do it like the manual says to do it. They have a lot more engineers than we do :P |
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Title: Re: setting top dead center Post by YonuhAdisi on 01/13/08 at 13:42:32 6C594E4C741D1E1B2B0 wrote:
While I was inspecting my engine the other day I also noticed that the starter gears make it difficult to rotate engine backwards. I noticed this because I have both engine side covers off and one of the gears the starter engages fell off and I was suddenly able to spin the engine both directions with no difficulty, but once I put that particular gear back on, I could only rotate the engine in the proper direction easily. So I would surmise that turning the engine backwards can damage the starter. (Just my thought on the matter, I could be completely wrong) |
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Title: Re: setting top dead center Post by justin_o_guy2 on 01/13/08 at 13:51:07 Good answers, thanks guys. |
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Title: Re: setting top dead center Post by viper on 01/13/08 at 14:12:03 Good advice don"t ever turn any Overhead valve engine in reverse direction be it a push rod or cam over valve assy. The slack in the chain and the cam profile (overlap) may allow a valve to stay open If the piston head contacts the valve edge valve head may become bent. This becomes a very expensive misfire. Or no start condition. Ride on Ride safely Viper |
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Title: Re: setting top dead center Post by rocky on 01/13/08 at 14:31:59 Thanks Greg, your explanation enabled me to get it right. This was my first try. Thanks again to all |
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Title: Re: setting top dead center Post by T Mack 1 on 01/13/08 at 14:59:41 1F2928332E07222F352F460 wrote:
This is the starter clutch (behind the flywheel) which free wheels in one direction. I don't think it will really hurt the starter, but boy oh boy, it sure make it hard to turn.... |
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Title: Re: setting top dead center Post by Savage_Greg on 01/14/08 at 05:15:58 504F494E53546555655D4F43083A0 wrote:
Justin, I don't think that being slightly off the mark on the timing marks will make much difference as far as the valve clearance is concerned. The camshaft diameter is probably pretty much the same whether you are +/- a few degrees on the crankshaft...considering that the crankshaft rotates at a ratio of 2:1 with the camshaft. So, 1.0 degree on the crankshaft would only be 0.5 degrees rotation on the camshaft anyway. It's just my habit to only quote the exact specs and never use the words like "close" when discussing an engine setting. After all, what I call "close" might mean "in the ball park" to someone else. Know what I mean? Besides, just how accurate is the paint mark? Could there be a small amount of error in it? Which direction is the error? BTW - If I ever find a good set of v-blocks, I might have to check the camshaft profile with an indicator. Just to see how much is "close". From my CD, here is the paint mark... http://i21.photobucket.com/albums/b277/gmdinusa/Alternator_Cover_4.jpg Now if you really want to get accurate, you could always remove the alternator cover :P http://i21.photobucket.com/albums/b277/gmdinusa/Crankshaft_Timing.jpg |
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