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Old 09-21-2011, 08:47 PM
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TDC issue, compression or exhaust stroke?

ok, Ill try and make this specific and short for you guys> I appreciate the time taken so Ill make it consise.

I put in a new distributor.
Nothing has changed on my timing at all, except putting in a new dist.

I need to find tru TDC.

With #1 plug removed,
I rotated the crankshaft, and notice there are 2 points of "TDC" but obviously not true TDC. 1 point of assumed TDC, I get ALOT of compression for a little before my dampner marker hits 0 degrees on the timing cover

However, when I continue to rotate the crankshaft bolt clockwise, air sucks in AFTER "tdc".. odd... then a little after that roughly 180 degrees later, I get TINY compression. like just a little air blown out the cylinder.

This doesnt make sense... arent you supposed to get vacuum before TDC not after? but isnt TDC.. true TDC ( compression) supposed to be the most air blowing out..?

and btw the IDIOT at pep boys , this "top level tech" to find tdc just look at the timing mark.. im like.. uh.. stuff can go wrong.. I didnt even explain details.. and he insisted that when the line is lined up its tdc.. im like.. trying to dispute it.. and he asks what year and model my car is.. how irrelevant is that to finding tdc.. this isnt a dirt bike..
then to make it worse, I go, well doesnt that marker go twice on the timing mark.. hes like no, 1 full rotation and youll get TDC.. and walked awway back intot he shop.. probably to go screw someones car up and tell them its their fault because they didnt adjust their metric air.

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Old 09-21-2011, 09:21 PM
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One way to tell is to pull a valve cover. At TDC, finishing exhaust, and starting intake stroke, the valve rockers are rocking.
If you are at TDC, finishing compression, starting power stroke, the valve rockers will not be moving,
Unless the throttle is wide open, it will be easier for air to enter through an open spark plug hole, than past a closed throttle plate..
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Old 09-21-2011, 10:01 PM
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Quote:
Originally Posted by Ocean-Angler
ok, Ill try and make this specific and short for you guys> I appreciate the time taken so Ill make it consise.

I put in a new distributor.
Nothing has changed on my timing at all, except putting in a new dist.

I need to find tru TDC.

With #1 plug removed,
I rotated the crankshaft, and notice there are 2 points of "TDC" but obviously not true TDC. 1 point of assumed TDC, I get ALOT of compression for a little before my dampner marker hits 0 degrees on the timing cover

However, when I continue to rotate the crankshaft bolt clockwise, air sucks in AFTER "tdc".. odd... then a little after that roughly 180 degrees later, I get TINY compression. like just a little air blown out the cylinder.

This doesnt make sense... arent you supposed to get vacuum before TDC not after? but isnt TDC.. true TDC ( compression) supposed to be the most air blowing out..?

and btw the IDIOT at pep boys , this "top level tech" to find tdc just look at the timing mark.. im like.. uh.. stuff can go wrong.. I didnt even explain details.. and he insisted that when the line is lined up its tdc.. im like.. trying to dispute it.. and he asks what year and model my car is.. how irrelevant is that to finding tdc.. this isnt a dirt bike..
then to make it worse, I go, well doesnt that marker go twice on the timing mark.. hes like no, 1 full rotation and youll get TDC.. and walked awway back intot he shop.. probably to go screw someones car up and tell them its their fault because they didnt adjust their metric air.
For the most part you are right,the guy is an idiot. He is right that everytime you line up the marks you get TDC,but only every other time it is on compression stroke.The reason for a slight suction after the pressure is valve timing overlap. Just turn the engine untill you get the more forcefull pressure then line up the timing marks and you have TDC compression stroke within factory tolerance. If you want 100% true TDC you need to use a piston stop.Turn the engine either direction just a little past TDC and install the piston stop.Turn the crank slowly until the piston hits the stop and mark the timming tab where the balancer mark lines up. Then turn the engine the opposite direction very carefully until the piston hits the stop again and mark the tab in the same way.Measure the center of these 2 marks and you have true TDC.Hope this helps.A piston stop can be made by breaking the ceramic out of an old spark plug and taping 3/8" threads in it. Then get a Bolt about3" long with threads all the way or a piece of allthread rod and screw it in with a jam nut to lock it once it is in place.
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Old 09-22-2011, 06:55 AM
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Next time line up the old distributor on #1 first. This will make life a lot easier when you drop in the new one.
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Old 09-22-2011, 09:45 AM
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Quote:
Originally Posted by T-bucket23
Next time line up the old distributor on #1 first. This will make life a lot easier when you drop in the new one.
I would agree 100%.. except, I kinda did that..
My old dizzy - the groove below the gear, where it rests in the oil pump shaft .. was basically inline with the rotor.. they faced the same direction
but the MSD, the rotor points about 45 degrees out to the side from the groove below the cam gear.
So I guess I still woulda had to turn the cap to line it up there..
basically that totally threw me off when I saw that. LOL
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Old 09-22-2011, 10:22 AM
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Quote:
Originally Posted by Ocean-Angler
I would agree 100%.. except, I kinda did that..
My old dizzy - the groove below the gear, where it rests in the oil pump shaft .. was basically inline with the rotor.. they faced the same direction
but the MSD, the rotor points about 45 degrees out to the side from the groove below the cam gear.
So I guess I still woulda had to turn the cap to line it up there..
basically that totally threw me off when I saw that. LOL
No you take a long screwdriver and turn the Oil pump drive to where you need it to be
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Old 09-22-2011, 03:12 PM
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Quote:
Originally Posted by Ocean-Angler
I need to find tru TDC.
Here is the procedure.....
http://www.crankshaftcoalition.com/w...op_dead_center

Quote:
Originally Posted by Ocean-Angler
With #1 plug removed,
I rotated the crankshaft, and notice there are 2 points of "TDC" but obviously not true TDC. 1 point of assumed TDC, I get ALOT of compression for a little before my dampner marker hits 0 degrees on the timing cover

However, when I continue to rotate the crankshaft bolt clockwise, air sucks in AFTER "tdc".. odd... then a little after that roughly 180 degrees later, I get TINY compression. like just a little air blown out the cylinder.

This doesnt make sense... arent you supposed to get vacuum before TDC not after? but isnt TDC.. true TDC ( compression) supposed to be the most air blowing out..?
Nobody is explaining to you that there are 720 crankshaft degrees of revolution in each complete 4-cycle event of a 4-cycle motor. The piston comes to the top of the bore twice in the complete cycle.

If we begin at TDC on the intake stroke, the intake valve is just beginning to open on the intake stroke. The exhaust valve is just closing, because we have just finished the exhaust stroke part of the four cycle event.

At 100-115 crankshaft degrees after TDC, we reach the max lift of the intake valve lobe on the camshaft. This is called the intake centerline. As we continue rotating the crank, the motor is filling up with air or air/fuel mixture depending on the fueling arrangement, carb or EFI. As a vacuum is created by the piston decending in the bore, the atmosphere is pushing air or air/fuel into the cylinder. Nature abhors a vacuum, so it will fill any vacuum it can see and equal it with atmospheric pressure (14.7 lbs per square inch at sea level). Try to get wrapped around this idea of the atmosphere filling the vacuum in the cylinder with pressure rather than the motor "sucking" air in.

At some point between 20 degrees after bottom dead center and 70 degrees after bottom dead center, the intake valve will close. This will be 200 to 250 degrees after TDC, where we started. Of course, the piston has reversed its direction at the bottom of the bore (BDC) and is now headed back up the bore to compress the mixture that was just pushed into the cylinder through the open intake valve by the weight of the surrounding atmosphere. Compression cannot begin however, until we close the intake valve and have a sealed cylinder. We leave the intake valve open past the 180 degree BDC point because the intake slug has inertia and will continue to pack the cylinder even with the piston coming up the bore in opposition to the intake slug. This is the most important point of camshaft design, to leave the intake valve open long enough to get all the mixture into the cylinder that is possible, without pushing it back up the intake tract with the ascending piston. When that happens, it's called reversion (the piston pushing the intake slug back up the intake tract through the "still open a little" intake valve. This confuses the venturi in the carburetor because it is seeing flow going both ways and doesn't know whether to shytte or go blind. The result is rump-rump at idle. As the motor gains rpm's, the motor will "come up on the cam" and get into a speed range that packs the cylinder properly, preventing the mixture from being blown back up the intake tract for that particular grind of camshaft intake lobe and the motor smooths out. This is why a cam must be matched to the static compression ratio of the motor. There is a sweet spot for each combination and it can be engineered to occur early or late in the rpm range.

OK, so we have closed the intake valve at 200-250 ATDC. The piston comes to the top of the bore and the plug fires somewhere before TDC, usually around 340-350 degrees ATDC where we started. The piston comes to the top of the bore and reverses direction as the plug fires, now heading down in the bore and applying pressure to the crankshaft. The exhaust valve will open to evacuate the cylinder of the burnt gases at some point before BDC, maybe 450-480 degrees ATDC from where we began. We want to keep the exhaust valve closed for as long as possible so that we don't lose any of the pressure that's forcing the piston down in the bore, but not so long that we begin reversion back into the cylinder from the exhaust system (that would add work to the piston in trying to evacuate the exhaust from the cylinder, called pumping loss). Max lift of the exhaust valve will occur around 560 to 590 degrees ATDC from where we started. This is called the exhaust centerline of the exhaust lobe on the camshaft. We'll close the exhaust valve somewhere at or after TDC, or 720 degrees ATDC, or two complete crankshaft revolutions from where we started.

I hope this clears up the matter for you.

Last edited by techinspector1; 09-22-2011 at 03:21 PM.
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Old 09-28-2011, 01:36 PM
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Put your finger in the spark plug hole and turn it over.....you will know when it is on compression stroke.
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