Post by Dan MacMillan on Nov 30, 2012 0:55:23 GMT -5
Does your engine have the correct pulley on it? Are the marks correct?
To set timing with a stroboscopic timing light you will need the correct timing mark on the pulley. A degree wheel is great but not necessary. However to adj valves you just have to be close as to crankshaft position. The cyl being adjusted must be on compression but there is a rather large amount of crank movement when both valves are closed.
In the above image you will notice that the base circle for the intake and exh lobes covers a large portion of the cam. When the lifters are riding on the base circle the valves are closed. Looking at the above diagram there is approx 90 deg of cam rotation where the 2 valves are closed at the same time. Given that the crank turns 2 revolutions for each revolution of the cam we end up with roughly 180 deg of crank rotation where the valves are both closed. This gives a lot of leeway before and after tdc compression where both valves are in the closed position, roughly 90 deg give or take{varies due to cam specs}.
So while it is very important to know cam position for spark initiation {1 or 2 deg will greatly affect performance}, it is very forgiving when adjusting valves.
The proceedure we use when we have to know exact tdc compression on diesel engines where there is no access to the piston can be used on any gas engine. Start by bringing the piston into the compression stroke by turning the crank clockwise while watching the rockers. When the rockers for cyl 1 are not rocking but both are rocking for the companion cyl {cyl 3 in the case of VW} are both rocking we know that cyl 1 is on compression. turn the crank about 90 deg counter clockwise. Adjust one of the valves on cyl 1 really tight thus holding it open. Slowly rotate the engine clockwise until the piston just touches the valve. At this point mark the pulley in line with the case seam. Next rotate the engine counter clockwise until the piston again just touches the valve. Make a second mark on the pulley. Loosen off the valve. Find the center between the two marks you made, this is exactly tdc. You can now erase the first two marks you made. This can be done to verify that your degree wheel is accurate.
Finding companion cylinders is very easy on any engine. Take the firing order {1,4,3,2 in the case of VW} and divide it in half. 1,4 and 3,2. place the second half directly below the first half.
1,4
3,2
Reading the numbers vertically will pair up the companion cylinders. so for VW it is 1 and 3, 2 and 4
Corvair would go like this
1,4,5,2,3,6
1,4,5
2,3,6
therefore 1 and 2, 4 and 3, 5 and 6 are companion cylinders.
{the usual wooden dowel used to feel when the piston is going up then down is relatively close but not exact as there are a few degrees of crank rotation when the piston does not move.}
From this point you can easily find where your ignition timing mark will be by finding the circumference of the pulley.
Circumferance = Pi x diameter
For example if the diameter is 10 inches (the diameter of the pulley you are using will have to be substituted in these equations as the number 10 is an arbitrary number} then C=3.1415x10
C=31.45 inches.
As there are 360 deg in a circle then 31.45" divided by 360 deg=.087" per degree. If your ignition timing for the distributor you are using calls for 7 deg before tdc then 7 x .087= .61 inches. Measure .61 inches to the right of your tdc mark. and make another mark. This is your timing mark at idle.While you are at it do the math for 29 degrees. Measure around the pulley that amount to the right of your tdc mark and make another mark. This is your timing mark at full advance 2000rpm with dist vac hoses disconnected and plugged. For 009 dist with no vac hoses it is the same value just no hoses to disconnect.
To set timing with a stroboscopic timing light you will need the correct timing mark on the pulley. A degree wheel is great but not necessary. However to adj valves you just have to be close as to crankshaft position. The cyl being adjusted must be on compression but there is a rather large amount of crank movement when both valves are closed.
In the above image you will notice that the base circle for the intake and exh lobes covers a large portion of the cam. When the lifters are riding on the base circle the valves are closed. Looking at the above diagram there is approx 90 deg of cam rotation where the 2 valves are closed at the same time. Given that the crank turns 2 revolutions for each revolution of the cam we end up with roughly 180 deg of crank rotation where the valves are both closed. This gives a lot of leeway before and after tdc compression where both valves are in the closed position, roughly 90 deg give or take{varies due to cam specs}.
So while it is very important to know cam position for spark initiation {1 or 2 deg will greatly affect performance}, it is very forgiving when adjusting valves.
The proceedure we use when we have to know exact tdc compression on diesel engines where there is no access to the piston can be used on any gas engine. Start by bringing the piston into the compression stroke by turning the crank clockwise while watching the rockers. When the rockers for cyl 1 are not rocking but both are rocking for the companion cyl {cyl 3 in the case of VW} are both rocking we know that cyl 1 is on compression. turn the crank about 90 deg counter clockwise. Adjust one of the valves on cyl 1 really tight thus holding it open. Slowly rotate the engine clockwise until the piston just touches the valve. At this point mark the pulley in line with the case seam. Next rotate the engine counter clockwise until the piston again just touches the valve. Make a second mark on the pulley. Loosen off the valve. Find the center between the two marks you made, this is exactly tdc. You can now erase the first two marks you made. This can be done to verify that your degree wheel is accurate.
Finding companion cylinders is very easy on any engine. Take the firing order {1,4,3,2 in the case of VW} and divide it in half. 1,4 and 3,2. place the second half directly below the first half.
1,4
3,2
Reading the numbers vertically will pair up the companion cylinders. so for VW it is 1 and 3, 2 and 4
Corvair would go like this
1,4,5,2,3,6
1,4,5
2,3,6
therefore 1 and 2, 4 and 3, 5 and 6 are companion cylinders.
{the usual wooden dowel used to feel when the piston is going up then down is relatively close but not exact as there are a few degrees of crank rotation when the piston does not move.}
From this point you can easily find where your ignition timing mark will be by finding the circumference of the pulley.
Circumferance = Pi x diameter
For example if the diameter is 10 inches (the diameter of the pulley you are using will have to be substituted in these equations as the number 10 is an arbitrary number} then C=3.1415x10
C=31.45 inches.
As there are 360 deg in a circle then 31.45" divided by 360 deg=.087" per degree. If your ignition timing for the distributor you are using calls for 7 deg before tdc then 7 x .087= .61 inches. Measure .61 inches to the right of your tdc mark. and make another mark. This is your timing mark at idle.While you are at it do the math for 29 degrees. Measure around the pulley that amount to the right of your tdc mark and make another mark. This is your timing mark at full advance 2000rpm with dist vac hoses disconnected and plugged. For 009 dist with no vac hoses it is the same value just no hoses to disconnect.