Tming - TDC / rotor before or after?

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Tming - TDC / rotor before or after?

Postby Connecticut Calvin » Mon Jun 09, 2014 3:39 pm

Hi everyone - just wondering if the ignition is supposed to fire exactly when the cylinder is at TDC, a little before or a little after. I'm static timing my '54 a bit. As I swivel the distributor counterclockwise (when looking at it from the operator's seat) I know my farthest range as the engine begins to choke out. Seems there's a sweet spot at idle, which is about mid-range from swiveling the farthest counterclockwise to clockwise, but that sweet spot seems like it's low on power. So I'm guessing the ignition is supposed to fire a bit after TDC when the valves are about ready to open...but where's that sweet spot for maximum power? I know I know...get a timing light. And I will...this is really so I understand the mechanics of the engine. THANK YOU! :D
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Re: Tming - TDC / rotor before or after?

Postby Eugene » Mon Jun 09, 2014 4:40 pm

Connecticut Calvin wrote:I'm guessing the ignition is supposed to fire a bit after TDC when the valves are about ready to open.
Actually the ignition system is designed to fire the piston when it reaches TDC. The mechanical advance fires the coil a bit before the piston reaches TDC so that the spark arrives at the spark plug when the piston reaches TDC.

For most of the compression/combustion cycle both valves are closed. Otherwise the air fuel mixture would not be compressed. And the valves have to be closed during and for a period of time after the combustion - other wise the explosion would not force/drive the piston down - - - - - no power.
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Re: Tming - TDC / rotor before or after?

Postby Connecticut Calvin » Mon Jun 09, 2014 5:48 pm

Ah! That would make sense. So I'm firing a bit late right now. Need to back it off a bit. :thanx:
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Re: Tming - TDC / rotor before or after?

Postby Trent M » Mon Jun 09, 2014 11:47 pm

You are static timing the ignition you say? Are you sure? :wink: Do it properly and leave it there. The spec timing will produce maximum power. I find the best/easiest way is the method Eugene showed me at a CubFest several years ago using a multimeter and checking continuity to determine just when the points break.

I also have a timing light but I haven't used it in a long time. I have static timed many Cubs and haven't found one where the advance worked improperly, checked with timing light and the advance mark on the pulley.
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Re: Tming - TDC / rotor before or after?

Postby Scrivet » Tue Jun 10, 2014 7:48 am

Trent M wrote:......... I find the best/easiest way is the method Eugene showed me at a CubFest several years ago using a multimeter and checking continuity to determine just when the points break..................
I think I understand the method, but just to be clear, where exactly is the multimeter connected? And, if you want to go into a little more detail on "the method" I won't mind :D just to make sure my thinking is spot on.
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Re: Tming - TDC / rotor before or after?

Postby Jim Becker » Tue Jun 10, 2014 9:22 am

Just connect the multimeter (or test light) at either end of the wire from the coil to the distributor. As you crank the engine over (or rotate the distributor) the connection will go "hot" when the points break.
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Re: Tming - TDC / rotor before or after?

Postby Matt Kirsch » Tue Jun 10, 2014 9:40 am

Static timing is when the engine is NOT running, BTW. AFAIK this is how it's done:

Get the engine up at TDC on the compression stroke on the #1 (front) cylinder.
Take the plug out of #1, put it back in the wire, then lay the threads against the engine block.
Loosen the distributor to where you can just rotate it.
Turn on the ignition.
Rotate the distributor slowly until the plug sparks.
Turn off the ignition.
Tighten down the distributor bolts.
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Re: Tming - TDC / rotor before or after?

Postby Trent M » Tue Jun 10, 2014 9:42 am

Sure. Place the #1 piston at TDC on compression stroke (pointer lined up with notch on the pulley, second notch if there are 2). Loosen distributor so you can turn it. Set multimeter to test for continuity, i.e., resistance (ohms or greek omega symbol on the meter). I like mine because there is an audible tone when you have continuity so I don't have to look at it. Put positive lead from multimeter on the outside connection of the distributor that goes to the coil and the negative lead to ground. I just find a place on the frame where I get good ground. Turn the distributor back and forth to find the exact spot where the points break, i.e., continuity is broken, the tone stops. Bingo, you are in time. Although I have done it by myself, I find it helps to have an extra set of hands. You are dealing with the meter, 2 leads, and turning the distributor all at the same time.

Before doing this I generally (always on a Cub I haven't timed before) take the distributor off and check the condition of the points and rotor. Also set the point gap properly.

I never try to hand crank a tractor that I haven't timed myself. I'm sure a lot of injuries have happened by hand cranking tractors that are not properly timed.
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Re: Tming - TDC / rotor before or after?

Postby Connecticut Calvin » Tue Jun 10, 2014 2:37 pm

Trent M wrote:Turn the distributor back and forth to find the exact spot where the points break, i.e., continuity is broken, the tone stops. Bingo, you are in time.


So two observations here.

1) Only problem with this scenario is that there are two locations for that "bingo" moment. Isn't it important to consider which "bingo" is the right one i.e. we turn the distributor all the way counterclockwise, then clockwise, we get continuity OR we turn it all the way clockwise, then counterclockwise we get continuity.

I'd presume that we turn the distributor counterclockwise (while looking from the operator's seat) until the continuity is broken...meaning the rotor is now to the left of the point; as the rotor turns clockwise with the engine running. Now we bring it back clockwise until we make continuity (or the plug sparks if we follow the plug-in-the-wire route). Only you're saying that we want continuity broken at TDC, so we then have to turn it back to counterclockwise until our meter beeps (or the plug-in-the-wire stops firing). But how can we be in time if the plug is not firing?

:lost:

2) The other guys are saying that we want ignition at TDC so the timing shouldn't be when the beep stops but when the beep starts.

I wasn't clear earlier. I was timing with the tractor off and #1 at TDC...but I played with it afterwards while running and messed it up. haha
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Re: Tming - TDC / rotor before or after?

Postby Eugene » Tue Jun 10, 2014 3:17 pm

Connecticut Calvin wrote:2) The other guys are saying that we want ignition at TDC so the timing shouldn't be when the beep stops but when the beep starts.
I'm not sure what the other guys are telling you. The coil fires the spark plugs when the ignition points open. Repeat, ignition fires when the points open.

Static timing. This is one task that takes much less time to demonstrate than to write up the procedure(s).

There are 3 common procedures for static timing the ignition to the engine. The general procedure is the same. Understanding the function of the test light, multimeter, or spark, is the difference.
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Re: Tming - TDC / rotor before or after?

Postby Trent M » Tue Jun 10, 2014 3:24 pm

Rotate the top of the distributor towards you, away from the engine to begin. You should show continuity (hear the tone on my multimeter) because the points are closed. Rotate the top of the distributor back towards the engine and set it just as the points break, i.e., where you no longer show continuity, or hear the tone. Doing it this way simulates the rotation of the distributor rotor drive and is the direction causing the points to open when running.
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Re: Tming - TDC / rotor before or after?

Postby Trent M » Tue Jun 10, 2014 3:31 pm

Eugene wrote:Repeat, ignition fires when the points open.


That is why you set to just when the points open, i.e., the continuity is broken, the beep stops.

Eugene is also correct that it is much easier to demonstrate the method than to do a write-up. I think it took me about 45-60 seconds to grasp the concept when he showed me, and I'm not the sharpest tool in the shed!
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Re: Tming - TDC / rotor before or after?

Postby danovercash » Tue Jun 10, 2014 3:37 pm

Engine fires at TDC at idle, distributor advances timing at fast idle. That's why some crankshaft pulleys have two marks, one for static and one to check advance with light. (I think that's right)
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Re: Tming - TDC / rotor before or after?

Postby Scrivet » Tue Jun 10, 2014 5:52 pm

I mainly asked the question to get just a little more detail for anyone reading this that may not understand. BOY, sometimes you get more than you bargained for :D :lol:
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Re: Tming - TDC / rotor before or after?

Postby Connecticut Calvin » Tue Jun 10, 2014 9:06 pm

Oh boy. My cub runs. I ain't touchin' it! :)
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