Rotor position at TDC

F

fried okra

Senior Member
I swapped out the distributor on my '31 Tudor 12v engine.

Next I was gonna set the timing using the Nu-Rex tool. I am visibly at TDC on #1 piston and get a slight motion on the timing pin against the timing gear but it does not readily go into the timing gear hole. I assume the timing pin is just too tight a fit.

Anyway, I was expecting to see the rotor at the #1 cylinder pin in the distributor cap. Instead the rotor is 180 off from where I expected it to be.

Is this rotor position normal or am I missing something?
 
You dropped the distributor in 180 degrees out of sync. Lift it up and spin the rotor to #1 cylinder and drop it back in.
 
Thanks....I've cranked the engine trying to start it since dropping the distributor in.

Does that make a difference possibly?
 
The model A distributor is rather unique in that the breaker cam can be positioned in any location. The cam gears don't always have a deep dimple to indicate position. Some are so shallow that they can be hard to detect. Roll it up on the #1 cylinder compression stroke and start feeling for it before it reaches TDC. Make certain it's on compression and not exhaust. Once it is positioned, the breaker cam can be positioned to fire #1. Be sure the spark control lever is properly positioned before setting the breaker opening point. The distributor rotor is set with the breaker cam so it should align with the #1 terminal.
 
rotorwrench....I saw the piston coming up flush with the top of the block, so I assume that is on compression stroke?
 
Okra - when the piston is rising before TDC, on compression stroke both valves are shut tight. After you pass TDC both valves remain shut tight. This is the TDC you want to set the timing. I’ve found that the NuRex tool does a fine job when used as directed.

When on exhaust stroke, when the piston is rising before TDC the exhaust valve is open. After TDC the intake valve opens.

You can watch the valves through the spark plug hole.

Silly point, but you are rotating the engine clockwise (when viewed from the front)?
 
Thanks JayJay....will need to check if can see the valves at TDC....just been looking at the piston.

Should the distributor rotor be at #1 plug contact in distributor on both exhaust and compression stroke when piston at TDC?

Yep, been been rotating engine clockwise when viewed from the front....thx.
 
fried okra said:
Thanks JayJay....will need to check if can see the valves at TDC....just been looking at the piston.

Should the distributor rotor be at #1 plug contact in distributor on both exhaust and compression stroke when piston at TDC?

Yep, been been rotating engine clockwise when viewed from the front....thx.
Click to expand...

Distributor is driven by the camshaft. One revolution of the camshaft (and one revolution of the distributor) equals two revolutions of the crankshaft. So no, TDC of #1 cylinder on compression the distributor rotor points to #1, and at TDC of #1 cylinder on exhaust (which is TDC on compression of #4 cylinder) the rotor points to #4.
 
fried okra said:
Thanks....I've cranked the engine trying to start it since dropping the distributor in.

Does that make a difference possibly?
Click to expand...
You can lift it anywhere and turn it 180 degrees so it drops back into the drive slot. When the pin is in the dimple it's always at TDC.
 
fried okra said:
Should the distributor rotor be at #1 plug contact in distributor on both exhaust and compression stroke when piston at TDC?
Click to expand...
No it only points at #1 when it fires on the compression stroke.
It turns 1/2 crank speed so it fires the plug every other up stroke of the piston.
 
fried okra,
I have found this to be the most helpful information source when it concerns ignition timing, tune-ups, etc.

http://www.modelabasics.com/Ignition.htm
 
ModelA29 said:
You dropped the distributor in 180 degrees out of sync. Lift it up and spin the rotor to #1 cylinder and drop it back in.
Click to expand...

Nope. The slot on the bottom of the distributor is offset. The distributor will only drop down into the slot in one position. If he is 180° off he will have to loosen the rotor and turn just the rotor 180° to line up with TDC on #1 compression stroke. No need to lift the distributor.
 
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y-blockhead said:
nope. The slot on the bottom of the distributor is offset. The distributor will only drop down into the slot in one position. If he is 180° off he will have to loosen the rotor and turn just the rotor 180° to line up with tdc on #1 compression stroke. No need to lift the distributor.
Click to expand...

bingo
 
Trying to start the engine with the timing out 180 degrees can be a lot of fun. It will produce a loud bang out the exhaust pipe or maybe flow the muffler apart. Put a juice can on the exhaust pipe to see how far it can be launched. Advance the spark so that you are sure to fire the plugs when the exhaust valve is open. Remove all but one spark plug leads so that the exhaust system is full of air/fuel when the plug fires.

I used to do this on an old tractor with no muffler and an exhaust pipe that was pointed straight up. A can protected the exhaust from rain. The can would shoot up 100 feet or so.
 
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This picture shows where the rotor belongs for timing the ignition.
 

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