TDC and Points

[Rj67cj6]

Hello all,
I'm at that point in my build where I will be turning the key on the engine in a long time. I believe I have positioned the #1 cylinder at TDC, but when I look at the points on my distributor the contacts are closed, or touching. Is this correct? What is the relationship between the points and TDC?

Thank you,

 

[73CJ]

As the points open that triggers the spark.

This should happen 5-15 degrees depending on HP cam or stock cam before the piston reaches TDC. Remember the compression stroke and points are 180 degrees off.

Ballpark.

 

[Lumpy Grits]

Point gap is measured at the highest point of the dizzy's cam lobe.
TDC is reference for set'n the ign timing. Not the point's gap.

LG

 

[Rj67cj6]

Gotcha, so you still have to set one independent of the other. Giving it the ole eye wouldn't necessarily tell anything.

 

[Posi]

My TDC tool, easy way to find TDC on the exhaust stroke. You can use a cork or your finger but it isn't as much fun.

 

[Lumpy Grits]

On that f-head, I would use a wood dowel and just measure it's highest point for TDC.
You set the point gap BEFORE, you set timing.
Make sure you have a new condenser installed also.
LG

 

[mstar]

As the points open that triggers the spark.

. Remember the compression stroke and points are 180 degrees off.

Ballpark.

Could you explain this in a little more detail.I am totally confused by this.

I have been doing tune-up work for 50+years and recieved training at GM training center . Just not sure what you mean. mike

 

[73CJ]

Point gap is measured at the highest point of the dizzy's cam lobe.
TDC is reference for set'n the ign timing. Not the point's gap.

LG

Point gap will get you close but I prefer the dwell meter and I know you are old enough to remember dwell meters.

I was not very clear "This should happen 5-15 degrees depending on HP cam or stock cam before the piston reaches TDC." I was referring to the engine cam not the distributor cam. HP=high performance Ever set the timing to stock with a HP cam? That's why I ball parked the 15deg timing. Stock is close to 5 deg stock vacuum line disconnected and plug engine side.

If TDC is referring to point gap then we better shut the hood and find a shop.

 

[73CJ]

Could you explain this in a little more detail.I am totally confused by this.

I have been doing tune-up work for 50+years and recieved training at GM training center . Just not sure what you mean. mike

OK a coil is a high ratio step up transformer 12volts to 20,000volts +. Transformers work as the magnetic fields collapse and develop. AC in AC out under normal transformer function. I.e. Radios and other electronic devices.

But wait the Jeep is 12v DC or a constant current flow in one direction. Turn the power on. That will give a pulse of current - once until you turn the power off and the magnetic field collapses. One more pulse or current flow. But the current must flow long enough to "saturate" the coils magnetic field to be strong enough. That's where dwell comes from.

So the points are turning the power to the coil on and off at the proper time causing pulses of current flow - spark!

Now 2 things are important:

1. How long the current flows - adjusted by the gap or dwell
2. When the spark occurs - adjusted by rotating the distributor

Note: set the dwell first and then the timing

Others can explain it better but this will give you a general idea. I am not an electrical design engineer but a mechanical engineer working as an electrical certified quality engineer (ASQ) in a Tractor electronics manufacturing plant. Been setting points since 1964.

Best advice is get a HEI distributor and run one power wire. My stock system would drown out in deep water even after lots of silicon spray and silicon sealer. The stock vacuum advance had a slot that could be sealed without interfering with the arm letting water in.

There many 1975+ 258s in junk yard that can donate the stock electronic ignition system too. You only need the distributor and module The coil is the same stock one.

 

[73CJ]

My TDC tool, easy way to find TDC on the exhaust stroke. You can use a cork or your finger but it isn't as much fun.

Even mashed your finger tip? Once in a lifetime is enough for me.

I use a timing light. That will tell you exactly what's going on. It will take into account worn distributor shaft bushings, worn distributor drive gears or a worn timing chain.

 

[Rj67cj6]

Thanks all for the information. 73CJ, you mentioned 1975+ stock electric distributor, is that as simple as pull the old one, replace with new, and your done? Any extra wires or pieces needed to complete this change?

 

[73CJ]

Pretty simple

Replace the distributor and wire the ignition box up with 2 wires running to the distributor, 12v and ground. NAPA had new distributors for $60 not long ago then you need the ign box too.

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The HEI can be had for a similar total price. But you only run 1power wire and drop it in and time it. HEI really helps on starting and the coil is in the cap. Uses GM parts so,they are inexpensive and available. The unit is totally sealed and I have never drown the CJ with a wet ignition.

JEGS Performance Products 40020 JEGS AMC/Jeep HEI Distributor - Free Shipping on All Orders @ JEGS

 

[mstar]

OK a coil is a high ratio step up transformer 12volts to 20,000volts +. Transformers work as the magnetic fields collapse and develop. AC in AC out under normal transformer function. I.e. Radios and other electronic devices.

But wait the Jeep is 12v DC or a constant current flow in one direction. Turn the power on. That will give a pulse of current - once until you turn the power off and the magnetic field collapses. One more pulse or current flow. But the current must flow long enough to "saturate" the coils magnetic field to be strong enough. That's where dwell comes from.

So the points are turning the power to the coil on and off at the proper time causing pulses of current flow - spark!

Now 2 things are important:

1. How long the current flows - adjusted by the gap or dwell
2. When the spark occurs - adjusted by rotating the distributor

Note: set the dwell first and then the timing

Others can explain it better but this will give you a general idea. I am not an electrical design engineer but a mechanical engineer working as an electrical certified quality engineer (ASQ) in a Tractor electronics manufacturing plant. Been setting points since 1964.

Best advice is get a HEI distributor and run one power wire. My stock system would drown out in deep water even after lots of silicon spray and silicon sealer. The stock vacuum advance had a slot that could be sealed without interfering with the arm letting water in.

There many 1975+ 258s in junk yard that can donate the stock electronic ignition system too. You only need the distributor and module The coil is the same stock one.

Well that is everything that I learned from the GM electrical engineer in 1965 but thanks for the class for the younger readers.The thing that is bothering me is this "on compression stroke the points are 180* off" or something like that.As I recall the points are mounted to a breaker plate and move slightly with vacuum advance.Maybe you are reffering to the fact that the cam turns 180* to the crank turning360* either way it really has no bearing on setting point gap and timming. mike

 

[mstar]

As A engineer I am surprised that you still are using that old technology.GM came out with the HEI in 1975 ,39 years ago.
There are a lot better options out there after all it's only a trigger with a coil on top.I suppose the fact that there cheap is a driving factor for
some. mike

 

[Lumpy Grits]

OK a coil is a high ratio step up transformer 12volts to 20,000volts +. Transformers work as the magnetic fields collapse and develop. AC in AC out under normal transformer function. I.e. Radios and other electronic devices.

But wait the Jeep is 12v DC or a constant current flow in one direction. Turn the power on. That will give a pulse of current - once until you turn the power off and the magnetic field collapses. One more pulse or current flow. But the current must flow long enough to "saturate" the coils magnetic field to be strong enough. That's where dwell comes from.

So the points are turning the power to the coil on and off at the proper time causing pulses of current flow - spark!

Now 2 things are important:

1. How long the current flows - adjusted by the gap or dwell
2. When the spark occurs - adjusted by rotating the distributor

Note: set the dwell first and then the timing

Others can explain it better but this will give you a general idea. I am not an electrical design engineer but a mechanical engineer working as an electrical certified quality engineer (ASQ) in a Tractor electronics manufacturing plant. Been setting points since 1964.

Best advice is get a HEI distributor and run one power wire. My stock system would drown out in deep water even after lots of silicon spray and silicon sealer. The stock vacuum advance had a slot that could be sealed without interfering with the arm letting water in.

There many 1975+ 258s in junk yard that can donate the stock electronic ignition system too. You only need the distributor and module The coil is the same stock one.

'Our' coils are DC powered. As per Tesla's design
Not sure if you can even buy a dwell meter any more. Mine is 30+ years old and was made by Sun.
LG

 

[73CJ]

Maybe you are reffering to the fact that the cam turns 180* to the crank turning360* either way it really has no bearing on setting point gap and timming. mike

Yes the rotor must point at the proper cap tower at the top dead center of the compression stroke.

 

[Rj67cj6]

Alright, now that you guys have given me a headache thinking about all of that.
Check the issue I'm having now on my new thread.

Thanks for the help/advice.

 

[73CJ]

As A engineer I am surprised that you still are using that old technology.GM came out with the HEI in 1975 ,39 years ago.
There are a lot better options out there after all it's only a trigger with a coil on top.I suppose the fact that there cheap is a driving factor for
some. mike

Ouch!

My CJ is 41 years and mine for 38 of those. I am running Holley fuel injection with a CPU. HEI too. Runs great.

But wait a minute this is not a daily driver or long distance racer. It's my toy that I enjoy kicking butt in my sloughs with 4' of water or 4' of snow. No tar baby or trailer queen. She always makes it back to the barn.

They sure do sell a ton of them and high performance parts for them?

As far as the ignition electronics at work, they are world class, Tier 4 level emissions engine controls with 160 pins on 3 connectors that control many Fortune 500 companies engines at 600 hp.

 

[73CJ]

Not sure if you can even buy a dwell meter any more. Mine is 30+ years old and was made by Sun.
LG

Well if you're going to work on them you need the proper tools. . .

 

[Lumpy Grits]

Alright, now that you guys have given me a headache thinking about all of that.
Check the issue I'm having now on my new thread.

Thanks for the help/advice.

Including a link would help some--
LG