Tachometer

And I found out it is important to have the right ballast resistor too. My B110 had the original equipment resistor mounted on the firewall. The porcelain was cracked and I thought I would replace it. The new resistor "looked right" but the truck would hardly start and run with it. I ended up taping the old resistor together and it is still running perfect 25 years later.

If I remember correctly there really isn't any such thing as a 6V or 12V coil per say. It's the total resistance in the primary circuit that is critical. In a 6v you should be looking for around 1-3ohms of resistance. A 12V system should have a total resistance value of around 4-6ohms. A high impedence coil should not require a resistor in a 12V system. It will run in a 6v system, but will have a weaker spark. You can run a low impedence coil in 12V system as long as your ballast resistor is on a high enough resistance to get to around 5ohms. I'll have to dig around for the article I read on coils a while back. It was very informative.

That's what I recall, I'm sure someone will correct me if I'm off base.

If I remember correctly there really isn't any such thing as a 6V or 12V coil per say. It's the total resistance in the primary circuit that is critical. In a 6v you should be looking for around 1-3ohms of resistance. A 12V system should have a total resistance value of around 4-6ohms. A high impedence coil should not require a resistor in a 12V system. It will run in a 6v system, but will have a weaker spark. You can run a low impedence coil in 12V system as long as your ballast resistor is on a high enough resistance to get to around 5ohms. I'll have to dig around for the article I read on coils a while back. It was very informative.

That's what I recall, I'm sure someone will correct me if I'm off base.

While the total impeadance (effective resistance) is important, just adding a resistor to a 6 volt coil does not make it suitable for 12 volt. The winding of the coil and the expected supply voltage determine what the ballast resistor should be.
A coils effective resistance changes as the coil charges, this is called impedance and it starts low but increases as flux biulds in the coil.
While coil theroy is all well and good, as a practical matter all you need to know is what voltage the coil was designed for and if an external resistor was called for and what that resistor is.
A ballast is used to allow the coil to charge to its maximum magnitic flux in the shortest period of time. It is basicly a crude current limiter that allows for a lower impeadance coil to be used on a higher voltage then if the same coil was used without the resistor. It keeps the current more or less steady as the coil charges and the impeadance changes. If the same coil is used without the resistor, the spark will weaken as the engine speed increases.
Is important to use the resistor and coil (where external resistors are used) as a matched set. The buyers guide for the coil will call out the correct resistor for that coil.
By useing an internally ballasted coil, you illiminate any possable mis-match.

I believe the coil was replaced when the guy converted it to 12V but if it's hooked up backwards how is the truck even running?

The coil will operate if hooked up backwards with slightly reduced output an the spark will jump backwards at the plug, wearing the side electrode instead of the center.

http://mgaguru.com/mgtech/ignition/ig104.htm

Thanks guys! ill switch them up tomorrow!

In old, non-vented points sets, the contacts would end up leaving a "deposit" of metal from one contact to the other. I was looking for a diagram illustrating this, but couldn't find one...but I recall in years past seeing a diagram illustrating the polarity of the points and how it affects the deposits...whether they traveled from the stationary to to moving contact, or vise versa.

Maybe someone else can find that illustration.

I need to replace my points soon they're slipping a bit

In old, non-vented points sets, the contacts would end up leaving a "deposit" of metal from one contact to the other.

With that being said, really only proper way to adjust points is with a dwell meter, especially when adjusting a set of used points. Now before there are comments from the ruffled feather crowd, yes, I know it will run and run very well after having the points set with a feeler gauge.