I just went through this with my B-125 engine.

Get the engine rotated to TDC #1 compression (the magneto coupling should be horizontal), then rotate the magneto  until #1 fires, then rotate the magneto slightly backwards enough to engage the impulse, then rotate it forward just enough to get the coupling horizontal, then install the magneto with the top of the magneto rotated toward the engine.

Tighten the magneto bolts lightly so as to allow the magneto to still turn. Then CAREFULLY rotate the top of the magneto away from the engine until the impulse trips. If all went well this will be correct, then re-test and fine tune as needed. If you are able to rotate the engine backwards with the fan about a half-turn then you won't need to go forward the full two revolutions. I like to go forward a half-turn and, using a spark tester, check #2, then #4 and #3 in sequence and finally back to #1 just to verify that sparks are indeed happening when and where they should.

Brinky wrote: ... I have new 50mm cables. When the start button is pressed I have 4.5v at the starter is this too low?

Did you perhaps mean 5mm cables? 4.5v is a bit low but 5mm cables are pretty small, 50mm is huge.

Brinky wrote: 50mm² , 345 amp

Ah, thank you for the edification, I missed the little 'square' symbol We do some things a bit differently on this side of the pond

Those cables should be adequate; you could do a voltage drop test on them to verify.

Put one test lead of your voltmeter directly on the battery terminal and the other lead directly on the terminal stud at the starter. While cranking, read the voltage drop, it should be a tenth of a volt (0.10 Volt) or thereabouts. This will test the conductivity of the entire cable and the the connections.

Do the same on the ground cable, testing from the actual battery terminal to a good ground near the battery. The voltage drop needs to be very low on this one too.

Brinky wrote: I did a drop test today, fully charged battery measured 6.5 volts, when turning over on the starter stud it dropped to 0.5 volts, and on the ground it dropped to 4.5 volts

Perhaps I was not clear enough as to the purpose of the voltage drop test and how to perform it.

The intent is to test how much voltage is dropped (or lost) through the actual cable itself.

This is done by placing one probe on one end of the cable, and the other probe on the opposite end of the same cable. When no current is flowing, there will be no voltage drop at all. When the starter is cranking and current is flowing, there will be at least some voltage drop, but we want it to be very, very low.

If this is how you actually performed the test and found 0.5 Volts drop on one cable, that is cause for concern.

A drop to 4.5 Volts (losing a full 2 Volts) across the starter seems a bit much to me.

Did you do a drop test on both cables (one at a time)? If each cable is losing 0.5  Volts then you have too much resistance somewhere in the cable circuit.

The battery may also be weak as another member mentioned above.

Brinky wrote: Hi thanks for the swift reply. Yes I tested each cable individually as you explained the test on the starter stud is dropping to 0.5 volts, that’s a 6 volt drop.

Just to clarify so that I am understanding correctly; Are you measuring from the starter stud to ground? Or from the starter stud to the battery?

Brinky wrote: I have the starter solenoid mounted in the tool box, so the cable goes from the battery to solenoid then from solenoid to the starter. It is a 6 volt solenoid used on early v8 fords.

This adds another set of connections that could cause high resistance. A voltage drop test across the solenoid itself is in order.

Brinky wrote: Have had the battery on two different charges neither indicating a fault. Thanks again

Slow cranking can be caused by:

Weak Battery

Poor connectivity (high resistance anywhere in the cabling including the ground return path)

Dragging Starter

Excessive friction in the engine being cranked

Voltage drop tests are important in order to locate poor connectivity.

A clamp-on style ammeter is also very useful to determine actual current draw to pinpoint if the battery is weak or if the load on the starter is excessive.

Brinky wrote: From the starter stud to the positive terminal on the battery, its negative ground

0.5 Volt drop from the starter stud through the solenoid and to the battery terminal is more than I like to see but also isn't horrifying.

If you are also losing 0.5 Volts on the ground cable then you are losing 1 full volt through the cables that isn't doing any good.

Even though the battery measures 6.5 volts doesn't mean that it can deliver full amps; a proper load test can be informative.

An amp draw test while cranking is the best test.

Brinky wrote: I have a 6 volt drop from the battery to starter stud . 6.5 volts to 0.5 volts

That indicates a MAJOR high resistance in that circuit In fact, it is so major that it is difficult to imagine the starter being able to even budge the engine.

To clarify once again; with one meter lead on the battery terminal and the other meter lead on the starter stud, the meter is reading a 6 Volt drop while cranking?

It is important to take the reading only while cranking. Attempting to take this reading when not cranking will give erroneous readings.

steve(ill) wrote: if you can pull the battery down to 4 v during cranking... it dont sound like the battery is in very good condition....

Yup.

At this point it is difficult to pinpoint exactly without a current draw measurement while cranking. Load testing the battery would be helpful.