1936 ohv250

[courtney]

Hello,

I've just acquired the above bike and have a couple of questions.

Firstly, I have promptly lost one of the screws from the dynamo terminal. I've attached a picture... hopefully. Does anyone know what size thread this is please?

Secondly, the wires and dynamo were not attached when I got the bike. The wires were labelled with which terminal they should go to (POS and FI), however, they were on old masking tape that came off both wires when I pulled them though from where they were tucked up. Does it matter which terminal these wires go to? If so, could someone please advise as to how I find out which is which using a multimeter?

Many thanks in advance!

[militaryron]

Usually those terminal screws are standard 2BA.  Ron

[Bicycle repair man]

The screws are 2BA as mentioned.
It does matter which wire goes to which terminal. The POS wire goes to the ammeter (your bike is negative earth). The FI wire goes to the terminal on the 4 position light switch that carries the wire wound resistance. 
Attached is a wiring diagram.

[courtney]

Thank you both, that's very helpful indeed.

Take care

[courtney]

I'm now experiencing a starting problem. When I got the bike the other day it started up quite easily. I went for a little ride yesterday for about 45 minute and it was a bit harder to start, but started ok and ran well. I went to go for a ride today and I could not start it. I don't seem to be getting a spark.

I'm new to magnetos a cm bikes like this. Can anyone suggest what the problem might be?

[Steve.S]

You might want to invest in a Spark Plug Tester. This will tell you if the spark is strong enough to withstand compression.
Check the points are clean and set to 0.0012". Check the HT pickup brush and slip ring are clean and grease free. Make sure the Points leaf spring is not touching the cam. You might want to remove the points base plate by unscrewing the centre screw and wiggling it about to release the taper. Note that it's  located with a keyway. Then you can put an Ohmmeter across the points to check resistance etc. and check that the brush on the base plate is grease free.
You may find the BrightSpark website useful in diagnosing problems, but if your spark won't jump 5/16" in air then it may be time for an armature rewind and condenser.