B won't start.

I can't get my '39 B started, crank start.  Have rebuilt mag. , newer spark plug wires, new spark plugs. All the plugs fire.  When we pull the tractor with a pickup it just back fires/won't start. There are no timing marks on the flywheel , we have looked very carefully. We have cranked to get cylinder #1 to TDC with the spark plug out and timed with mag.  It is getting gas. Suggestions???

Sounds like the mag is 180 degrees off. Find compression stroke on #1. You can do it with your thumb over the hole. Once you're on compression stroke, you can use a piece of stiff wire like a coat hanger, turn the engine until the wire stops rising out of the plug hole. The timing marks are on the flywheel, they just aren't stamped very deep. From there put your mag on with the rotor pointing at #1 and the timing should be close enough to get the engine started. I can't remember if it says FIRE or just an F that you center in the window. Once you find the mark, a dab of white paint makes it easy to see.

I brought home a 39 B last week same deal hand start had been parked in a pole barn for last 25 years it also will not start but it has no spark from the mag. The fly wheel has an F on it for fire #1 you can find it thru the inspection hole on left side of housing the F on mine was very faint and kind of hard to see, I put a piece of close line rope in the spark plug hole and crank it over till I can feel the piston holding the rope then start looking for the timing mark have to look close to find it. 

With the magneto (off the tractor) there is no firing order to think about. The rotor points to the terminals in order . When you have a spark ant number four terminal the next terminal to fire is number one. When bringing the number one piston up it will not want a spark to occur until it is at the very top. That is why I said (as soon as you feel pressure at the number one plug hole to stop and install the magneto. When you then continue to turn the engine over and the piston reaches the very top it will trip the impulse and the terminal that fires will be number one.

Could Be that the impulse spring is broken or the impulse pawls have the corners rounded off to where it will not catch to load up the impulse spring. (could be)

how would the plugs fire if the impulse didn't work?

Two guys asking different questions on the same thread Charlie. The impulse thing was for Larry B.

And I'll say it another way. The impulse gives a quick spin of the mag rotor so you can have the 3rd ingrediant in making electricity which is motion between a conductor and a magnet. This will cause current to flow in the primary windings and when the points break current will flow in the secondary windings. The impulse coupling pawl stops are set so the whole process will occurr in a retarded state for easier starting and reduced kick back. After a specified rpm is reached (almost instantaneously) during start centrifical forces keep the pawls away from the stops and return the mag to a preset timing. On some very well tuned engines it is possible to make it idle down far enough to cycle on the impulse. I wouldn't recommend it though as its bound to not be good on the impulse.

Will this method of timing the magneto work on a WD45?