I restored this 675 and had it running like silk. Then it suddenly stopped. When I looked at it, everything was in place but the valve gear was jamming. I repositioned the crank rods several times and it seemed free again. But when I put it upright it would jam again. I made sure the crank screws were tight but still the same result. The question is what position should both cranks be in relative to one another
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With the counterweight of the wheel at the top, the crank is slightly forward of the axle. It should be keyed with either a 2-pin eccentric crank or half moon (early) crank.
When upside down its free when up right jams can’t find the jam
Look for a loose wheel. All of the side rod screw holes should be in a perfectly straight line on both sides. If not there is probably a wheel moving on the axle.
Finally got it pinned down. The motor brush plate dropped a screw I should have thought of it since when I turned over and then back down which caused the problem. The screw is probably on the track somewhere
Haha Magne-traction is really good at finding those screws for you! 
sometimes the magnet picks them up and tosses them up inside the works
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