Why won't my bike run backwards?

I mean in reverse. It is nothing new long ago I stopped being prepared for a reverse running engine when it kicks back and starts at the same time. I am not looking for advice to aleviate the problem just the logic of what is happening. I figure if it kicks back the crankshaft must be rotating the wrong way and the spark would be after top center so how does it reverse itself and run the proper way?

Fran Ha ha I can't figure out how to edit the title or delete the post to add a letter next time.

Fran,
I had a 78 250OR I bought new in 78. Back around 82-83 after lots of desert racing miles on it.
I was out puliing ribbon after a race one day and when I stopped to grab the ribbon my engine nearly stalled.
I grabbed the throttle and kept it running, but when I started to ride away it felt like my backwheel was in a hole. When I looked down at the back wheel and let out the clutch the wheel was turning backwards.
My theory is the crank was very loose causing the piston to hang at TDC as the crank pin moved across the bearing before it began to pull the piston downward.
When it caught just before it stalled, the piston was not quite at TDC and it fired and ran backwards. I shut it off, gave it a kick and was on my way.
When I thought about later, I remember it didn't seem to rev like it should, but it was running.
Ron

I had that happen on a 73 Yamaha RT 360 with a compression release,as I was coming down a steep long downhill and when I got to the bottom the engine I thought had stalled but it caught and was running,so after was sitting around waiting for my buddy to come down and we went to take off I revved it and took off I nearly took out the family jewels on the gas cap. After that I always made sure it was going in the right direction.

my 87 xc 250 did it too quite a few times...something was screwy with the sem timing ....swapped in a motoplat and never had a problem again

Timing too advanced BTDC, (before top dead center) . If timing is too far BTDC and ignition occurs then its going to force the piston back down (running backwards instead of it braking over TDC and continuing the power stroke. Which also causes detonation. Same thing causes kickback. When they say a auto "diesels" during shut down it is actually compression ignition of the junk fuel trying to combust on the hot dome of the piston that is coming up to TDC on compression stroke, forcing it down and engine then rotates backwards.

Engine may not ping at higher RPMS because the engine can take that advanced timing at greater RPMs. Usually retarding the timing a little will help alot. Most factory timing settings are pretty far advanced in reality esp. the big bores.

I know exactly how the timing is set on this one. I made measurments with the cylinder off to find top center, then had a dial indicator in the spark plug hole as it is centered in the model pictured below. I have had the timing light on it to see how accurate the pin in the hole is compared to the spec. I am pretty sure I have settled on 12 degrees before center, not the 18 in the spec and I am pretty close to as retarded as possible without enlarging the slots for adjustment. It kicked back twice yesterday, one of which it started as well, but not anywhere as hard as before I made this wood protractor.

Most of my riding of these bikes has been with the made in spain motoplat pictured. I kind of think it says mini 6 on it but can't see that in the picture. I am confident the xc500 1983 version would occasionally do the kick back and start at the same time and it ran until the crank finally gave out without ever touching the timing adjustment from new. After posting yesterday I was wondering if the spark might not occur in the exact same place when turning backwards so it would never run in reverse for even a full revolution.

Fran