I have a 90, quadracer with many mods, the motor pulls so hard, it pulls the bearing carrier forward and loosens my chain.
I even bought hardend bolts, tighten them to 80 Ft lbs, still moves.

Does anyone have any ideas????
Is this common?

I was thinking about a block in front of the carrier, wont move then.

 

I will assume that you have a good set of sprockets and an o-ring chain that isn't stretched. If these items aren't in good condition then you are fighting a losing battle.
I've got an '88 Zilla that had a problem with a perfectly good chain coming loose. That was before I began to use a torque wrench to ensure the proper torque was achieved. My manual says that the two smaller bolts should be around 60 ft-lbs and the two larger ones should be approximately 85 ft-lbs (the 250 may be different). Are you sure you aren't reading the N-m gauge on your torque wrench instead of the ft-lbs? That could account for your problem. Also you mentioned getting hardened bolts. That will only hurt your situation. Part of what creates the "holding power" of the bolts is the very slight yielding they experience. The bolts you bought aren't going to stretch and therefore may slowly loosen back up. I don't think these wrong bolts are causing your problems however since it is happening so fast. Be sure you are following the proper torque specifications for each size bolt and that the chain adjusting nuts (and lock nuts) are tightened. There is no way your 250 is "pulling" as hard as my 500 (which always has a perfectly adjusted chain).

 

I have a new o-ring chain, and good sprockets.
The stock bolts stripped out at 60 ft lbs, so i went to harder, and can get like 100 ft lbs out of them.
I got an idea for a brace in front of the housing so it wont pull forward even if the bolts come loose.
After closer in spection, the carrier does not come forward too much, it pushes the brake rotor side backward, therefore the sprocket is slightly forward, and on an angle, causing my chain to pop.
I think i need a brace so that side does not move rearward also.

 

THIS IS WHY I HATE THE FACTORY SWINGARMS AND CHANGED MINE OVER TO A LAGAERS WITH THE ROUND HOUSING. REMOVE YOUR CARRIER AND INSPECT THE 2 REAR BOLTS THAT COME OUT OF IT AND MAKE SURE THEY ARE TIGHT. IF NEED BE REMOVE THEM AND INSTALL NEW ONES WITH RED LOC-TIGHT. THESE 2 BOLT HOLD THE CARRIER BACK. ALSO LOOK TO MAKE SURE YOU CARRIER IS TIGHT IN THE ARM AND DOESN'T HAVE A TON OF PLAY. WHEN YOU GOT TO LINE THE AXLE UP TAKE A CENTER PUNCH AND STICK IT IN THE SWING ARM BOLT AND USE A TAPE MEASURE TO MEASURE BACK FROM THE PUNCH TO THE AXLE. DO THIS ON BOTH SIDES TO MAKE SURE YOUR AXLE IS STRAIGHT. THE LITTLE MARKS ON THE ARM ARE IN ACCURATE IN SOME CASES. IF YOU AXLE IS NOT STRAIGHT THEM YOU BIKE WILL NOT TRACK STRAIGHT AND IT WILL WIGGLE THE HOUSING AND CAUSE LOOSING BACK THERE. WATCH PUTTING ANYTHING IN THE SPACE BETWEEN THE FRONT OF THE CARRIER AND THE ARM THAT YOU CAN'T PROPERLY SECURE SINCE YOU DON'T WANT IT TO COME LOOSE AND JAM THE SPROCKET. IF YOU NEED AN EXTRA ADJUSTER I HAVE A WAY TO DO IT AND COULD FAX YOU A DRAWING OF IT. I WILL HAVE TO DRAW IT AGAIN SINCE I RUN THE LEGAERS ON BOTH ZILLAS BUT I CAN SHOW YOU HOW TO PUT AN ADJUSTER IN FRONT ON THE CARRIER. OH AND IF YOU OR ANY OF YOUR BUDS NEED A SWING ARM I GOT 2 OF THEM FOR SALE. ALONG WITH A TON OF OTHER PARTS.