I am tearing down a 4 screw SU carb. There is a disc/bolt on the bottom of the carb that appears to screw into the bolt housing that attaches the carb float housing to the main body. I don't understand the purpose of this except to seal the hole on the bottom of the main housing. Anyone care to enlighten me?

I think what you're talking about is on the carb bottom and looks like a thumbtack on steriods.It is called the jet adjustment limiter.As the name implies it provides a stop at the maximum adjustment point for the jet. Have Fun!! Daniel

Yes, it is a thumb tack on steroids. I'll have to look at that again. I didn't see where it affects the jet. The thumb tack bolt does have a nut on the end that limits the bolt penetration into the bottom of the carb. body cavity in which the bolt that attaches the float resides.

If you were to unscrew the jet too far during carb adjustment.The jet adj. would hit the stop.Its the round "nut" on carb bottom with little balls on it.The bolt on the stop is to keep it securely in place and stop the factory from screwing it too deep into the carb and cracking it.There is no reason to touch the stops.They do not require any maintenance and are factory set. Have Fun!! Daniel