Guys I think I finally figured out where coolant has been going in Bugsy my '68 Sprite with a 1098. One of those eureka moments that seem to occur in the middle of the night. O was tweaking the carbs getting ready for a car show last week and all of a sudden noticed a coolant leak coming from the heater shutoff valve. A snealy leak that was slow but not dripping on the ground as coolant would run down valve and hit the hot engine block and immediately evaporate. Not enough to smell just a slow steady drip, drip, drip. You needed to be actually looking at the valve to notice it.

Took valve apart yesterday and found rubber O ring is shot. Thanks to Frank I was able to find a #5 O ring 1/4" ID, 3/8" OD at Ace Hardware. Took 3 Hardware stores to find the correct washer but it looks like Ace is the place for this one.

Further reflection reminded me of Kim with his overheating problem after runing up into the mountains for 20-30 minutes of hi speed climbing. You may be having the same issue. Leaking around the heater valve. Not fast enough to be easily visible but happenns as altitude changes.