About five hundred years ago, I had uncontrolled fuel leak/flooding on the center carb of the Holley Tri-Power on the 'Camino.
I tightened the needle 'n' seat assembly, removed and cleaned the needle and seat...nothing helped.
Got to looking at the thing. The stupid rubber O-ring on the outside of the needle/seat assembly was essentially deformed--crushed--and couldn't seal the seat housing to the float bowl.
Holley sells a "kit" to fix this. They DON'T sell just the stupid O-ring. I didn't need a "kit", I needed an O-ring.
That was my motivation to research O-ring sizing and materials. The Holley O-rings are not the "premium" material, which is Viton. I think the Holley O-rings are cheap neoprene.
Viton has fabulous temperature rating, and it resists cold-flow (permanently distorting due to being compressed between the float bowl and the seat housing). A downside to Viton is that it gives off toxic fumes when burned. Viton, I guess, is kinda related to Teflon and has many of the same qualities--compatible with many fluids, stable, etc.
Anyway, I ordered the minimum quantity (a hundred) Viton O-rings in the proper size for Holley needle/seat assemblies, used three on the Tri-Power, sent ten or twenty to a guy in Arizona; and have somehow managed to lose all the rest of 'em. Oh, I'm sure there nice 'n' safe...somewhere...out in the Shop.
Point is, you can figure out the sizing of the O-rings you need, and then either buy a Viton O-ring assortment, or buy the specific Viton O-rings you need from a supplier.
If you just go to an auto-parts store and try to buy O-rings from the big metal assortment they keep behind the counter, you're almost sure to get neoprene O-rings not Viton. But--as said--they've probably got a specialty "kit" of O-rings and such specifically for your fuel system, at silly prices using quality O-ring material--probably Viton.
