This summer for the first time since I have owned this car from 1971, the fuel is foaming (boiling) in the carbuetor bowl even though EVERYTHING electrical and mechanical checks out ok. The idle stability comes and goes when the ambient air temperature is above 80 F. This only started when I topped off the tank that had 7 gallons of winter gas left with premium summer gas. Since then I tried regular summer gas (same brand as the premium Mobile) with the same idle issue. Turns out summer gas has a lower vapor pressure (less than 14.7 psi) than winter gas (greater than 14.7 psi) to help minimize air polution, etc. I am actually starting to wonder if this year's summer gas has a new formulation and optimized for fuel injection. I will know when I get a chance to run this car on BP summer gas and BP winter gas late this fall. Also, I am wondering if different brands have different formulations for summer gas which may effect Opel idle. The last 2 tanks were Mobile. Before that (last year) I was using BP, Shell and Marathon without any idle problems. Thinking maybe we should start a thread listing gasoline brands where we are having problems and not having problems.