Yes that seems about right. When I had my old GTS engine the best I could get was twelve miles per gallon. But it was almost one hundred horsepower more too, so I didn't complain. Most times I was only getting eight with it.
You nay want to check your distributor timing. Especialy if your engine is running hot. There is a thread here where I asked about the car dieseling or running on after I turned the switch off. George said it may be timing and by golly I am betting almost anything he is right. Because my milage has fallen way off again and it is not running nearly as good as it did. I am going to check it this weekend. Unfortunetly to check the timing you have to remove the interior of the car. Most of it anyway. So it is not as easy as on your Sunbeam.
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