I have a 1987 300 ZX non turbo. A couple of months ago, I noticed a drastic fall in my MPG's. I used to get at least 300 mpg and now I'm lucky to get 150. With gas prices what they are - this really sucks. I decided to sell it, but it failed smog. The oxygen level was 10.21 and only .73 is allowed. The timing registered at 18. Isn't that kind of high? What could be causing this?