I went through the same thing you did last month...my HC levels in my 300E were just a tiny bit high. What Duke recommended is correct but there are several things you can do.
1. Manage the test. Make sure you can keep the revs up in the 2500 range until the technician gets into the car to commence the test.
2. Clean your spark plugs.
3. I used the stuff you can buy at the auto parts store that you put into the gas tank before the test. It's a fuel cleaner and they guarantee that you will pass the test or they will refund your money.
4. An oil change would help too.

Good luck.

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1987 300E
1967 Jaguar E-type Series I, 2+2