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If you think you'll have problems - Dry Gas - about 4 to 6 cans. It may be a rare commodity in the land of perpetual summer, but anything with methanol or alcohol in it might work as well. Have it hot - deep hot - a 20 mile trip prior to testing should do it. Good Luck.
If your car is running smoothly, you should not have a problem at 70K miles. If you are concerned, there are smog stations that advertise: "No charge if you don't pass". You pay about $10 extra, if you pass. Beware, as these stations also repair smog related items to get your vette through the process. It would be advised to validate the shop by asking anyone who has used their services for a recommendation.
I don't think you will have any problems, but if you think you need help I have heard of some people having good luck with a product called guaranteed to pass.
Put the engine in field test mode and see if the O2 sensor is OK.
Some dry gas helps.
Make sure it is up to operating temp when you go in for the test.
From: San Diego , CA Double Yellow DirtBags 1985..Z51..6-speed
Re: SMOG TEST ANY TIPS TO HELP PASS (65Z01)
Worn out catalytic converters will give you high hydrocarbon levels (HC).
Retard the timing two degrees.. that will help burn the fuel better.
Put in a clean air filter, and 87 octane gas.
This will keep your Hydrocarbon emmisions low. I did neither of these on my Ford, and ended up failing.
I just passed the smog test with my '92 about a month ago. She has 101,000 miles on her; only thing I have replaced is the Oxygen sensors. You shouldn't have any problems if yours is running well.