I recently brought a mazda rx7 with a 12a bridgeport
The previous owner removed the oil injection system on it and said to premix the oil with the petrol in the tank, does anyone know what the petrol-oil ratio it should be?

 

Try a quick search, that should provide you with everything you need to know. Oh, and don't limit your search to the 1st gen section.

 

Between 100:1 and 150:1 should be fine for a street car. You can add more oil if you race, but it will probably smoke. I typically use 128:1 because its easy to remember - one oz for every gal of gas.

 

One time I dumped a quart of motor oil in with 10 gallons (that is 40:1!) because I forgot to buy two stroke oil. Didn't smoke. Then again, who's to say the oil actually dissolved in the fuel... Ehh. I drove for 2000mi with no OMP or premix with no ill effects. Then I realized the problem and started premixing. (Who knows how long the previous owner drove it like that!)

When I would remember, I'd just add one 8oz bottle for every 10 gallon tank. Add it first so the fuel going in gives a mixing action. If you have an '81-85 car then you have a larger tank and you might want to dump in say one 16 ounce bottle.

Aw crap, just realized... Metric. The 100-150:1 range is just peachy. For reference, the stock OMP injects anywhere from 400:1 to 150:1 depending on load, although it is recommended to increase the rate as you bring power (revs) up.