Hey guys. Every time i take the fuel cap off, it lets go with heaps of air pressure. Maybe my car has wind? No seriously. Is this a big deal? Is it an easy fix. Or run to the mechanic? Cheers Simon

Your fuel is evaporating faster than what you're using, try driving faster

It will only become a problem if the fueltank pressure exceeds that of what your carbs float can handle. AFAIK some dattos have a valve that controlls the pressure but can become faulty. I just got a fancy fuelcap that does the same thing.

some cars have a carbon canister to collect that vapor and burn it later, like we have in CA.

most likely just a blocked hose from years of soot. take the fuel tank out and give all the hoses that run to the carbon canister a good dose of compressed air.

this is dangerous..get it looked at.

Newer cars use a carbon canister that then vents to the low pressure side of the intake, so the vapours are burned inside the engine.

Old cars used breathers that simply went higher than the tank, and looped around a bit, then simply vented to the atmosphere.

Simply put. you should have neither vaccuum or pressure in your fuel tank.

I have seen tanks split from excess pressure.

I've also seen a 2 tank XJ6 jaguar pump the petrol from one tank into the other due to an airlock. The petrol was blasting out of the filler and all over the back guard and side of the car...an absolute 'fireball' waiting to happen...

Hi. Thanks for all of that!
Err I think I'll get it addressed quickly. I spend enough money on this car without getting a split fuel tank or some sort of servo fireball fuel filling nightmare !!
Thanks heaps guys
Has anyone seen one of those fancy fuel caps that can release pressure.

For a Datsun 1200, don't get a cap with pressure relief, you'll end up with petrol on your fender.

B210s and newer cars use a pressure-relief cap.

For a 1200:
- Some cars use a loop line (like Dattodude says). Apparently all Aussie coupes and sedans
- Others have a pressure valve in the engine compartment

Fulll details are here: Fume Control. This includes a picture of the loop system, and also discusses the other kind.

I've been dealing with the same thing. High pressure when the car sits in the sunlight, a vacuum after driving around for a few minutes. Enough of a vac that the car would start running wierd.
I found that the vent line that goes from the back to the front is clogged on my car. I've tried compressed air and even tried to stuff some safety wire through it but it's still clogged. I have to replace the whole tube.
In the mean time, i hooked up a rubber tube and vent it out thru the trunk.

yeah that gave me a prob. a while back , I didn't know that extra line went to a valve mponted to the fire wall . when I put the A-14 in the car I thought it was a return line like the 78 B-210 GX like it came out of ...........wrong lol. I could drive about a mile , then would act like she was running out of gas , couldn't breath. Thanx to ddgonzal I figured it out rather quick.