View Full Version : '74 Midget - that fuel vent line in the trunk

09-15-2010, 06:52 PM
You know that small (1/4"?) fuel vent line that comes out of the tank and to part of the evaporative control? It's on the right side, next to the fill hose.

Mine's cracked about right where it goes into the fuel tank, and lately I've noticed a gas smell on filling the tank.

Is that just a standard (albeit with wire braid around the outside) piece of fuel line that i can replace myself? Or is it a specialized line? I couldn't find it in the Moss Motors catalog.

09-15-2010, 10:32 PM
Just replace it - nothing special as I recall

09-16-2010, 12:58 PM
That's the anti vapor lock line. I didn't know earlier cars had it too.

09-16-2010, 03:02 PM
VAPOR LOCK! Sadly, I now know what that is. :wall:

If your MK III fuel system is not vented (mine wasn't), "vapor lock" may occur: without venting, as the fuel tank drains a vacuum is created in the system. This vacuum will cause your fuel pump to seize. Then, you're stranded! I had to call a flat bed because of this kooky anomaly. It only occurred after driving about 20 miles or so, then the engine would die from fuel starvation. It appears like a fuel pump failure, but it isn't! The car would start again after about 20 minutes or so, then it would happen again.

This problem about drove me crazy. My fuel pump is under the car, and it's practically new. Everything engine-wise is sorted and in great condition: rebuilt carbs, new lines/hoses, engine tuned, etc. I tried all the roadside remedies: whacking the fuel pump, etc. Finally, I found the SU Fuel Pump "fuel delivery shooting guide" (see link, below) and "vapor lock" was listed on the first page! So, I drilled a small hole in my gas cap, problem solved! And thanks to my AAA membership, this was a cheap fix.

A test (and cure) for vapor lock is to open the gas cap quickly: if you hear a "whoosh" sound of air rushing when the cap opens, your vapor lock problem is solved (e.g., by letting air back into the system). If removing the cap cures your problem, leave it off and go purchase a vented cap.

What an obscure problem . . . :yesnod:

SU Fuel pump articles: https://homepages.donobi.net/sufuelpumps/sufuelpumparticles.html

09-16-2010, 05:54 PM
I've read a few comments on other sites mentioning just plugging up this vent line, but only if you have a venting gas cap.

09-16-2010, 07:19 PM
I don't think you'll have a problem. I have an earlier cap on mine that isn't vented either and my line is essentially plugged up.