Cawi Gas Cap Troubles

If its not one thing its another. I recently got a 68 360 Viking running thats been a heap of parts for some time and when I first started it everything was great. Started on the first kick, idled good, sounded good. So the next day I trucked it to the desert for a test ride but it wouldn't start. The motor popped once when kicking it over and that was it, nothing else. After getting it back to the garage I noticed a pool of black oil dripping from the exhaust manifold and immediately thought a crank seal had failed. So I started the tear down process by removing the Cawi cap to drain the tank and pressure rushed out. Once the top end was off the motor I found that the crank case had about 4oz. of gas laying at the bottom.



Pressure in the tank had forced gas past the carburetor float valve seat when the petcock was on after the initial start up and while trying to start it in the desert. Its been 100 degrees here lately which causes gasoline to vaporize quickly. I took the gas cap apart and found quite a bit of rust and corrosion that had caused it to stop venting properly.



The gas cap comes from a stock pile of caps from different tanks I acquired over 20 years ago. I rechromed them at one point in time but I don't think the plating process caused the rust problem as much as the cap being exposed to moisture in the tank which it came off of.



This plugged gas cap episode brings to mind threads I've read on this site of folks having problems with fuel flow. I can see how a problem could develop, even from a partially plugged gas cap, due to too much pressure in the tank pushing the fuel past the carb valve seat as well as creating a vacuum in cold conditions that would not allow fuel to flow at a normal rate.



Thinking about how rusty some tanks get inside it stands to reason that when the moisture vents out through the gas cap it could condense inside the cap where the resulting water droplets would rust things up.

Check out the outside perimeter of the cap in the first picture and you can see tiny amounts of rust in the vent holes.

Good job diagnosing the concern, you'd never realize all that rust and corrosion would be hiding in there. Good for the rest of us to know on the old Huskys too. Funny story, in my automotive life, I once had to diagnose a Fire Truck that would run for a distance then stall. Turned out that someone had plugged the fuel cap vent out of concern for having fuel vapors catch fire. So, the idea that fuel starvation could occur (as you noted) is legitimate as well. Thanks for the heads up.

Something else to watch here: Fuel with Ethanol in it will cause all rubber seals in the old Cawi caps to swell - blocking the breather path.

We have 150 NEW vented caps in stock and are waiting for a special order of caps with the vent tube fitted. Should be here within the next month...

Andy

Andy, show us the caps with the vent tube when you get them done. I need one for my 79.

I have to weigh in on this older post. Learning the hard way. I had my 74 mag motor completely rebuilt. The gas cap that was on the bike when I got it looked original, but it turned out to be a cheap (non vented) copy. Two seizures and much heart ache later, I found the cause of the seizures was a non vented gas cap! Replaced it with a original circa 70's after market vented cap and the problem was solved.

Sorry you had to endure such an expense due to the wrong gas cap. In any event thanks for sharing. Your story reinforces the importance of a well vent fuel tank.

my method provisional
for tank 79/1983
air chamber valve !