Built a 1915 Roadster truck and mounted a round tank in the bed behind the seat for better head pressure. thought that was a good idea although the "purists" will probably cringe.
When the tank is full I get "blow back" (or whatever) and I get gas out of the vent hole. Enough to puddle up filling the bung cap and run down the side of the tank. Any ideas how to fix this other than less gas and relocating the tank?
Maybe modify the gas cap by adding a coiled tube?
My first question is how full must the tank be for this to happen? Perhaps just not putting in that last .5 gallon might do the trick.
We had a similar problem on the speedster. I had placed the round tank on top of part of the boot and the filler is right behind the passengers head. My son in law was passenger when I was driving after a fill up, and of course he complained loudly. I solve the problem by soldering a 2" length of fine tubing inside the cap. It seemed to have solved the problem.
I filled the tank to just under the cap.
Coil tube? Place it how?
Originally the gas cap was made in two parts, an inner sheet metal part with a small hole, then the cap with another small hole for venting. With this design sloshing gas inside is less likely to come out of the venting hole than with the currently available solid repro caps. Unfortunately most originals were made out of pot metal and has rarely survived until now.
Attach the tube to the gas cap by your method of choice (braze, epoxy) so that it sticks above the tank as shown in my crude drawing. This wouldn't work for a tank under the seat, but since yours is in the bed it would work.
Thanks Mark, Roger and Tony. I'll give it a shot!
I'm getting an engineering bug. I want to see if there's a similar fix for tanks under the seat. Another project to add to the list. As if my girlfriend didn't add enough projects already!
As a temporary fix you might call Lang's and order the later gas cap if it fits your tank. It has a chamber below the top and offset venting holes from the top to the chamber and from the bottom to the chamber. Good venting and no slosh out the vent hole which is otherwise common. I also found an "o" ring that fit the cap nicely under the edge lip and I just put the cap on until I feel the drag of the o ring. My gas cap is totally dry and doesn't come loose. OK its a later cap than my '23 touring car originally came with but it ended the gas leaking out the cap.
Sometimes you can find the missing piece of the original cap that Roger mentioned in the bottom of your gas tank. It is a slightly dished steel piece with a very small hole in it. Usually the original pot metal cap its too deteriorated to hold it in place anymore and it will fall off again if you try to use it.
I have had good luck using a cup type freeze plug that fits in the underside of the cap. Drill a small hole in it and use some JB Weld to hold it in place.
With the tank exposed to the sunlight, the gas will expand quite a bit on a warm day and if the tank is near full you will have gas coming out the vent even if there are two parts to your gas cap. The two part cap will usually prevent gas sloshing out of a full tank when rounding a left hand turn though. Best bet is to not fill the tank to the top. Fuel storage tanks are limited to 90% fill to allow for expansion.
The fix I used for this problem was to replace the gas cap with one with a splash guard. The link below shows one from Lang's but I am sure all the venders care it.
Very nice. Didn't even know they existed.
Cheaper to just not fill it to the top, John ;>)
"I have had good luck using a cup type freeze plug that fits in the underside of the cap. Drill a small hole in it and use some JB Weld to hold it in place."
I repaired one w/JB weld like that. It was fine for a couple of years until I put in some gas w/ethanol in it, then the gas started to soften the JB weld.
Or you can get this kit;